Transcript
CHEM1031 Study Notes Assumed Knowledge
Acid - proton donor Base - proton acceptor Acidic oxides (non-metals) react with water to make acids or bases to form salts (CO 2). Basic oxides (metals) react with acids to form salts but do not react with alkaline solutions (CuO, e 2O!). Amphoteric oxides (Al, "n, #b, $n) react with acids or bases to form salts. %eutral oxides (CO, % 2O) don&t react. acid ' metal 2 ' salt acid ' carbonate CO 2 ' 2O ' salt
Gases
*istin+uishin+ properties of +ases er compressible /ow rapidl tak take shap shape e of and and 0ll 0ll a co cont ntai aine nerr (li1 (li1ui uids ds onl onl tak take shape) expand and contract with temperature chan+es (more so than li1uids, solids is near ne+li+ible) in0nitel expandable (unlike li1uids, solids) low densit as ariables Pressure (#a) 3for 3force ce4a 4arrea ea.. *ue *ue to part partic icle less in moti motion on,, co coll llid idin in+ + with with 2 momentum into each other and walls. 5#a 3 5%4m 3 564m! (5% 3 5k+m4s2) 5atm 3 789mm+4:orr >anometer - measures di?erence in pressure 3 595!2;#a 3 595.!2;k#a 595.!2;k #a 3 5.95!2;bar 3 5<.7psi • • •
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Barometer - measures atmospheric pressure •
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Volume (m! - 59!=)
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number4Amount (mass - k+, moles) Temperature (alwas in @elin absolute temperature)
:hese are dependent upon each other in the three mpirical as =aws Boyle’s Boyle’s aw - V ! (or #55 3 #22) - as pressure increases, olume decreases (or 3 ) - as tem emp per erat atur ure e C"a#le C"a#les’ s’ aw aw - V ! T (or increases, olume increases (also o a a-=us -=ussa sac& c&ss - foun found d when when A$ogad# ad#o’s o’s aw (als +ase +asess rea eact cted ed olu olume metr tric ic rati ratios os wer were sm smal alll whol whole e numbers - a stochiometric ratio) % V ! n (or 3 ) - as the number of moles increases, so does the olume Combinin+ Bole&s and Charles& =aw or 3 PV ! T Combinin+ all three forms the &deal Gas aw PV ! nT •
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or Constant mol-5
PV ' n(T where
D
3
Eniersal
as
3 F.!5<; 6 mol mol-5 @ -5 ($G) 3 9.9 9.9F2 F29; 9;7 7 = atm @ -5
$tandard :emperature and #ressure ($:#) 9 oC (27!.5;@) and 5 bar (5.99H59 (5.99H59;#a or 9.IFatm). 5 mole of +as at $:# is 22.7=. Je can also sub in n3m4> and densit (K - rho) 3 m4 to inte+rate other alues. )alton’s aw o* Pa#t+al P#essu#es - in a mixture of +ases, total pressure is the sum of the pressure each +as would exert if alon al one e un unde derr th the e sam ame e co con ndi diti tion onss (ass ssu umi min n+ the +as ase es ar are e independent and do not react) # : 3 #a ' #b ' #c ' L Mole ,#a-t+on - for each component A in a mixture, the mole fraction is (a alue between 9 and 5 - not percenta+e - to express the Mpercenta+eN of moles of that substance in a mixture) A 3 #artial #ressure of A #A 3 A# : ach +as also obes the Gdeal as =aw independentl as if the took up all the olume, and hence were # :3#A'#B, #A3nD: and #B3nD:. oweer these conclusions in the 57 th-5Ith centur, and it wasn&t until the 5I th-29th centur that a theor of atoms be+an to form, so these laws all looked at macroscopic ideas, in/uenced b what we know to be properties of microscopic atoms. @inetic :heor of ases mole molecu cule le siPe siPe is ne+l ne+li+ i+ib ible le co comp mpar ared ed to dist distan ance ce between them mole molecu cule less moe moe ra ran ndoml doml in strai trai+h +htt line liness in all directions at arious speeds forces forces of attrac attractio tion4r n4repu epulsio lsion n are are ne+li ne+li+ib +ible le (becau (because se the are er weak) except in collisions •
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number4Amount (mass - k+, moles) Temperature (alwas in @elin absolute temperature)
:hese are dependent upon each other in the three mpirical as =aws Boyle’s Boyle’s aw - V ! (or #55 3 #22) - as pressure increases, olume decreases (or 3 ) - as tem emp per erat atur ure e C"a#le C"a#les’ s’ aw aw - V ! T (or increases, olume increases (also o a a-=us -=ussa sac& c&ss - foun found d when when A$ogad# ad#o’s o’s aw (als +ase +asess rea eact cted ed olu olume metr tric ic rati ratios os wer were sm smal alll whol whole e numbers - a stochiometric ratio) % V ! n (or 3 ) - as the number of moles increases, so does the olume Combinin+ Bole&s and Charles& =aw or 3 PV ! T Combinin+ all three forms the &deal Gas aw PV ! nT •
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or Constant mol-5
PV ' n(T where
D
3
Eniersal
as
3 F.!5<; 6 mol mol-5 @ -5 ($G) 3 9.9 9.9F2 F29; 9;7 7 = atm @ -5
$tandard :emperature and #ressure ($:#) 9 oC (27!.5;@) and 5 bar (5.99H59 (5.99H59;#a or 9.IFatm). 5 mole of +as at $:# is 22.7=. Je can also sub in n3m4> and densit (K - rho) 3 m4 to inte+rate other alues. )alton’s aw o* Pa#t+al P#essu#es - in a mixture of +ases, total pressure is the sum of the pressure each +as would exert if alon al one e un unde derr th the e sam ame e co con ndi diti tion onss (ass ssu umi min n+ the +as ase es ar are e independent and do not react) # : 3 #a ' #b ' #c ' L Mole ,#a-t+on - for each component A in a mixture, the mole fraction is (a alue between 9 and 5 - not percenta+e - to express the Mpercenta+eN of moles of that substance in a mixture) A 3 #artial #ressure of A #A 3 A# : ach +as also obes the Gdeal as =aw independentl as if the took up all the olume, and hence were # :3#A'#B, #A3nD: and #B3nD:. oweer these conclusions in the 57 th-5Ith centur, and it wasn&t until the 5I th-29th centur that a theor of atoms be+an to form, so these laws all looked at macroscopic ideas, in/uenced b what we know to be properties of microscopic atoms. @inetic :heor of ases mole molecu cule le siPe siPe is ne+l ne+li+ i+ib ible le co comp mpar ared ed to dist distan ance ce between them mole molecu cule less moe moe ra ran ndoml doml in strai trai+h +htt line liness in all directions at arious speeds forces forces of attrac attractio tion4r n4repu epulsio lsion n are are ne+li ne+li+ib +ible le (becau (because se the are er weak) except in collisions •
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+as particle collisions are perfectl elastic k a Q absolute temperature :his explains Bole&s =aw as less less space means more *#e.uent collisions, and hence hi+her pressure (as collisions result in a force applied), and Charles& =aw as increasin+ temperature, kinetic ener+ (molec (molecule ule speed) speed) incre increase ases, s, so collis collision ionss become become more more *#e.uent and with g#eate# *o#-e. @inetic theor states k a is onl dependent on temperature, not +as tpe, and di?erence +ases at the same temperature hae the same aera+e kinetic ener+. As R k 3 mS242, heaier +ases will trael more slowl with the same ener+. Gt can be found that (don&t need to know deriation) Rk 3 %A is Ao+adro&s number. Demember this is per molecule, so to 0nd per mole multipl b Ao+adro&s number. Combinin+ this with our other formula for Rk Date of as >oement >oement Srms 3 Doot-mean-s1uare (rms) simpl means we hae s1uare-rooted the mean alue. E/us+on - escape of molecules throu+h a hole of molecular dimensions (assumin+ no collisions between molecules) )+/us+on - mixin+ of +ases until the mixture is homo+eneous Esin+ the aboe rate and these ideas (in di?usion it could be two +ases reactin+ and producin+ a colour located at a particular point and speeds) we can determine molecular mass. G#a"am’s aw - :he rates of e?usion (and di?usion) of two +ase +asess at the the sa same me temp temper erat atur ure e and and pres pressu surre are are ine iners rsel el propo proporti rtiona onall to the s1uar s1uare e roots roots of their their densit densities ies (note (note time time is inersel proportional to rate) 3 3 3 All +ases are actuall non-ideal all particles do hae olume - becomes si+ni0cant at hi+h hi+h pres pressu surre (rea (reall olu olume me T idea ideall olu olume me as idea ideall olume hits Pero) the hae attractie forces - si+ni0cant at low temperatures (real olume U ideal olume as particles are are brou brou+h +htt to+e to+eth ther er +ase +asess with with low low inte intera rato tomi micc dispersion forces like e do not experience this) partic particles les do intera interact ct - ne+li+ ne+li+ibl ible e at hi+h hi+h temper temperatu ature re (enou+h ener+ to keep bonds apart), but si+ni0cant at other temperatures (real pressure U ideal +as pressure as ther there e ar are e les less molec olecu ules les - chem chemic ical all l bond bonded ed to+ether - and hence less collisions) All known life depends upon the atmosphere, howeer the atmosphere doesn&t hae a de0nite end, with IIV within !9km, Mouter spaceN at W59,999km. • •
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Atom+- St#u-tu#e
#a+e !
