Homework 1-solutions - tran (pt4954) – Homework 1 –...

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Unformatted text preview: tran (pt4954) – Homework 1 – sutcliffe – (51045) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points If Δ G ◦ rxn is positive, then the forward reaction is (spontaneous / nonspontaneous) and K is (less / greater) than one. 1. nonspontaneous; less correct 2. nonspontaneous; greater 3. spontaneous, greater 4. spontaneous, less 5. None of these; Δ G is not directly related to K . Explanation: A positive Δ G ◦ rxn (standard reaction free energy) denotes an endothermic reation, which is nonspontaneous. Also, Δ G ◦ rxn =- RT ln K , so a positive Δ G ◦ rxn would result in a K that is between the values of zero and one. 002 10.0 points Which combination of Δ G ◦ and K is im- possible? 1. Δ G ◦ = 16 J, K = 635 correct 2. Δ G ◦ =- 37 . 4 kJ, K = 3 . 57 × 10 6 3. Δ G ◦ = 18 . 5 J, K = 0 . 993 4. Δ G ◦ = 86 . 4 kJ, K = 7 . 31 × 10 − 16 5. Δ G ◦ =- 72 J, K = 1 . 03 Explanation: If Δ G ◦ is positive, K must be less than 1; if Δ G ◦ is negative, K must be greater than 1. Also, if Δ G ◦ is small (compared to RT), then K will be close to 1, and if Δ G ◦ is large, K will be much smaller or greater than 1. 003 10.0 points When Δ G is calculated for a proposed reac- tion, the process being described is 1. any mixture of reactants converted to equilibrium mixture. 2. 100% reactants converted to equilibrium mixture. 3. any mixture of reactants and products converted to equilibrium mixture. 4. 100% reactants converted to 100% prod- ucts. correct Explanation: Δ G = Δ G final- Δ G initial = Δ G products- Δ G reactants 004 10.0 points Note: The ’reaction progress’ axis reads from all reactants (far L) to all products (far R) The figure represents a reaction at 298 K. b b b b b G A B C D E rxn progress Based on the figure, the standard reaction is 1. nonspontaneous. 2. spontaneous. correct Explanation: Δ G ◦ is negative (point E is lower free en- ergy than point A), so the standard reaction is spontaneous. 005 10.0 points A gas-phase reaction has K p = 3 . 99 × 10 20 at 25 ◦ C. Calculate Δ G for this reaction. Correct answer:- 117 . 549 kJ / mol rxn. Explanation: tran (pt4954) – Homework 1 – sutcliffe – (51045) 2 K p = 3 . 99 × 10 20 T = 25 ◦ C + 273 = 298 K Δ G =- RT ln K p =- (8 . 314 J / mol · K) (298 K) × ln ( 3 . 99 × 10 20 ) =- 117 . 549 kJ / mol rxn 006 10.0 points Consider the reaction 2 Fe 2 O 3 (s) + 3 C(s) → 4 Fe(s) + 3 CO 2 (g) , Δ H ◦ = 462 kJ , Δ S ◦ = 558 J · K − 1 . Calcu- late the equilibrium constant for this reaction at 525 ◦ C....
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This note was uploaded on 09/20/2011 for the course CALCULUS 7234832 taught by Professor Campsisi during the Spring '11 term at University of Texas.

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Homework 1-solutions - tran (pt4954) – Homework 1 –...

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