6. A scientist constructs a galvanic cell as shown in the diagram. As the cell operates, the Zn(s) e — Electrochemistry, Galvanic Cells, Standard Reduction Potentials Chemistry Question
Problem Context
- A scientist constructs a galvanic cell as shown in the diagram. As the cell operates, the Zn(s) electrode increases in mass and the Al(s) electrode decreases in mass. A data table with the standard reduction potentials for the substances follows the diagram.
A. Write the half-reaction for the oxidation that occurs at the anode.
Model Answer
The correct equation (state symbols are not required) is:
Al(s) → Al³⁺(aq) + 3 e⁻ [1].
B. Write the balanced net ionic equation for the overall reaction that occurs in the galvanic cell.
Model Answer
The correct balanced net ionic equation (state symbols are not required)
is:
2 Al(s) + 3 Zn²⁺(aq) → 2 Al³⁺(aq) + 3 Zn(s) [1].
C. Initially, each electrode has a mass of 50.0 g. The cell is allowed to run for a period of time and is then stopped. Which electrode’s mass changed the most? Justify your answer with a calculation.
Model Answer
Zn experiences a greater change in mass [2]. The response must include a
valid justification comparing the masses of Al and Zn based on their molar
masses and the balanced equation's stoichiometry [1]. Acceptable justifications
include:
* Assuming the entire Al anode reacts: 50.0 g Al × (1 mol Al / 26.98 g Al) ×
(3 mol Zn / 2 mol Al) × (65.38 g Zn / 1 mol Zn) = 182 g Zn [2].
* Calculating per mole of reaction: 3 mol Zn × (65.38 g Zn / 1 mol Zn) = 196.1
g Zn / 1 mol rxn; and 2 mol Al × (26.98 g Al / 1 mol Al) = 53.96 g Al / 1 mol
rxn. Thus, for however many moles of reaction that proceed, the mass of Zn
produced will be greater than the mass of Al consumed [2].
* Explaining that as the reaction proceeds, three moles of Zn are used for
every two moles of Al. Thus, for every 196 g of Zn that are produced, 54 g of Al
are consumed [3].
D. The standard Zn/Al cell has a value of E°cell equal to 0.90 V. The scientist needs a galvanic cell that produces a greater voltage. The scientist has access to the chemical systems in the table. If the scientist uses the Zn half-cell and one of the other options from the table, what is the MAXIMUM voltage that could be generated at standard conditions?
Model Answer
The correct calculated value is 2.26 V [3]. Acceptable responses
include:
* E°cell = 1.50 V + 0.76 V = 2.26 V [3].
* Listing the half-reactions and their potentials before providing the total:
Au³⁺(aq) + 3 e⁻ → Au(s) (1.50 V)
Zn(s) → Zn²⁺(aq) + 2 e⁻ (0.76 V)
E°cell = 2.26 V [3].
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