Conceptual Problems. For these five problems, you do not need to justify or explain your answer. Each problem is worth 5 points

1. What is the OH^- concentration (in units of moles per liter) of an aqueous solution of pH 6?

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2. The oxidation/reduction reaction

Cu²⁺(aq)+ SO₄^2-+ Zn(s) ß à Cu(s) + Zn²⁺(aq) + SO₄^2-(aq)

is spontaneous. Is the standard potential for this reaction greater than zero, less than zero, equal to zero or impossible to determine?

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3. In a galvanic cell such as a chemical battery, does an oxidation reaction occur at the positive electrode, the negative electrode, both electrodes or neither electrode?               Answer
4. What are the units of the rate constant for the irreversible reaction A à B+C?

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5. In the irreversible reaction A + B à C, if one increases the concentration of C, will the rate of change in the concentration of A increase, decrease, remain the same or is it impossible to tell?

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Numerical Problems. You must show all your work for complete credit.

6. (20 points) Consider an aqueous solution which contains 0.010 M acetic acid (CH₃COOH), 0.010 M sodium acetate (Na⁺ CH₃COO^-) and 0.010 M sodium chloride (Na⁺ Cl^-). The pK_A of acetic acid is 4.75.

1. Assuming that all activity coefficients are one, what is the pH of the solution ? Justify your answer with an equation.
2. Determine the ionic strength of the solution.
3. Determine the average activity coefficient for sodium acetate in the solution.
4. Using the activity coefficient that you calculated, estimate the pH that you would actually measure of this solution using a pH meter.

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7. (20 points) Let's say that a galvanic cell (spontaneous oxidation/reduction reaction like a battery) is constructed using the half reactions Cu²⁺(aq) + 2e^- à Cu(s) (E⁰ = +0.34V) and Zn²⁺ + 2e^- à Zn(s) (E⁰ = -0.76V). What ratio of concentrations, Zn²⁺/Cu²⁺, in the electrolyte will be required if one wants a 1.10 V zero current potential? Assume that the temperature is 298 K and that all activity coefficients are one. Hint: What is Q in the Nernst Equation for this reaction?

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8. (20 points) For the elementary reaction series A+B à C ß à D, for components C and D only, give

1. the rate equations (differential equations describing the rate of change in concentration of each component with time),
2. the values of the initial rates of change of C and D and
3. the final concentrations of C and D when the reaction comes to equilibrium.

Note that the initial concentrations are A₀ = B₀= 0.30 M, C₀ = 0.20 M, D₀ = 0.00 M. The rate constant for the first reaction, k₁, is 30 M^-1 s^-1, the forward rate constant for the second reaction, k_f2, is 15 s^-1 and the reverse rate constant, k_b2, is 10 s^-1.

Applied Problem. You must show all your work for complete credit.

9. (15 points) The purpose of a catalyst is to lower the activation energy of a reaction (in fact, that is all that a catalyst does). The enzymes in your body which mediate chemical reactions are catalysts. One of these enzymes is called catalase and it catalyzes the breakdown of hydrogen peroxide: 2H₂O₂ (hydrogen peroxide) à 2H₂O + O₂. At 20 C, the uncatalyzed rate of this reaction is quite slow, taking days to weeks for an open bottle of hydrogen peroxide to decay. When the enzyme is added, the rate increases by a factor of 10⁸ at the same temperature (in other words, ). Assuming that the rate is related to the activation energy in the usual way (k is proportional to ), determine how much the enzyme lowers the activation energy for this reaction.

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