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, nonspontaneous

\displaystyle \small -1.1 V, spontaneous

\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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