[grodanboll]

According to the Nernst equation the voltage of a galvanic cell should change depending on the electrolytes' concentration and I'm wondering why this happens. The way I see it it would make sense that lower concentrations result in less voltage as there's fewer ions reacting, but that doesn't explain why the voltage only changes when the concentrations are different.

[Borek]

. The way I see it it would make sense that lower concentrations result in less voltage as there's fewer ions reacting, but that doesn't explain why the voltage only changes when the concentrations are different.

No idea what you mean - you say "voltage should change with the concentration" and then you say "I don't get why it changes when the concentration changes".

In general it is based on thermodynamics, see for example Gibbs free energy and cell voltage.

[grodanboll]

Sorry, should have been more specific, I don't get why in the Nernst equation the voltage only changes when the electrolyte concentrations in the two half-cells are different from each other. I feel that the voltage should be lower if the concentrations in both cells are 0.1M compared to 1M for example, but according to the Nernst equation the voltage should then be the same.

[Borek]

I believe you refer to the concentration cell. The only source of the energy is the concentration difference - when the concentrations are identical, system is at equilibrium, so there is no reason for the oxidation/reduction reaction (same reaction, just going in different directions) to proceed. This is thermodynamics 101.

Basically every source of energy requires some kind of a gradient/difference. If there is no parameter that is different for two parts/states of the system, system is at equilibrium and there is no energy to be gathered.

Reasonably good analogy are two communicating vessels - when there is a liquid level difference, liquid will flow till the level is identical in both vessels. That's the same what happens in the concentration cell - flowing current is a way of "transferring concentration" between cells and will stop flowing once the concentration becomes identical. Level difference of liquid in communicating vessels is the equivalent of the potential difference between cells with different concentrations.