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ThermochemistryThermochemistry

AP Review Unit 5AP Review Unit 5

Chapters 6 and 18Chapters 6 and 18

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Enthalpy, ∆H ∆H = q + w + ∆(PV) = U + ∆(PV) = q @ const. P Measured using calorimetry

Do the process underwater Measure ∆T for the water (and calorimeter) Calculate qwater + calorimeter = mc∆T qprocess = - qwater + calorimeter

Can be calculated from bond energies Add up bonds to be broken (endo, therefore +) Add up all bonds to be formed (exo, therefore -) ∆H = sum of the two

Can be calculated ∆Hgeneric= ∑∆Hf(prod) - ∑∆Hf(react)

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∆H (continued) ∆Hformation = heat for the formation of 1 mole of a

compound from the necessary elements in their reference form. Reference = most stable form at 25°C, 1 atm, 1M

Can’t be known absolutely; a relative quantity Must define standards arbitrarily

∆Hf = 0 for any element in reference form ∆Hf = 0 for the H+ ion

Other ∆Hf’s are tabulated ∆Hgeneric’s are calculated with Hess’ Law H° means “standard”, measured at 25°C, 1 atm, 1 M

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Entropy, S Def’n: measure of randomness, chaos, disorder An absolute quantity; no need for arbitrary stds

3rd Law: S = 0 for a perfect crystal at 0 Kelvin We still need arbitrary std for ions S = 0 for H+ ion

Different from enthalpy and Gibb’s free energy Tabulated as S, not Sf or ∆S Units are J/(mol K), not kJ/mol

Calculated the same ∆S = ∑S (prod) - ∑S (react)

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Examples of Entropy Which is higher in entropy? 1 mole of liquid water or 1 mole of water vapor 1 C8H18 molecule or 8 C atoms + 18 H atoms

1 C8H18 molecule or 1 CH4 molecule

1 gram of hot water or 1 gram of cold water 1 gram of compressed gas or 1 gram of expanded Poker deck (52 cards) or a pinochle deck (24 cards) New poker deck or a shuffled poker deck It’s all about the number of possible states

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Gibb’s Free Energy, ∆G Two ways to calculate

∆G = ∑∆Gf (prod) - ∑∆Gf (react)

∆G = ∆H - T∆S

Measure of spontaneity ∆G = 0 means you’re at equilibrium ∆G < 0 means process is spontaneous (forward faster) ∆G > 0 means process is non-spont (reverse is faster)

Calculating Temperature effects Use ∆G = ∆H - T∆S T at which ∆G = 0 is T at which process switches spontaneity

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∆H for Related Reactions Relationships are additive (K’s are multiplicative) ∆Hreverse = -∆Hforward (additive inverses)

∆Hdoubled = 2∆Hsingle = ∆Hsingle + ∆Hsingle

∆Hmultistep = ∆Hstep 1 + ∆Hstep 2 + ∆Hstep 3 …

Relationships are the same for the other state functions S and ∆G

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Miscellaneous ∆G = - maximum work a reaction can do

Spont process has neg ∆G and can do positive work

∆G = ∆G° + RT ln Q At equilibrium, ∆G = 0 and Q = Keq

∆G° = -RT ln K

At equilibrium, ∆G = 0, so ∆H = T ∆S Useful for equilibrium processes like boiling and melting Great lab technique for finding ∆S