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Unformatted text preview: purdy (sbp456) Homework 3 flowers (53885) 1 This print-out should have 32 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. Homework 3. Due 25 September 2008 by 10 PM 001 10.0 points Which of the statements concerning equilib- rium is NOT true? 1. Equilibrium in molecular systems is dy- namic, with two opposing processes balancing one another. 2. The value of the equilibrium constant for a given reaction is the same regardless of the direction from which equilibrium was attained. 3. A system that is disturbed from an equi- librium condition responds in a manner to restore equilibrium. 4. A system moves spontaneously toward a state of equilibrium. 5. The equilibrium constant usually is inde- pendent of temperature. correct Explanation: Equilibrium constants are temperature de- pendent. 002 10.0 points Explain why equilibium constants are dimen- sionless. 1. Every concentration or pressure that en- ters into K c or K p is really divided by the cor- responding concentration or pressure of the substance in its standard state. correct 2. They are not really dimensionless but we must treat them as such in order to be able to take ln K in the expression G =- RT ln K . 3. They are dimensionless because the pres- sures or concentrations we put in are all for the substances in their standard states. 4. The statement is not true. Equilibrium constants have units that involve some multi- ple of atmospheres or moles per liter. 5. They are dimensionless because concen- trations and pressures have no units. Explanation: The amount of each component is in terms of activity, which is the measured amount (concentration, pressure) divided by the amount of 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) Find the equilibrium constant for the re- action at 298 K if H =- 56 . 52 kJ and S =- 72 . 60 J K 1 at 298 K. 1. 3 . 23 10 4 2. 1 . 65 10 4 3. 8 . 08 10 9 4. 1 . 30 10 6 correct 5. 7 . 67 10 7 Explanation: G = H - T S =- 56520- (298)(- 72 . 60) =- 34885 . 2 J K = e G/ ( RT ) = e 34885 . 2 / (8 . 314 298) = 1 . 3033 10 6 004 10.0 points purdy (sbp456) Homework 3 flowers (53885) 2 If G rxn is positive, then the forward reaction is (spontaneous / nonspontaneous) and K is (less / greater) than one. 1. None of these; G is not directly related to K . 2. nonspontaneous; greater 3. nonspontaneous; less correct 4. spontaneous, less 5. spontaneous, greater Explanation: A positive G rxn (standard reaction free energy) denotes an endothermic reation, which is nonspontaneous. Also, G rxn =- RT ln K , so a positive G rxn would result in a K that is between the values of zero and one....
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- Spring '07