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Onl alence electrons determine chemical properties, and hence isotopes hae nearl identical chemical properties. =i+ht is electroma+netic radiation (a self-sustainin+ oscillation of electric and ma+netic fields), and is characterised b its fre1uenc (X - nu P or sec-5) or waelen+th (Y m or an+strom3Z359 -59m), which are related b c3XY, with isible li+ht bein+ !.I-7.9 59-7m, whilst +amma ras are around 59 -52m and lon+ radio waes 59 <. Mono-"#omat+- (ad+at+on - a selection of one fre1uenc (in practicalit, a narrow band of fre1uencies) for arious scienti0c measurements Poly-"#omat+- (ad+at+on - consistin+ of man fre1uencies =i+ht has tpical wae-like properties (refraction, di?raction and interference), howeer also exhibits the photoelectric e?ect, discoered in 5FF7 b ertP who found li+ht could e[ect electrons from the surface of a metal, and a current could /ow to another electrode in a acuum. e also found it re1uired a threshold fre1uenc that was dependent on the tpe of metal used which was independent of the intensit, howeer once aboe the threshold fre1uenc the intensit increased current siPe, and the ener+ of the electrons emitted depended on the fre1uenc. Gn 5I9;, instein realised li+ht comes in packets or 1uanta (+ot %oble #riPe in 5I25), where each 1uantum of ener+ is proportional to fre1uenc 3 hX where h 3 #lanck&s constant 3 8.828H59-!< 6s A photon with enou+h ener+ could be absorbed and e[ect an electron, producin+ a current, and onl one with enou+h ener+ could oercome the attraction of the atom, the remainder ener+ bein+ conerted to kinetic ( k 3 hX-J). :he ener+ of a particular orbital can be found b the Ddber+ 1uation 3 D where D 3 Ddber+ constant for (on data sheet) n5 3 lower shell n2 3 upper shell or hdro+en the shell is a +ood indicator of electron ener+ 3 -D4n2 Jhite (polchromatic) li+ht passin+ throu+h a +as composed of sin+le atoms +as lines (or speci0c fre1uencies) remoed, formin+ an aso#t+on se-t#um (the release of the photons once electrons fall is in all directions, and hence much weaker at the detectin+ screen a prism can be used to distin+uish between colours). Jhen heatin+ a +as b electrical dischar+e, it produces these series of lines in an em+ss+on se-t#um. :his is because electrons occup discrete ener+ states that the moe up or down. :he spectra ar with the +as used and pressure (proximit alters ener+ of shells).
#a+e <
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=man 3 E Balmer 3 isible li+ht #aschen 3 GD :he alue of n for each ener+ leel4shell is the #+n-+al .uantum nume#. :he g#ound state is an atom&s lowest state, howeer it can under+o transitions to hi+her e2-+ted states b heatin+ or collidin+ ener+eticall with other bodies. :hese are unstable and result in the lowerin+ of ener+ leels b emission of photons. Complete remoal of an electron means the electron has been moed to n3\. :he ener+ re1uired to moe a alence electron upwards is called the ionisation ener+. $olutions to the S-"#d+nge# e.uat+on hae exact analtical forms for the hdro+en atom ]^3^ where ] 3 amiltonian (an operator that corresponds to the total ener+ of the sstem - encompasses nature of proton and electron particles and their Coulombic attraction _the electrostatic attraction or repulsion between protons and electrons`) 3 ener+ of the state (a constant of proportionalit) ^ (psi) 3 the waefunction An ei+enstate (or orbital) is an allowed ener+ (or shell) under the contraints of the $chrdin+er e1uation (labelled b 1uantum numbers - the are the outcomes or results when solin+ the e1uation). :hese are wae-like states with !* shape and amplitude (this form is a direct conse1uence of the $chrdin+er e1uation. :he electron densit (probabilit of an electron bein+ at a certain point) is +ien b the s1uare of the waefunction (howeer the eisenber+ uncertaint principle limits the abilit to know both the position and ener+ (thus speed) of a 1uantum particle like an electron) ^^ ^2 As the electron could be at an distance from the nucleus (althou+h further is less probable, the olume with a I9V chance of an electron bein+ there is called the ounda#y su#*a-e, and this surface is thou+ht of as the spatial limit of the atomic orbital. :here are four 1uantum numbers to label each electron #+n-+al .uantum nume# (n) - 5, 2, !, L, \ a4+mut"al 5angula# momentum6 .uantum nume# (l) - 9, 5, 2, L, (n-5) magnet+- .uantum nume# (ml) - 9, 5, 2, L, l (but counted from ne+atie to positie -l, -l'5, L, 9, L, l-5, l) s+n .uantum nume# (ms) - A set of orbitals with the same n are called a s"ell (in hdro+en all orbitals are in the same shell as there is one electron), and a set of orbitals with the same n and l are part of the same sub-shell, labelled b letters (called orbital smbols) if l39 s o#+tal if l35 it&s a o#+tal Demember $padoof if l32 it&s a d o#+tal =
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#a+e ;
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if l3! it&s an * o#+tal then +, h, i and so on :his is written as (n)(orbital smbol)no. of electrons e.+. 2s2 s orbitals are sphericall smmetric, howeer the inner orbitals often de/ect the outer orbitals (b Coulombic repulsion) resultin+ in their electron densities peakin+ further awa. p orbitals consist of two lobes of electron densit on opposite sides of the nucleus with a nodal plane (Pero electron densit) between them. As there are three ml alues, there are three tpes of orbitals px, p and pP. d orbitals hae either a cloerleaf shape (d x, dP, dxP, dx - ) or two lobes and a torus (d P ). • •
2
2
2
:he Coulombic attraction between the nucleus and electrons leads to a contraction of shells as ou moe to the ri+ht of the periodic table, re1uirin+ more ener+ to pull the electron out due to the increasin+ nu-lea# -"a#ge. >ulti-electron atoms are more dicult to obtain analtical solutions of throu+h the $chrdin+er e1uation as the electrons repel each other, howeer we can still approximate orbitals that resemble the hdro+en atom. :hese repulsions are considered ele-t#on+- s"+eld+ng, and do not a?ect the electrons of an outer shell e1uall s electrons of an outer shall hae at least one smaller lobe of densit closer to the nucleus (inside the re+ion of shieldin+ electron) and hence are less a?ected as the are more often closer in and not further out the e?ect is smaller for p orbitals, then d, then f, and so on :hus the de+ree of shieldin+ +oes sUpUdUf (opposite is de+ree of penetration) •
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Jhen determinin+ electron con0+uration (a particular arran+ement of electrons), orbitals 0ll up from lowest ener+ to hi+hest, and this can be remembered b the 0lin+ from the top ri+ht (0rst arrow, then second, and so on). :he au*au (buildin+ up) principle assembles atoms b addin+ one electron for eer proton (and usuall neutron) into the lowest ener+ orbital free. :his follows two rules Paul+ e2-lus+on #+n-+le - two electrons cannot hae all the same 1uantum numbers (and hence a maximum of two electrons in the same sub-orbital (same m l), hain+ two di?erent m s) Hund’s #ule - in a subshell of orbitals (same l or orbital smbol, but di?erent ml) electrons distribute one electron in each orbital ali+nment (the 0rst spin 1uantum number) before the +o back and 0ll it up the remainin+ orbital ali+nments with the other 1uantum number o the onl exception is when ou can +et a half-0lled instead of a nearl 0lled as that is more stable, and this onl occurs in two transition metals and the ones below them (roup G and G) Cr (Chromium) _Ar`
