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University of Toronto St. George

Chemistry

CHM135H1

Kris Quinlan

Summer

Description

LECTURE
13
Slide No. Notes
14 - nothing to do with rate
- spontaneous reaction can be slow or fast
- for instance, diamond to graphite is a very slow yet spontaneous
reaction
- if the forward reaction is spontaneous, the reverse is non-
spontaneous
15 - delta H is not enough to determine spontaneity
16 - S is a measure of disorder
- Entropy goes from localized to dispersion for spontaneous reactions
17 - gas particles are very spread out
- whereas solid particles are in ordered structures
18 - Ans1: B
- Ans2: A
- Ans3: B
- Ans4: A
- Second question:
- A) A2 (gas) + 2B (gas) -> 2AB (gas)
- B) negative S
19 - imagine two coins
- combinations of results are only HH; HT; TH; TT
- therefore 2 to the power of 2 is four
- imagine three coins
- combinations of results can be HHH; TTT; HHT; HTH; THT; TTH;
THH; HTT
- therefore 2 to the power of 3, which is 8
20 - solids vibrate faster, therefore higher entropy
- at BP, big jump in entropy because gas formed, particles very
spread out
- recall that at phase changes, the temperature does NOT change
21 - more entropy in drop of water than a bucket because entropy is an
extensive property
- Delta S = S of CaO plus S of CO2 subtract S of CaCO3 =
1mol(39.7J/Kmol) + 1mol times 213.6J/Kmol – 1 mol times
92.9J/molK = 160J/K
- This answer was expected to be positive so check!
22 - spontaneous – exogonic
- non-spontaneous – endogonic
- in Gibbs, surr incorporated by delta H of system
23 - note that the second sign is for delta S only, not the whole second
term including T
- negative H, positive S – negative G therefore reaction spontaneous
www.notesolution.com - positive H, negative S – positive G therefore reaction non-
spontaneous
- both positive – depends on temperature; at high temperatures,
reaction is spontaneous
- both negative – depends on temperature; at low temperatures,
reaction is spontaneous
- Ex. negative S; reaction spontaneous at lower temperatures
24 - Delta H = 2(H for Fe) + 3(H for water vapour) – H for iron oxide –
3(H for Hydrogen gas) = 0 + 2(-241.8) – (-824.2) – 0 = 98.8kJ
- Delta S = same format but for S instead of H = 141.5J/K
- Delta G = 98.8 kJ – 298K (141.5J/K)(kJ/1000J) = 56.6kJ
- Delta G is 0 when reaction switches from spontaneous to non-
spontaneous
- To determine T at that moment, isolate for T when 0 = Delta H

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