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Why are cell potentials the same for reactions with same overal equation?

e.g. hydrogen-oxygen fuel cell in acidic and alkaline conditions

It is from the chemistry past paper

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mystreet123
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Actually, cell potentials will vary according to changed conditions. This is outlined by something called the Nernst equation, which states that $$E_\text{cell}=E^\circ_\text{cell} - \frac{RT}{zF}\ln Q_\mathrm r$$ Because of this, a change in $\mathrm{pH}$ could affect the $\ln Q_\mathrm r$ term, causing a change in cell potential. In addition, the values for reduction potentials that you look up online or in a textbook are for a reaction occurring at $25\ \mathrm{^\circ C}$, meaning that your reduction potential will likely change with a change in temperature.

Niels Kornerup
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  • please see my added pictures of the question thanks for replying! @Benzene – mystreet123 Jun 01 '16 at 10:45
  • Given the above fuel cell, a change in pH would have no effect on the concentrations of the reducing or oxidizing agents, as neither of them are protic. As such, the reaction remains the same and the value of Q is unchanged by the change in pH. – Niels Kornerup Jun 01 '16 at 14:59