Long free-response question

A nickel–silver galvanic cell

Units 4 and 9
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

A multipart question, often built around a lab setup or a data table, that mixes several topics. You calculate, explain and justify your answers across many lettered parts. On the exam: 3 questions (Questions 1–3), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

A student builds a galvanic cell at 25 °C. One beaker contains a nickel electrode in 1.00 M Ni(NO₃)₂(aq); the other contains a silver electrode in 1.00 M AgNO₃(aq). The electrodes are connected by a wire through a voltmeter, and the solutions are connected by a salt bridge containing KNO₃(aq).

Table 1. Standard reduction potentials at 25 °C

Half-reaction	E° (V)
Ag⁺(aq) + e⁻ → Ag(s)	+0.80
Ni²⁺(aq) + 2 e⁻ → Ni(s)	−0.25

Source: Standard values, rounded

Suggested time: 23 minutes

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Part (a)

1 point

Write the balanced net ionic equation for the overall reaction that occurs as the cell operates.

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Part (b)

1 point

Calculate the standard cell potential, E°cell.

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Part (c)

1 point

Calculate ΔG° for the cell reaction, in kJ/mol_rxn.

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Part (d)

1 point

Identify the anode in the cell, and state the direction in which electrons flow through the wire.

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Part (e)

1 point

As the cell operates, toward which beaker do the NO₃⁻ ions in the salt bridge migrate? Explain why this migration is necessary.

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Part (f)

2 points

The cell is operated at a constant current of 0.150 A for 45.0 minutes. Calculate the change in mass of the silver electrode, and state whether the electrode gains or loses mass.

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Part (g)

1 point

A second cell is identical to the first except that the silver half-cell contains 0.10 M AgNO₃ instead of 1.00 M AgNO₃. Will the initial cell potential of the second cell be greater than, less than, or equal to 1.05 V? Justify your answer.

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Part (h)

1 point

When the original cell operates for a long time, its voltage gradually decreases and eventually reaches zero. Explain this observation in terms of Q and K.

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Part (i)

1 point

Is the equilibrium constant, K, for the cell reaction greater than 1 or less than 1? Justify your answer.

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