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CHM135H1 (333)

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University of Toronto St. George
Kris Quinlan

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 - 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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