inimise formal char+es (formal char+e 3 no. of alence electrons - number assi+ned in =ewis structure, where lone pair32, bond35 summin+ the formal char+es +ies the oerall char+e if unable to balance formal char+es, ne+atie char+es +o on most electrone+atie, positie char+es on least electrone+atie) lements in the third period and below can hae an e2anded $alen-e s"all due to the aailabilit of d orbitals. Gn contrast, some elements (B, Be, Al) cannot form =ewis octets in certain circumstances, and become ele-t#on de7-+ent se-+es, not hain+ a total of F electrons (e.+. BeCl2 or BeCl!). $ome molecules are stable with an odd number of electrons, leain+ an unpaired *#ee #ad+-al ele-t#on (e.+. %O2). Jhen completin+ =ewis structures, there ma be more than one wa to minimise formal char+es, and this is called #esonan-e, written as shown in the No9 o* Geomet#+-al dia+ram (or in one structure Name Pa+#s S"ae and statin+ how man other 2 (A2) =inear structures possible). %ote no electron pair is [umpin+ all the wa around, but instead ! (A!) :ri+onal the [ump from the atom to #lanar bein+ bonded and back. Gn realit the bonds do not /ip back and worth, and each %O bond len+th is somewhere :etrahedr < (A ) < between a sin+le and double al bond (approximatel calculated b aera+in+ the ; (A;) len+ths in one of the Axial3+ree :ri+onal resonance structures). n Biprami :o 0nd !* shape, we 1uatorial d use Valen-e S"ell Ele-t#on 3blue #a+e 59
8 (A8)
Octahedr Olier Bo+danoski al
Pa+# (euls+on 5VSEP(6, which uses =ewis structures and the
repulsion of bonded (B#) or lone pairs (=#) of electrons. $#D makes no distinction between sin+le, double or triple bonds. Gnstead of a bond to a new atom, a blue or +reen sphere could also represent a lone pair. :he +eneral form of a central atom (A) surrounded b other atoms () is A n, or if there are lone pairs (), An-mm where n is the number of electron pairs (bonded and4or lone), m is the number of =#. :his +eometr is modi0ed due the hierarch of electron pair repulsions (from stron+est to weakest) =#=#T=#B#TB#B#. :his means two nei+hbourin+ pairs will de/ect, and dependin+ on what is surroundin+ it, ma cause the an+le to widen (and shorten the others). Onl in repulsion are multiple bonds considered multiple bonds are shorter and fatter clouds of electrons, and thus hae stron+er repulsion than sin+le bonds. Because lone pairs can replace outer atoms, and the take the place furthest from other lone pairs and bonded pairs, the actual shape of the molecule is di?erent to its +eometr (hain+ one or more outer atoms remoed). Comou nd Class
Geomet#y
A<
tri+onal planar tetrahedral tetrahedral tri+onal bipramid
A!2
tri+onal bipramid
A2 A! A22
S"ae
E2am le
)+ol e Mome nt
bent
$O2
es
tri+onal pramidal bent seesaw (remoe e1uatorial) :-shaped (remoe two e1uatorials)
%! 2O
es es
$<
es
Cl!
es
G!tri+onal A2! linear (triiodid %o bipramid e) A; octahedral s1uare pramid Cl; es A<2 octahedral s1uare planar e< %o A dipole moment (j, measured in coulomb metres C m oerall asmmetric distribution of electron densit) onl occurs in atoms where the outer atoms are identical and pull in a net direction, or hae di?erent electrone+atie char+es (bein+ di?erent atoms) sucient enou+h to produce a dipole moment, howeer this is not somethin+ we particular care about (onl the shape part). >olecules with a dipole ali+n in an electrical 0eld (as opposites attract). *ipole moments tpicall span 9-7H59-!9C m, and the alues are directl related (but not exactl proportional) to electrone+atiit di?erence. Gn $#D, resonance should be treated with an aera+e ond o#de# (bein+ 5.! in %O!-), and these are treated as one bond, but #a+e 55
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when lookin+ at repulsions as an intermediate between a sin+le bond and double bond. Gn lar+er molecules like hdrocarbons, $#D rules appl to each central atom (each carbon in the case of hdrocarbons), and the should be treated independentl. $#D produces 1uite accurate results, except for transition metals and some other molecules, howeer we don&t need to know these. :he an+les in $#D do not match up those in the arin+ orbital ali+nments (which are often at I9o to each other) and hence a new model formed b Valen-e Bond T"eo#y 5VBT6 is re1uired. As electrons can be considered waes, their orbitals hae wae-like properties, and orbital interactions are analo+ous to the superimposition of waes. :hat is, when two atoms bond and oerlap sli+htl, the electron densit is the sum of the two parts. :he new orbital is a composite of the old orbital, and is called "y#+d+sat+on. :his leads to the e1ualisation of ener+ies of the alence orbitals (as the become one orbital) and allows for the +reatest possible number of unpaired electrons for bondin+. %ote how positie (blue) with positie enhances the 0nal blue (as the are bein+ added constructie), whilst when added to a ne+atie (pink) it is destructie. Also note that the number of orbitals remains the same (in this case three p&s and one s), and the notation in that the three ! p&s become p . Oerall in methane
Outer atoms (that aren&t hdro+en) also hbridise before bondin+ (e.+. C2Cl2 - both chlorines hbridise !s and !p orbitals to +enerate sp!-chlorine). :his has been for four electron pairs, howeer in cases like B! there are onl three bonds to be made as there are onl three free alence electrons, and so an sp2-orbital is made (with one electron within each box, and the composite bein+ an electron from the /uorine). oweer, as one p-orbital ali+nment is i+nored (sa pP) it does not conform to the
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new ener+ orbital and remains empt at a sli+htl hi+her ener+ state. :his atomic p P orbital remains acant. $imilarl in BeCl2, as onl two orbitals are re1uired, the 2s and 2px hbridise to form sp and leain+ a acant 2p and 2pP (note how because hbridisation occurs in the alence shell there is no number outside of the hbrid orbital). Hy#+d+sa t+on
Sets o* Ele-t#ons 5ea-" gold loe6
:nused %o#+tals Geomet 5lue;+n #y <6
sp
2
2
linear
sp2
!
5
tri+onal planar
sp!
<
9
tetrahed ral
)+ag#am
$ets of electrons is the number of lone pairs plus the number of atoms it is bonded to (so multiple bonds count as much as sin+le bonds). Gn elements below the 2 nd period, d orbitals come necessar to form sp!d and so on. :o use B: we 5. draw =ewis dot structure 2. appl $#D to determine shape !. choose the hbridisation model that 0ts (sp, sp2, sp!) <. construct -bondin+ assembl from partiall filled unhbridised p-orbitals Je can extend the B: model to account for double or triple bonds usin+ the unused p-orbitals. or example, in ethene
#a+e 5!
