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Dario56

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- TL;DR Summary
- Why is Enthalpy of Reaction Used as Energy Input in Galvanic Cells When Defining its Efficiency?

For electrochemical energy devices such as batteries or fuel cells working reversibly, efficiency is defined as:

η=ΔrG⦵/ΔrH⦵

Since cell operates reversibly Gibbs energy change of the system is equal to electrical work done by the cell, so we can instead just use standard Gibbs energy of reaction as energy output. What I am not clear about is why is enthalpy of reaction used as energy input? Since electrochemical energy devices transform chemical energy of electroactive materials to electrical energy, energy input should be chemical energy, but I am not sure why enthalpy of reaction represents chemical energy? Enthalpy of reaction is given by this equation:

ΔrH⦵=ΔrG⦵+TΔrS⦵

Since cell operates reversibly, ΔrG⦵ represents reversible electrical work (maximum non-PV work done by the cell) and TΔrS⦵represents heat exchanged with surroundings to keep cell at constant T in reversible operation since change in Gibbs energy as measure of non-PV work is only applicable for closed systems at constant p and T.

η=ΔrG⦵/ΔrH⦵

Since cell operates reversibly Gibbs energy change of the system is equal to electrical work done by the cell, so we can instead just use standard Gibbs energy of reaction as energy output. What I am not clear about is why is enthalpy of reaction used as energy input? Since electrochemical energy devices transform chemical energy of electroactive materials to electrical energy, energy input should be chemical energy, but I am not sure why enthalpy of reaction represents chemical energy? Enthalpy of reaction is given by this equation:

ΔrH⦵=ΔrG⦵+TΔrS⦵

Since cell operates reversibly, ΔrG⦵ represents reversible electrical work (maximum non-PV work done by the cell) and TΔrS⦵represents heat exchanged with surroundings to keep cell at constant T in reversible operation since change in Gibbs energy as measure of non-PV work is only applicable for closed systems at constant p and T.