5 Steps to a 5 AP Chemistry 2019

360 ❯ STEP 5. Build Your Test-Taking Confidence

❯ AP Chemistry Practice Exam 2, Section II (Free Response)

Time—1 hour and 45 minutes

Answer the following questions in the time allowed. You may use a calculator and the resources at the back

of the book. Write the answers on separate sheets of paper.

Pb

38% H 2 SO 4 solution

Pb + PbO 2

Question 1

A simplified diagram of one cell of a lead-acid battery as used in most automobiles is shown above. The

half-reactions are:

PbO 2 (s) + HSO 4 - (aq) + 3 H+(aq) + 2 e- → PbSO 4 (s) + 2 H 2 O(l)

Pb(s) + HSO 4 - (aq) → PbSO 4 (aq) + H+(aq) + 2 e-

The standard cell potential is +2.04 V at 25°C. Initially the electrode on the left is pure lead, and the

electrode on the right is pure lead coated with lead(IV) oxide.

(a) The standard reduction potential for the PbO 2 half-reaction is 1.68 V.

(i) Write the overall reaction for the lead-acid battery.

(ii) Calculate the value of the standard reduction potential for the Pb half-reaction.

(b) A student constructs a brand-new cell and it is fully charged.

(i) Calculate the initial value of ΔG° for the cell.

(ii) Calculate the value of K for the cell.

(iii) The cell is allowed to operate until it reaches equilibrium. What is the value of ΔG° for the cell

at equilibrium?

(c) In an automobile, the water from the electrolyte evaporates. Explain how this will affect the cell potential.

(d) Why is the PbO 2 electrode made of Pb coated with PbO 2 and not pure PbO 2?

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