Why does a mercury button cell maintain an exceptionally constant electromotive force (approximately 1.35 V) throughout its entire operational lifetime?
Answer and explanation
Correct answer: D. The overall cell reaction does not involve any dissolved ions in solution whose concentration could change over time
- Anode:
- Cathode:
- Cathode:
- Step 1: The electrode reactions in a mercury cell are:
- Step 2: The overall cell reaction is: .
- Step 3: Notice that the overall chemical equation contains only solid and pure liquid phases, with zero dissolved ions in solution whose concentration changes during discharge.
- Step 4: According to the Nernst equation, because the reaction quotient is invariant, the potential remains constant at ~1.35 V until the cell is exhausted.
Answer
The overall cell reaction does not involve any dissolved ions in solution whose concentration could change over time
Why not the others: 'It is connected to an internal miniature transformer...' is technologically fictitious for a button cell; 'Mercury atoms act as superconducting pathways...' is false because superconductivity occurs only near absolute zero; 'It continuously absorbs atmospheric carbon dioxide...' is false because button cells are hermetically sealed.
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