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:he electron in the second bond of the double bond sits in the 2p P orbital, and these two ad[acent orbitals form a =%ond, which accounts for the second bond. %ormal, direct sin+le bonds (called >%ond) hae far +reater orbital oerlap, and hence are stron+er. Gn addition, the closer the ener+ies between the bonds the stron+er the -bond. -bonds hae poorer orbital oerlap and hence weaker bondin+, and the lie around the -bond. Gn a triple bond, there are two -bonds (in addition to the -bond) which occur at I9 o to each other. Gn resonance structures these are called delo-al+sed =%onds. Gn lar+er molecules such as #Cl ; (tri+onal bipramid) and $ 8 (octahedral) hbridisation can +o beond s- and p-orbitals, and into d-orbitals. or example, in $8, the sulfur must be able to present 8 free places to bond with, and hence one s orbital, three p orbitals and two d orbitals are used to make sp!d2. E2tended =%ond+ng or extended -orbital oerlap occurs when two sets of -bonds are close to each other (as in butadiene, C23C-C3C 2, or +rapheme, where alternatin+ double bonds result in extended -bonds), allowin+ it to conduct electricit due to the delocalisation of electrons (that is, a lar+er -cloud of electrons). oweer this cannot be explained b B:, and hence we use another new model. Mole-ula# ?#+tal 5M?6 T"eo#y is used to account for this, in addition to the structure of molecules like diborane (in dia+ram), and the apparent parama+netism of some simple molecules like O 2, when it is predicted the should be diama+netic. Gn >O :heor, eer two atomic alence orbitals that combine form another two orbitals (unlike B: where onl one was formed, and hence the are di?erent). >O :heor is based upon the oerlap of atomic orbitals. Atomic orbitals (AOs) are mathematical probabilit for re+ions of electron densit (determined b s1uarin+ the waefunction), and as the are waes hae positie and ne+atie amplitude, which simpl tell us the side the electron is on. Bein+ waes, when combinin+ the can be constructie when in phase (called ond+ng - when two like si+ns combine) or #a+e 5<
Olier Bo+danoski
destructie when out of phase ( ant+ond+ng - when two opposite si+ns combine positie and ne+atie). B: does not account for antibondin+. :hese can be calculated for 2 b the e1uations ^bond 3 (^(5s)A ' ^(5s)B) ^antibond ' (^(5s)A - ^(5s)B) :his can be shown dia+rammaticall Antibondin+ orbitals (indicated b an asterisk after the ) alwas hae one more node than their bondin+ counterparts. Gn the bondin+ of 2, two e1ual-ener+ orbitals (5s) can produce either a bond (which has a lower ener+, and hence is more stable) or antibond (which has a hi+her ener+ and is less stable). :his new electron con0+uration is written as 5s . As there are onl two electrons, it is 0lled from the bottom 0rst, and as 5s is more stable it is 0lled 0rst (labelled as 5s as there are two electrons). Bond order is also rede0ned Bond Order 3 Gn the case of 2, as there are two bondin+ electrons and no antibondin+ electrons, bond order 3 (2-9)42 3 5. Gf bond order is 9, there is no bond (as can be seen in the case of e 2 below, howeer e 2' does exist). Gt cannot be U9 (as bondin+ electrons alwas 0ll 0rst). :his has all been for elements in the 0rst period, but it works much the same in the second period (in the dia+ram the p-orbitals hae been +eneralised, howeer if it was period 2 ou would use 2p P, 2p, 2p, 2p, 2p and 2p). or formalit&s sake, p P is used for horiPontal -bonds, whilst px and p are used for ertical bonds. %ote how the number of nodes is the same before and afterwards in constructie, and there is one more in destructie. :his model can be used to explain the parama+netism of ox+en as usin+ the aufbau principle (and abidin+ b #auli&s xclusion #rinciple and und&s Dule) we +et lone electrons that aren&t paired. 2
2
#a+e 5;
Olier Bo+danoski
:his is di?erent to B: as in B: electrons are inoled from the be+innin+ in the hbridisation of orbitals (whilst now the are considered two at a time), and hence we neer think of an MemptN hbridised orbital (whilst >O theor re1uires un0lled or partiall un0lled antibondin+ orbitals), and there is no >O anti-bondin+ e1uialent in B:. :he bond order in >O is analo+ous to sin+le, double and triple bonds in more simple bondin+ theories, so as bond order increases, bond ener+4enthalp (the ener+ re1uired to break the bond) also increases as there are more bonds to break, howeer bond len+th decreases due to electrostatic attraction Bond Order Bond ner+ Bond =en+th :here are two critical factors that determine whether an >O will form and how stable the bondin+ orbital will be (or how unstable to antibondin+ orbital is) 5) de+ree of oerlap (more oerlap (T-bonds)3+reater bondin+4antibondin+ character) 2) similarit in ener+ies of contributin+ orbitals (a 5s orbital will form a lower ener+ bondin+ orbital or een a hi+her ener+ anti-bondin+ orbital with another 5s rather than +oin+ with a 2s) nd 2 period elements often exist as "omonu-lea# d+atom+-s (=i2, B2, etc. - all except Be and %e as antibondin+ cancels with bondin+). oweer, b this theor, B 2 should be diama+netic when it shows parama+netic properties. :his is because the 2s and 2p orbitals are similar in radii, and hence ener+ies, causin+ some oerlap (particularl in those before and includin+ %) resultin+ in o#+tal m+2+ng (or s-p hbridisation) and - crossoer. =i and Be are done normall, but B, C and % hae a =G##* -bondin+ and -bondin+ order, so the -bond is actuall filled up first. :his stabilises the 2s (s& is [ust the diamond of orbitals 2nd shell, so alence), whilst destabilisin+ the 2p (from the 2p P orbital), or in other words causes them to moe further apart in ener+. As the resultin+ orbitals are not deried from one atomic orbital from each atom in the bond, the labels s and are used to show the predominant tpe. As shown in the dia+ram of ener+ leels, mixin+ occurs most where the ener+ies are closest. Gt is important to note that the number of molecular orbitals is
#a+e 58
Olier Bo+danoski
e1ual to the number of atomic orbitals that made them, the bondin+ orbitals are lower than the atomic orbitals from which the were formed whilst antibondin+ is hi+her. Gt follows both #auli exclusion principle and und&s rule, and the form best when composed of atomic orbitals of like ener+ies. Gn heteronuclear diatomics, the more electrone+atie atom will hae lower atomic orbital ener+ies (as ener+ies are ne+atie and it re1uires more ener+ to pull out an atom). :he bi++er the di?erence in ener+ies (or electrone+atiit), the +reater the chance of orbital mixin+ (so %O does not hae orbital mixin+, whilst CO does) as the di?erent orbitals (e.+. the 5s of one and the 2pP of another) are likel to cross. Additionall, if both atoms are below nitro+en (as in C%-) there will also be orbital mixin+. Another means of deducin+ this is b considerin+ what somethin+ is isoelectronic (hain+ the same electron con0+uration) with. $o CO would be the same as %2, and hence has orbital mixin+. &nte#mole-ula# ,o#-es
:he state of a substance (the boilin+4meltin+ point) depends on the stren+th and nature of the forces between atoms4molecules of a substance )+se#s+on *o#-es - a molecule with no e#manent dipole that can become temporaril polar when collisions induce unsmmetrical electron distributions (distortions of the electron cloud), occurrin+ in all molecules polarisabilit is the ease of distortion and determines the stren+th of the force, increasin+ with the number of electrons present (as more electrons means lar+er molecule which means less electrone+atie) molecular shape also determines stren+th of the force, with a hi+her $A ratio (so more $A) bein+ hi+her forces as the are more polarisable )+ole%d+ole - two molecules with e#manent dipole moments attract each other b the approach of oppositel char+ed ends, which can be caused b asmmetric structure or peripheral atoms of di?erin+ electrone+atiit, and we label this with ' and - or an arrow pointin+ from positiene+atie with a perpendicular line throu+h it at the positie end Hyd#ogen ond+ng - the stron+est intermolecular force is the result of a dipole-dipole interaction between a stron+l polarised (due to hi+h electrone+atiit of O4%4-) and an O, % or (note re1uires two of these hi+h electrone+atiit atoms, one to create the dipole, the other to bond with it is almost 59H stron+er than other intermolecular forces, dominatin+ molecular properties (eident in binar compounds as boilin+ point increases drasticall despite smaller molecular siPe) has the form donor-qqqacceptor (the donor makes the ', •
