For a closed circuit Daniell cell, which of the following plots is the accurate one at a given temperature?

Solution

Chemistry ma’am · 4 steps

Look at the degree sign on the axis: this is E°_(cell), and it does not change with time.

1. What E° means. E°_(cell) is the standard cell potential, measured with every ion at 1 M. For the Daniell cell it is E°_(Cu²⁺/Cu) - E°_(Zn²⁺/Zn) = 0.34 - (-0.76) = 1.10 V. It is a property of the two couples and depends only on temperature.
2. What running changes. In a closed circuit the cell reaction Zn + Cu²⁺ → Zn²⁺ + Cu keeps going. So [Zn²⁺] rises and [Cu²⁺] falls as time passes.

   E_(cell) = E°_(cell) - 0.059/2 log[Zn²⁺]/[Cu²⁺]

3. Which term moves. The changing concentrations sit only in the log term, so only E_(cell) drops with time. E°_(cell) is a fixed number at a given temperature, whatever the concentrations are.
4. So. A plot of E°_(cell) against time is a horizontal straight line, which is option (d).

The idea to keep. E° is fixed by the electrode couples and the temperature. Concentrations change only E_(cell), through the Nernst log term. Always check whether the axis says E or E°.

Answer from the NTA final answer key. Solution by PrepWiser.