February 28, 2021, 01:09:11 PM
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Topic: The effect of temperature on EMF of a voltaic cell  (Read 320 times)

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Offline cvgjnh

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The effect of temperature on EMF of a voltaic cell
« on: February 01, 2020, 04:34:35 PM »
I understand why changing the concentrations of the electrolytes in a voltaic cell affects the voltage. Given a zinc-copper cell for example, if I were to increase the concentration of the zinc sulphate electrolyte, the equilibrium between zinc ions and zinc metal would shift towards the side of the zinc metal. This would cause the reduction potential for zinc ions to increase, decreasing the overall cell potential. Please correct me if I’m wrong as this is how I understand it!

However, I don’t quite understand why temperature affects the voltage the way it does. According to the Nernst equation, if the reaction quotient is greater than one (meaning that the zinc sulphate concentration is greater than the copper sulphate concentration in this example), increasing temperature decreases voltage. Conversely, if the reaction quotient is less than one, increasing temperature increases voltage.

I am familiar with Le Chatlier's principle, but if I were to apply it to this scenario, given that the redox reaction taking place is exothermic, it would suggest that increasing temperature would always decrease the voltage produced since it would cause a shift towards the reactant side. This, of course, does not correspond with the relationship predicted by the Nernst equation.

I have looked all over the internet but I could not find any answer to this seemingly straightforward question. Any help is greatly appreciated!

Offline mjc123

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Re: The effect of temperature on EMF of a voltaic cell
« Reply #1 on: February 05, 2020, 08:22:31 AM »
Are you thinking of E = E° - RT/2F*ln([Zn2+]/[Cu2+])?
It's not just the last term that has a T-dependence. How does E° vary with temperature?

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