AP Chemistry · Unit 9: Thermodynamics and Electrochemistry
9.9 Cell Potential and Free Energy
In short
This topic focuses on the quantitative and conceptual relationship between standard cell potential (E°) and the standard Gibbs free energy change (ΔG°). You are expected to calculate the standard cell potential of electrochemical cells by identifying the oxidation and reduction half-reactions and their respective standard reduction potentials.
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What else do you need to know about Cell Potential and Free Energy?
You must understand that a thermodynamically favored reaction results in a positive voltage (E° > 0) and a negative ΔG°, whereas a thermodynamically unfavored reaction results in a negative voltage (E° < 0) and a positive ΔG°, requiring an externally applied potential to proceed. The mathematical relationship is defined by the equation ΔG° = −nFE°.
Can you do these?
- I can explain that thermodynamically favored redox reactions produce a positive voltage, while unfavored reactions produce a negative voltage and require an externally applied potential.
- I can calculate the standard cell potential of an electrochemical cell by identifying the oxidation and reduction half-reactions and their standard reduction potentials.
- I can use the equation ΔG° = −nFE° to relate standard cell potential to standard Gibbs free energy change, and use the sign of E° to determine if a reaction is thermodynamically favored.
How is Cell Potential and Free Energy tested on the exam?
This topic is assessed using the suggested skill 5.F (Mathematical Routines). You may be asked to calculate, estimate, or predict an unknown quantity (such as ΔG° or E°) from known quantities by selecting and following a logical computational pathway. Mark schemes will expect you to attend to precision, perform dimensional analysis, and use correct significant figures. You must also be able to provide written explanations justifying whether a cell is thermodynamically favored based on its standard cell potential.
Free practice questions with full explanations, and instant marking of your free-response answers.
Cell Potential and Free Energy: common questions
What do you need to know about Cell Potential and Free Energy for AP Chemistry?
This topic focuses on the quantitative and conceptual relationship between standard cell potential (E°) and the standard Gibbs free energy change (ΔG°). You are expected to calculate the standard cell potential of electrochemical cells by identifying the oxidation and reduction half-reactions and their respective standard reduction potentials.
How is Cell Potential and Free Energy tested on the AP Chemistry exam?
This topic is assessed using the suggested skill 5.F (Mathematical Routines). You may be asked to calculate, estimate, or predict an unknown quantity (such as ΔG° or E°) from known quantities by selecting and following a logical computational pathway.
How do I know I have mastered Cell Potential and Free Energy?
Check you can do each of these: explain that thermodynamically favored redox reactions produce a positive voltage, while unfavored reactions produce a negative voltage and require an externally applied potential; calculate the standard cell potential of an electrochemical cell by identifying the oxidation and reduction half-reactions and their standard reduction potentials; use the equation ΔG° = −nFE° to relate standard cell potential to standard Gibbs free energy change, and use the sign of E° to determine if a reaction is thermodynamically favored.
Where does Cell Potential and Free Energy fit in AP Chemistry?
Cell Potential and Free Energy is topic 9.9 in Unit 9: Thermodynamics and Electrochemistry of AP Chemistry.
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