•
•
#a+e 57
Olier Bo+danoski
and the acceptor has a lone pair of electrons for attraction)
Non%&deal
Gases
:he ideal +as laws make two assumptions there is no attractie force between atoms4molecules the olume of the atoms4molecules is ne+li+ible Gn a compressibilit isotherm (+raph), we can plot the +as e1uation ratio (which should alwas be 5 in an ideal +as) a+ainst pressure. At 9 pressure, the ratio is 5, howeer as it increases it deiates below 5 as intermolecular forces pla a role, attractin+ the molecules and reducin+ olume, and hence the ratio. oweer, as olume of the total +as decreases the olume of each particle becomes si+ni0cant, stoppin+ the olume from becomin+ an smaller until the ratio be+ins to increase (as pressure is still increasin+ but olume isn&t decreasin+ as much) so it surpasses 5. Gn noble +ases, there are so few intermolecular forces (as molecules are monatomic and hence dispersion forces small), so the don&t reall dip down. Gf forces aren&t as stron+ (for example, less aailable alence electrons in % 2 and C< compared to halo+ens), then the dip down is not as stron+. Gf the olume of the molecules is lar+er, then the e?ects of olume push up the ratio faster as the come into pla with a lar+er e?ect sooner. :o correct for this, we use the an der Jaals e1uation • •
:he pressure correction term (0rst bracket) takes into account the intermolecular forces (note a 3 stren+th of intermolecular forces lar+er a means stron+er added as pressure increases). :he olume correction (second bracket) accounts for the molecular olume (note b 3 olume of +as molecules lar+er b means lar+er molecules, less free olume subtracted as free olume decreases). #a+e 5F
Olier Bo+danoski
Both meltin+ and boilin+ point can be used as measures of intermolecular forces. At the boilin+ point of a substance, the intermolecular attractie forces balance with the kinetic ener+ of the substance, and if this occurs at #35.95!H59 ; #a (5atm) it is the no#mal o+l+ng o+nt. At the meltin+ point, the kinetic ener+ ener+ies of the molecules are at a point where solidi0cation and li1uidation are e1uall possible (also with a no#mal melt+ng o+nt). :hus the rates of escape and capture from a phase depends on the kinetic ener+ and intermolecular forces, lar+er intermolecular forces producin+ hi+her points. Condensed P"ases o* Matte# Sol+ds hae 0xed shape, hi+h densit and low compressibilit
due to the stron+ bondin+ between the atoms. :he atoms ma pack in a crstalline solid (lon+ ran+e order) like silica41uartP, or irre+ularl to +ie an amorphous solid (disorder) like +lass. Mole-ula# or atom+- solids are made of coalentl bonded atoms or free atoms, held to+ether b dispersion forces, dipole forces and4or hdro+en bondin+, and their meltin+ point is dependent on these forces (althou+h it is +enerall low). Atomic solids are held to+ether b [ust dispersion forces. Netwo#E=:G#= the @ alues to+ether. A small @ means the e1uilibrium faours the reactants, whilst a lar+e @ means the e1uilibrium faours the products. :he aboe formula can also be done in terms of pressure for a has (as pressure is proportional to concentration as #3nD: and we can diide b to +et #3cD:), +iin+ us @ p (note this number will be di?erent to @ c but works similarl). :o conert between them, sub in the alternate alue (from #3cD:) into the e1uilibrium constant expression (the bit @ e1uals) aboe, then simplif, which can be used to show (althou+h ou can [ust do this 0rst principles) @ p 3 @ c (D:)n+ where n+ 3 total mol(products) - total mol(reactants) %ote that the units of D chan+e with the units used. Gn hetero+eneous e1uilibria (where the reactants are not all in the same phase), if pure solids or li1uids are present the are not included in the e1uilibrium constant expressions (as the hae a concentration of 5). :herefore, as the e1uilibrium is independent of the amount of solids present, arin+ amounts will render the same pressure and concentration. :he #ea-t+on .uot+ent (v) is calculated in exactl the same as @, except it uses the current concentrations rather than the concentrations at e1uilibrium. :o calculate concentrations from the e1uilibrium constant use GC b writin+ out the chemical reaction and then underneath write the 5) &nitial concentrations 2) Chan+e in concentrations (b workin+ out which wa the reaction will +o, then addin+ or subtractin+ a multiple of x that re/ects the stochiometr of that substance in the e1uation) !) E1uilibrium 3 Gnitial ' Chan+e (+ies concentrations at e1uilibrium) <) Esin+ these alues as our concentrations, sub into the e1uilibrium constant expression, let it e1ual our @ alue, then sole for x (if ou hae a 1uadratic, 0nd which alue is impossible - a ne+atie alue means the reaction +oes the other wa and ma be impossible, or alues bi++er than the amount of what ou are subtractin+ from are impossible too) ;) $ubstitute in x to 0nd 0nal concentrations :o aoid hain+ to sole complex calculations, we can use small x approximations that is, lettin+ x39 when subtractin+ from lar+er numbers (rule of thumb is when xU;V of this lar+er number). Gf @ c is small (e.+. 59-F), the amount of product produced (x) is small compared to the amount of reactants left, so we can approximate it to 9 on the denominator. =ikewise, if @ c is lar+e, then it is likel it will be small on the top.
#a+e 2<
Olier Bo+danoski
e C"atel+e#’s P#+n-+le states
Gf a chan+e is imposed on a sstem at e1uilibrium, the position of the e1uilibrium will shift in a direction that tends to reduce that chan+e. Je can induce this in three di?erent was Con-ent#at+on - chan+in+ the concentration of the reactants or products chan+es the alue of v if vU@, more products need to be formed and the reaction will shift to the ri+ht, whilst if vT@, it will shift to the ri+ht (this can also be used to predict which wa a reaction will +o) P#essu#e - note three e?ects (works similarl to concentration) addin+4remoin+ +as or product (same e?ect as o doin+ so in terms of concentration as # Q c) o chan+in+ the olume of the container (note in dia+ram %O2 diminishes twice as fast as % 2O<) o addin+ an inert +as ( N? as whilst it E,,ECT chan+es total pressure of the sstem, it has no e?ect on the partial pressures of each species doesn&t increase number of collisions) Teme#atu#e - determine if reaction is endothermic or exothermic (U9 exothermic), then treat heat as part of the reaction b addin+ it to the appropriate side (endothermic is release, so add to products) disturbin+ e1uilibrium b chan+in+ temperature is fundamentall di?erent as it chan+es the e1uilibrium constant, related b the an&t o? e1uation which can be inte+rated (this can be used to calculate a new @ note in an exothermic reaction, increasin+ temperature will make a smaller @) •
•
•
Addin+ a catalst will +reatl accelerate a reaction without bein+ consumed, howeer the do not appear in the 0nal oerall reaction and therefore cannot a?ect the e1uilibrium position of the reaction, onl the rate of the reaction. :he do this b proidin+ an alternate reaction path4mechanism with a lower actiation ener+. :his is important for all modern chemical products, like the production of ammonia for fertiliser (#t catalst), remoin+ %O in ehicle exhaust (#d oxide), and crackin+ petroleum (Peolite), and the are important in biolo+.
