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20 Electrochemistry Solutions to Exercises (b) Q=1 if all reactants and products are at standard conditions. (c) If concentration of reactants increases, Q decreases, and E increases. 20.62 (a) No. As the spontaneous chemical reaction of the voltaic cell proceeds, the concentrations of products increase and the concentrations of reactants decrease, so standard conditions are not maintained. (b) Yes. The Nernst equation is applicable to cell emf at nonstandard conditions, so it must be applicable at temperatures other than 298 K. There are two terms in the Nernst Equation. First, values of E° at temperatures other than 298 K are required. Then, in the form of Equation [20.16], there is a variable for T in the second term. In the short-hand form of Equation [20.18], the value 0.0592 assumes 298 K. A different coefficient would apply to cells at temperatures other than 298 K. (c) If concentration of products increases, Q increases, and E decreases. 20.63 Analyze/Plan. Given a circumstance, determine its effect on cell emf. Each circumstance changes the value of Q. An increase in Q reduces emf; a decrease in Q increases emf. Solve. (a) PH₂ increases, Q increases, E decreases (b) [Zn²⁺] increases, Q increases, E decreases (c) decreases, Q increases, E decreases (d) No effect; does not appear in the Nernst equation 20.64 Any change that causes the reaction to be less spontaneous (that causes Q to increase and ultimately shifts the equilibrium to the left) will result in a less positive value for E. (a) Increases E by decreasing on the right side of the equation, which decreases Q. (b) No effect; the "concentrations" of pure solids and liquids do not influence the value of K for a heterogeneous equilibrium. (c) No effect; the concentration of Ag+ and the value of Q are unchanged. (d) Decreases E; forming AgCl(s) decreases the concentration of which increases Q. 20.65 Analyze/Plan. Follow the logic in Sample Exercise 20.11. Solve. (a) Ni(s) Zn(s) (aq) 629