Walther Hermann Nernst and EE’s

By Friday, February 26, 2016 , 0

You may have heard of the Nernst equation (named after guess who?!) and we will look at it in a moment but I always think it is good to put things into context.

Nernst lived from 1864 – 1941 and was a German Chemist. He is famous for proposing the third law of thermodynamics, that entropy change approaches zero as temperature approaches absolute zero (which I really like as it is kind of obvious but it takes a great mind to make this link!) – this won him a Nobel prize in 1920.

He also proposed the surprisingly unheard of ‘Nernst-Einstein equation’ (unheard of a school level that is) with the great man himself. I won’t go into what the Nernst Einstein equation is as it is far too complicated (and I’m not too sure if I understand it!) but it is a relationship between mobility and the diffusion coefficient (say no more!).

We come across him with regards to the ‘Nernst equation’, that fiendishly complicated and hard to understand equation that was covered in Option C, Energy.

The equation is:

E = Eo – (RT / nF) ln Q (told you it was complicated) but at least you don’t have to learn it as it is given in the data book.

So what does it do?

Well, like all good equations, it is quite simple, it is taught in the context of redox cells and it links together the concentration of the cell with voltage.

E is the voltage of the cell

Eis the emf of the redox cell under standard condtions

R is the gas constant (the chemists ‘Pi’ – more about this in a later post)

F is Faraday’s constant

T is the temperature in Kelvin

n is the number of electrons transferred

What I like about this equation is that it links in ‘Q’ – the reaction quotient as well.

In a redox cell, it is the concentration of ions being oxidized divided by the concentration of ions being reduced.

Under standard conditions E =Eo as Q = 1 and ln 1 = 0

But, in a redox cell the solutions of species being oxidized and reduced are kept separate so it is possible to change the concentration of them and hence change the cell voltage.

If you are looking for a good EE, the use of the Nerst equation in redox cells is a good one as it allows you to make predictions and test them – you could even plot a graph of predicted voltages and then test the predictions, potting another graph of actual values and trying to account for any differences in them.

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