HW3solutions - Niu(qn269 Homework 3 owers(53885 This...

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Niu (qn269) – Homework 3 – fowers – (53885) 1 This print-out should have 32 questions. Multiple-choice questions may continue on the next column or page – Fnd all choices be±ore answering. Homework 3. Due 25 September 2008 by 10 PM 001 10.0 points Which o± the statements concerning equilib- rium is NOT true? 1. A system moves spontaneously toward a state o± equilibrium. 2. The value o± the equilibrium constant ±or a given reaction is the same regardless o± the direction ±rom which equilibrium was attained. 3. Equilibrium in molecular systems is dy- namic, with two opposing processes balancing one another. 4. The equilibrium constant usually is inde- pendent o± temperature. correct 5. A system that is disturbed ±rom an equi- librium condition responds in a manner to restore equilibrium. Explanation: Equilibrium constants are temperature de- pendent. 002 10.0 points Explain why equilibium constants are dimen- sionless. 1. They are dimensionless because the pres- sures or concentrations we put in are all ±or the substances in their standard states. 2. The statement is not true. Equilibrium constants have units that involve some multi- ple o± atmospheres or moles per liter. 3. Every concentration or pressure that en- ters into K c or K p is really divided by the cor- responding concentration or pressure o± the substance in its standard state. correct 4. They are not really dimensionless but we must treat them as such in order to be able to take ln K in the expression Δ G 0 = - RT ln K . 5. They are dimensionless because concen- trations and pressures have no units. Explanation: The amount o± each component is in terms o± activity, which is the measured amount (concentration, pressure) divided by the amount o± that component in its standard state in that unit. The units in the numera- tor and denominator are identical and cancel out. 003 10.0 points Consider the reaction NO(g) + 1 2 O 2 (g) NO 2 (g) ²ind the equilibrium constant ±or the re- action at 298 K i± Δ H = - 56 . 52 kJ and Δ S = - 72 . 60 J · K 1 at 298 K. 1. 3 . 23 × 10 4 2. 1 . 30 × 10 6 correct 3. 1 . 65 × 10 4 4. 7 . 67 × 10 7 5. 8 . 08 × 10 9 Explanation: Δ G = Δ H - T Δ S = - 56520 - (298)( - 72 . 60) = - 34885 . 2 J K = e Δ G/ ( ) = e 34885 . 2 / (8 . 314 · 298) = 1 . 3033 × 10 6 004 10.0 points
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Niu (qn269) – Homework 3 – fowers – (53885) 2 IF Δ G rxn is positive, then the Forward reaction is (spontaneous / nonspontaneous) and K is (less / greater) than one. 1. spontaneous, greater 2. nonspontaneous; greater 3. nonspontaneous; less correct 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.
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HW3solutions - Niu(qn269 Homework 3 owers(53885 This...

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