#a+e 2;
Olier Bo+danoski
A-+ds and Bases A##"en+us&s de0nition of an acid was that it contains an
that ionises in water to +ie an ' ion, whilst a base contains an O that ionises in water to +ie an O - ion, howeer this didn&t explain %! bein+ basic. B#nsted%ow#y &s de0nition is that an acid is a proton donor, and thus a base is a proton acceptor, and thus the products are also con[u+ate acids and bases themseles (as the
species are in e1uilibrium). Jater is uni1ue as it is amphiprotic4amphoteric, actin+ as both acid and base. At 2; oC, the e1uilibrium constant (also called the auto#otolys+s -onstant *o# wate# due to its self-ionisation) for pure water is @ w 3 _!O'`_O-` 3 59-5< ' as a bare proton doesn&t reall exist, but it is sol$ated b surroundin+ water (all the exact nature of it is still the sub[ect of research) and forms hdro+en bonds with the ox+en from other water4hdronium molecules. 1uilibrium constant alues ar from 59-29 to 59!9, and due to its lar+e breadth, we use the lo+ scale (and appl a ne+atie to make the small alues we commonl deal with positie). ence p 3 -lo+_!O'` (or _!O'` 3 59-p) pO 3 -lo+_O-` p@ a 3 -lo+@ a rom this we can show that because _ !O'`_O-` 3 59-5< H ; ?H ' 1
$tron+ acids and bases dissociate completel in water (use arrow, not e1uilibrium), and hence their con[u+ate species do not react to an measureable extent with water. Jeak acids and bases react with water but dissociate incompletel as an e1uilibrium is established, and hence hae the e1uilibrium constants (acidit constant)
#a+e 28
Olier Bo+danoski
(basicit constant) Acids are stron+er (easier to remoe protons) with a hi+her @ a but lower p@ a, and similarl bases are stron+er with a hi+her @ b and lower p@ b. or an acid and its con[u+ate base (or this could be written ice ersa base with con[u+ate acid - same thin+) A ' 2O A- ' !O' @ a A- ' 2O A ' O - @ b 22O !O' ' O- @ a H @ b 3 @ w 3 59-5< :herefore K a ; K ' K w ' 1
>an acids (particularl or+anic ones) hae more than one proton that can react with water, and hence are oly#ot+-. As the protons ma hae di?erent bond stren+ths, the hae di?erent alues for p@ a. :hose attached to more electrone+atie atoms (like ox+en) will hae a lower p@ a and hence dissociate more (as the O can then -bond with !O'42O, 0llin+ in this space and limitin+ the abilit of an ' to come back and bond, and thus makin+ it a stron+ acid). en when there are identical protons, the p@ a for remoin+ the 0rst is di?erent to the second (as the molecule is now sli+htl ne+atie and has a ti+hter control on its '). :hese are written as p@ a (5), p@ a (2), and so on. Jhilst in the form A - (losin+ a proton in a diprotic acid), the become amphiprotic (becomin+ either 2A or A2-), and ou can determine whether the will +o acidic or basic b lookin+ at the p@ a (2) and p@ b (5< - p@ a (5)), conertin+ to @ a and @ b, and then the lar+er e1uilibrium constant will win out (as it produces more of those products), determinin+ if it is an acid or base. w+tte#+ons are molecules that contain both positie and ne+atie char+es (that do not combine), makin+ it oerall neutral (e.+. the amino acid +lcine). :his means it has both an acidic and a basic end. Jhen lookin+ at the p of salts, consider the anion and cation di?erentl. Gf the form part of a stron+ acid or base, the anion or cation (respectiel), the will be extremel weak con[u+ates, and hence not react with water nor alter p. Jeak acids and bases hae moderate-stron+ con[u+ates, which can raise or lower the p. oweer if both ions are weak, then we need to consider their @&s (%O: p@), and the one with the +reater e1uilibrium constant will produce more product and hence determine if it is acidic or basic (that is, if @ a T @ b, it is acidic). %ote that when solin+ e1uilibriums with acids and bases, we can onl make a small x approximation for _A` when the initial _A` is <99H@ a. Also, if the concentration of the acid or base is less than 59-;, then the autoprotolsis of water must be considered, and rather than usin+ the initial _ !O'` or _O-` of water as 9 (as we hae been approximatin+), we must now use 59 -7. Consider C!COO (a1) ' 2O (l) !O' (a1) ' C!COO- (a1). Gf we were the add C!COO%a, be =e Chatelier&s principle, the e1uilibrium will shift left to minimise the chan+e, and hence will
#a+e 27
Olier Bo+danoski
moe awa from disassociation and alwas tend towards neutral. :his is shift in e1uilibrium is called the -ommon +on e/e-t. Dearran+in+ our e1uilibrium constant expression for @ a in terms of !O' and takin+ the -lo+ of both sides we +et the Hende#son%Hasselal-" e.uat+on which tells use directl the p (useful for common ion e?ect uses initial conc.) p 3 p@ a ' lo+ 3 p@ a ' lo+ :he common ion e?ect also proides the basis of u/e#s. Consider A ' 2O A- ' !O' A- ' 2O A ' OAddin+ a small amount of base (O -) or acid (!O') will result in the e1uilibrium shiftin+ in the other direction (howeer 2O concentration increasin+ is irreleant as it is a li1uid and hence its concentration is alwas 5), and thus minimisin+ the disturbance of the acid or base. :he p of a bu?er can be calculated usin+ the enderson-asselbalch e1uation (or GC, but takes lon+er). :here are man important bu?ers in nature, for example the blood bu?er between carbonic acid and hdro+en carbonate (p@ a38.<), hdro+en phosphate and dihdro+en phosphate (p@ a37.2), and amino acids (which act as an acid, p@ a7 as arious). Blood outside the ran+e 7.2-7.8 is smptomatic, and fatal outside of 8.I-7.I. :o +et within these ran+es, _base` 3 59H_acid` (so p is correct for endersonasselbalch e1uation). An mixture of weak acid and con[u+ate base (or ice ersa) will produce a bu?er, howeer the capacit of the solution to resist a p chan+e depends on the relatie concentrations. :he bu?er is most stable the weak acid and con[u+ate base are added in e1ual concentrations as deiations on either side (addin+ either one) will be e1ual. :hus from the enderson-asselbalch e1uation, H'K a. :itrations are used to determine the concentration of an unknown solution b reactin+ a known olume with a standard solution (known concentration) of a reactant. Esuall the standard solution is in the burette and titrates a known olume that has been pipetted into a conical /ask. :he e1uialence point is the point at which ou hae the same number of moles of added O - or !O' (dependin+ on what ou are titratin+) as the amount of acid or base (respectiel) that ou had initiall. Gn stron+ acid-stron+ base4stron+ base-stron+ acid titrations it is 7, in weak acidstron+ base titrations it is T7, whilst in weak base-stron+ acid titrations it is U7. alfwa to the e1uialence point, the current _acid`3_base` (as half of the acid has been reacted into its con[u+ate base), meanin+ p3p@ a and hence it is a bu?er. #a+e 2F
Olier Bo+danoski
Acid-base indicators are weak acids or bases that chan+e colour with p, +enerall occurrin+ at p 3 p@ a (indicator) 5 (e.+. phenolphthalein has a p@ a of I.<). :he react with water (or the solution) to +ain or lose protons and hence chan+e colour. :he p@ Gn or endpoint should be close to the e1uialence point in a titration, and as little indicator used as possible is better to aoid chan+in+ the p. A ew+s acid is an electron-pair acceptor (inerse of proton donor), whilst a =ewis base is an electron-pair donor (inerse of proton acceptor), howeer the stren+ths of these acids and bases aren&t as readil 1uanti0ed as Brnstedy=owr counterparts. T"e#mo-"em+st#y
