AP Chemistry : Cell Potential and Free Energy

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #1 : Cell Potential And Free Energy

1. \(\displaystyle \small Zn \rightarrow Zn^{2+} + 2e^{-}\) \(\displaystyle \small \small E^{o} = 0.762 V\)

2. \(\displaystyle \small Cu \rightarrow Cu^{2+} + 2e^{-}\) \(\displaystyle \small \small E^{o}= -0.340 V\)

3. \(\displaystyle \small Zn^{2+} + Cu \rightarrow Zn + Cu^{2+}\)

What is \(\displaystyle \small \small E_{cell}^{o}\)? Is reaction 3 spontaneous?

Possible Answers:

\(\displaystyle \small -1.1 V\), spontaneous

\(\displaystyle \small -1.1 V\), nonspontaneous

\(\displaystyle \small 1.1 V\), nonspontaneous

\(\displaystyle \small 1.1 V\), spontaneous

Correct answer:

\(\displaystyle \small -1.1 V\), nonspontaneous

Explanation:

\(\displaystyle \small E^{o}_{cell} = E_{red}^{o} + E_{ox}^{o}\)

\(\displaystyle \small E_{cell}^{o} = -0.762 V - 0.340 V\)

\(\displaystyle \small E^{o}_{cell} = -1.1 V\)

There are two concepts to consider in this problem.  First, the question asks for the \(\displaystyle \small E^{o}_{cell}\).  Reaction 3 has \(\displaystyle \small Zn^{2+}\) being reduced so the potential for the half reaction becomes negative. \(\displaystyle \small Cu\) half reaction appears the same in reaction 3 so the potential is the same. Second, negative voltages indicate non-spontaneity and positive voltages are spontaneous.  

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