# Electrochemical potentials

I was carrying out an experiment into the effect of temperature changes on the standard potential of a cell.

$$\ce{Zn(s) +2Fe^{3+}->Zn^{2+} +2Fe^{2+}}$$ I used equi-molar concentrations of $\ce{Fe(CN)_6^{4-}}$and $\ce{Fe(CN)_6^{3-}}$. Thus the Nernst equation

$$\mathrm{E^{\theta}_{cell}=E_{cell}-{\frac{RT}{nF}}*ln1}$$ reduces to $$\mathrm{E^{\theta}_{cell}=E_{cell}}$$

We plotted our $\mathrm{E^{\theta}_{cell}}$ values versus T and noticed that our $\mathrm{E^{\theta}_{cell}}$ values were actually increasing with temperature.

I would have expected that these values go down as the concentration of the reactant falls.

I asked my demonstrator and sourced information from different web resources, but can't access a definitive answer.

Any hints even?

• @JohnSow use E^\circ for $E^\circ$ – RE60K Oct 2 '14 at 10:45
• My demonstrator told me that he expected the entropy change to be negative as well. I would have that with a larger atom $$Fe^{2+}$$, there would be less solvation and thus a positive change in entropy? – Edward Oct 4 '14 at 21:16

For starters, $\ce{Zn^{2+}}$ is also part of the reaction quotient, so the log term in the Nernst equation will not necessarily be zero if the hexacyanoferrate concentrations are equal, depending on the $\ce{Zn^{2+}}$ concentration:
$$E = E°+\frac{RT}{zF}\ln\frac{[\ce{Zn^2+}][\ce{Fe^2+}]^2}{[\ce{Fe^3+}]^2}$$ Depending on whether the reaction quotient is greater than or less than 1, the potential can have either a positive or negative temperature dependance.
• use \ln for $\ln$ – RE60K Oct 2 '14 at 10:45