:hermochemistr is the stud of ener+ chan+e in a chemical reaction, which helps us understand the direction of chemical chan+e (includin+ how to approach e1uilibrium) and how much ener+ it takes to drie a chemical reaction. %ote 5 calorie 3 <.5F<6 (the ener+ re1uired to heat 5+ of 2O b 5oC), whilst 5 Calorie35999 calories. eat is the amount of ener+ transferred between two ob[ects, whilst absolute4thermodnamic temperature is what can actuall be measured (in @). or example, heatin+ two di?erentl siPed bodies of water with the same amount of ener+ will result in a hi+her temperature in the smaller one. $imilarl, di?erent materials will ield di?erent temperatures for the same ener+. System - the reaction (or thin+) we are interested in (open sstems can pass ener+4mass across boundaries into surroundin+s, whilst closed sstems can pass ener+ but not mass, whilst isolated sstems cannot pass either mass or ener+) Su##ound+ngs - eerthin+ else (we onl worr about this if a?ected b the sstem, like in thermal contact) :n+$e#se - sstem ' surroundin+s Bounda#y - the place where the ener+ (e.+. heat) /ows across :here are four thermodnamic state functions (onl depend on current state, not a?ected b how it cam to be or will be, hence the chan+e in each is onl dependent on 0nal-initial, not the pathwa taken as opposed to at" functions that depend on the pathwa taken, like heat and work, as some method ma re1uire more of one or the other) &nte#nal Ene#gy (E) - sum of nuclear, electronic, ibrational, rotational, translational and interaction ener+ies of all the indiidual particles in a sample of matter absolute E cannot be measured, onl E (in the cases shown due to bond ener+ies) •
#a+e 2I
Olier Bo+danoski
•
•
•
() - related to the heat absorbed or produced in a chemical sstem, determined b measured temperature chan+e under constant pressure Ent#oy ($) - measure of the number of was ener+ is distributed throu+hout a chemical sstem (related to enthalp) G+s *#ee ene#gy () - relates enthalp and entrop Ent"aly
:he ,+#st aw o* T"e#modynam+-s states E 3 1 ' w where 1 3 heat absorbed b the sstem (6) w 3 work done b the sstem (6) Jork is the ener+ re1uired to moe somethin+ a+ainst a force. :his ener+ can be electrical, li+ht, sprin+ ener+ or piston ener+ (the main focus of thermochemistr). Gn pistons, if a compressed +as is placed under a piston, it will expand to atmospheric pressure and moe the piston up. ence work is dependent upon the chan+e in olume of the pistons and the opposin+ pressure, +ien b w 3 -p (don&t think we need to use this). :his comes from the conseration of ener+, as ener+ cannot be lost or +ained, simpl transferred, and hence is used in the form of heat or moement. eat can be determined from temperature b 1 3 c: where c 3 heat capacit (dependent upon both the tpe and amount of substance present). or pure substances 1 3 mc: 1 3 nC: where c 3 speci0c heat capacit (64@4+) C 3 molar heat capacit (64@4mol) Dearran+in+ E 3 1 - p +ies us the ent"aly or "eat o* #ea-t+on (31) 3 E (or ) ' # >an chemical reactions occur under constant pressure (not constant olume) like laborator experiments in open containers, biolo+ical reactions in liin+ sstems, atmospheric reactions and combustion reactions that aren&t in closed sstems, so measurin+ the heat chan+e would +ie , not E. Calo#+met#y can a bomb calorimeter to measure E (usuall for combustion reactions) b hain+ a thermall insulated constant olume from the rest of the unierse and a known heat capacit (and hence we can predict the e?ect of the surroundin+s). Alternatiel, to measure b hain+ constant pressure, we can use a Mco?ee#a+e !9
Olier Bo+danoski
cupN calorimeter that is also thermall insulted, and usuall used for li1uids in heat of dissolution, heat capacit of solids and a1ueous reactions. Je can hae enthalp of aporisation (apT9 as ou hae to oercome intermolecular forces) is the ener+ re1uired to +o from li1uid to +as (in 64mol), enthalp of combustion (cU9 as ou are releasin+ ener+ b formin+ new bonds) is burnin+ in ox+en, and enthalp of atomisation (atomT9 as ou hae to put in ener+ to break the bonds) is splittin+ a substance into each indiidual atom (not een into 2, but 2). Atomisation enthalpies can be found b summin+ all the indiidual bond enthalpies. Gn an chemical reaction (assumin+ states remain the same), such as combustion, we can approximate the enthalp b 0ndin+ the atomisation enthalp and then addin+ the ne+atie alues of the product&s bond enthalpies (basicall 0nal-initial but usin+ an alternate pathwa). :his is Hess’s aw if ou add up chemical e1uations to form a new oerall e1uation, then the oerall enthalp is the sum of the indiidual enthalpies. oweer in this case it is onl approximate as bond ener+ies depend on the entire molecule, not [ust the two atoms inoled, and the alues on tables are mere aera+es (for example we would predict CClBr 2 wouldn&t decompose in the stratosphere as bond ener+ies appear hi+her than the ener+ of the li+ht there, but in actualit the bond ener+ies are lower). $o instead of usin+ bond enthalpies (which are hard to measure as isolated atoms in the +as are dicult to measure experimentall), we use atoms in the state the are most commonl found in, or their standa#d state, occurrin+ at 5 bar (59 ; #a), 2IF@ and 5>, and this is denoted b a o (e.+. o). %ow we use ent"aly@"eat o* *o#mat+on (f o). or an element in its standard state (e.+. O2 (+), e (s)), f o 3 9. Gn this, we look at the ener+ re1uired to make the products and subtract the ener+ re1uired to make the reactants r 3 f (products) - f (reactants) Je can use this to predict the properties of di?erent substances, for example the thermite reaction (e 2O! ' 2Al) is hi+hl exothermic so we know the safet precautions to use. :he thermite reaction is used to weld railroad tracks (when there is no electricit), and a ariant is used as an i+niter for rocket fuel. A spontaneous reaction means once it has started it will continue on its own, whilst a non-spontaneous reaction must constantl hae ener+ applied for it to run, or otherwise it will stop. is not the onl criterion for spontaneit (despite most exothermic reactions (U9) bein+ spontaneous, and the more exothermic the more i+orous), as endothermic reactions like that between barium hdroxide and ammonium nitrate can also be spontaneous. :he other criterion is ent#oy. #a+e !5
Olier Bo+danoski
Gf we look at 2O (s) 2O (l), 38.92k64mol (hence endothermic, add heat to left). At :T27!@, it will shift to the ri+ht, whilst below this it will shift left. :his is because ener+ (heat) is /owin+ from the surroundin+s to the sstem (or ice ersa), and this ener+ /ow is ke to understandin+ spontaneous chan+e. ntrop is the tendenc for ener+ to spread out as far as possible (so hain+ a hot ob[ect next to a cold one will result in ener+ moin+ from the hot to the cold until the are e1ual - a result of random probabilit, like di?usion). :he ener+ can spread out in two main was the molecules themseles moe further apart, or the ener+ is spread across more molecules. :herefore entrop is +reater in +as T solid particles spread further solution T solid ' li1uidener+ localised in solid spreads +as ' li1uid T solution ener+ spreads een further in +as C28 T C< more bonds to spread ener+ across !mol T 2mol entrop Q amount (spreads across more molecules) 29@T59@ more kinetic ener+3collisions, spreads ener+ %ote that phase (position entrop) tends to dominate molecular complexit, as the molecules can spread further apart. @nowin+ which is +reater, we can deduce whether the $ of a reaction will be positie or ne+atie (b doin+ 0nal-initial). Standa#d ent#o+es ($9) are the entrop chan+e from :39@ (where $39) to :32IF@ (2; oC). Je can 0nd entrop b $ 3 :he propensit for ener+ to spread out is known in the Dnd aw o* T"emodynam+-s $unierse 3 $sstem ' $surroundin+s or an spontaneous process, $ unierse T 9. Esin+ data from tables (hain+ f 9 in k64mol and $ 9 in 64@4mol - D>>BD E%G:$), we can then determine the di?erence between reactants and products, and hence if a reaction will be spontaneous at 2IF@. Esin+ the aboe alue of $ for surroundin+s, and usin+ 13, it can be shown that for a spontaneous process 3 sstem - :$sstem U 9 :his is called G+s ,#ee Ene#gy, a measure of the spontaneit of a process and the useful ener+ aailable from it. or A== chemical reactions, a +raph of a+ainst the mole fraction of a reactant4product will hae a minimum, and that minimum occurs at e1uilibrium. As U9 for an spontaneous reaction, this makes sense as will onl decrease. Gf the sstem does not form an
#a+e !2
Olier Bo+danoski
e1uilibrium, it will [ust be a strai+ht line from the reactant to product (and dependin+ on which direction it +oes it could be dia+onall up, down, or horiPontal like water +oin+ between solid and li1uid at 9oC. As we can see in this case of the e1uilibrium, when one side has a hi+her alue of 9 than the other, it MraisesN the +raph and the e1uilibrium shifts towards the reactants (as more ener+ is re1uired to shift towards the raised end). Gf less raised, the e1uilibrium will be more centred, and alwas below both sides. :he relationship between o and the e1uilibrium constant (@) is o 3 -D: ln(@) where D 3 uniersal +as constant (64mol4@ - make sure uses same units) Jhen roT9, @U5, and the reactants are faoured, shown in the +raphs. Gal$an+- Cells
:he concept of oxidation from reactions with ox+en, in which the metal was oxidised to form an ionic compound (althou+h ori+inall thou+ht to be e1ual sharin+ of electrons, howeer we now know electron densit is hi+her on the ox+en). Oxidation is loss, reduction is +ain (of electrons). %ote that the reducin+ a+ent reduces the other species and it itself is oxidised (and ice ersa). Gn oxidation, the oxidation number decreases, whilst in reduction the oxidation number decreases (oxidation numbers are also used for coalent substances but are not ionic char+e, onl used for conenience). Oxidation numbers are used for namin+ compounds, deducin+ properties and identifin+ redox reactions. ree elements are neutral, and therefore hae oxidation numbers of 9, as do neutral molecules, but the parts within those molecules and ions are e1uialent to the ionic char+e (or char+e it would be if ionic), includin+ the si+n. :hen follow the rules in this order 5. is alwas -5 2. roup 5 is alwas '5, roup 2 alwas '2 !. O is usuall -2 (except in peroxides where it is -5) <. alo+ens are usuall -5 ;. is -5 with metals and '5 with non-metals :o balance redox reactions we 5. *etermine half-reactions 2. Balance atoms and char+es a. Balance atoms other than O and b. Balance O b addin+ 2O c. Balance b addin+ ' d. Balance char+e b addin+ e!. >ultipl each half-reaction b an inte+er for the same number of e<. Add half-reactions (includin+ states), cancel excess, check balanced a. Gf basic, add O- to cancel ' (cancel an additional 2O)
#a+e !!
Olier Bo+danoski
:he actiit series shows the order of most likel to be oxidised (or more correctl, the stron+est reducin+ a+ents). :he hi+her species on the reactiit series determines the direction of the spontaneous reaction. Gn redox reactions, electrons are transferred from one element to another, and we can harness these electrons b separatin+ the half-reactions in a gal$an+- -ell. As electrons are +ained (reduction) throu+h the circuitr in the cathode, cations in the solution bump into these electrons and become part of the solid. :he opposite occurs at the anode. :he shorthand nomenclature for standard cell notation is (note it shows the direction of reactions4/ow of electrons the middle salt brid+e ) Esin+ a table of cell potentials ( 9 ( or 64C - the number of [oules transported b 5 amp in 5 second)), we can add the cell potentials to+ether to 0nd the oerall potential of the reaction b determinin+ their direction, balancin+ electrons and summin+. A spontaneous reaction alwas has 9T9. :he tables proided are for reduction potentials (not oxidation), so eerthin+ will be backwards and upside down from $C. %ote that unlike the e1uilibrium constant, this does not depend on the stoichiometr, and hence multiplin+ b a speci0c number does not chan+e the cell potential. Jhen the electrodes themseles are part of the chemical reaction, the are called a-t+$e ele-t#odes. Je can also use inert materials like +raphite (often used for halo+en +ases) or platinum, called +na-t+$e ele-t#odes. :he conduct electrons, but do not partake in the reaction. enerall their reactions inole a +as or another ion. lectrodes are alwas placed on outside of the shorthand notation, re+ardless if actie or inactie. Gnitiall a standard hdro+en electrode was used to make measurements o?, howeer this is dicult to replicate accuratel (as concentration ma ar with the +as), so it was initiall replace with a normal and then saturated calomel electrode, howeer this contained mercur, so now we use a siler4siler chloride standard electrode, where 939.22. Esin+ a number line, and knowin+ which reaction is more positie, we can deduce the other electrode&s standard potential. As concentration a?ects cell potential, we use a standard concentration in 9, bein+ 5>. arin+ the concentration will inole =e Chatelier&s principle, and if it shifts to the ri+ht the cell potential
#a+e !<
Olier Bo+danoski
will increase, whilst shiftin+ to the left it will decrease. :his is 1uanti0ed in the Ne#nst E.uat+on cell 3 9 - H ln(v) where cell 3 the maximum potential a cell can +enerate () 9 3 cell potential if c35> D 3 F.!5< 64@4mol (uniersal +as constant - note units) : 3 temperature (@) n 3 number of electrons transferred per mole of rea+ent 3 arada constant 3 I8) and measurin+ the cell potential. :his is how p meters work (comparin+ 2' (a1, 5>) ' 2e 2 (+) or more commonl a siler4siler chloride reference to an unknown solution and measurin+ current can be used to calculate concentration and hence p). A similar thin+ can be done with other ions for ion selectie electrodes (to measure the concentration of particular ions). Concentration cells are also used in nere si+nallin+, ion pumps across cell membranes, and ener+ production and stora+e in cells. orcin+ the electrons in the opposite direction (that is, a+ainst the spontaneous reaction) is called ele-t#olys+s, formin+ an ele-t#olyt+-ell. As oxidation alwas occurs at the anode, the anode is now the other electrode which is becomin+ positie (as it is now losin+ electrons). Gn a +alanic cell, the anode was alread ne+atie, and electrons /owed from it to the cathode. →
#a+e !;
Olier Bo+danoski
,a#aday’s ,+#st aw o* Ele-t#olys+s states that the mass of
a substance (m in +rams) liberated at an electrode durin+ electrolsis is proportional to the 1uantit of char+e (1 in Coulombs) passin+ throu+h the electrolte m Q 1 ,a#aday’s Se-ond aw o* Ele-t#olys+s states the number of Coulombs needed to liberate one mole of di?erent products occurs in whole number ratios 1 3 Gt (G in amps, t in seconds). Je can diide this 1 (C) b arada&s constant (C4mol) to 0nd the number of moles of electrons, which can be used to deduce the number of moles of a substance bein+ reduced or oxidised. :here are three main classes of batteries P#+ma#y Batte#+es - non-reersible (can&t char+e) o )#y Cell - carbon cathode coated in >nO 2 (with +raphite powder for conductiit), a paste of % %O2 cathode ,uel Cells - fuels4chemicals can constantl be passed into the batter reactants are burnt (oerall like combustion), o howeer the two half reactions (anode 2, C<, etc. cathode O2 ' <' '
