[IBguy]

Hey, I'm doing a practical at school, and I've encountered a difficulty in explaining the data I got from the lab. I have a voltaic cell with Zinc and Copper electrodes, and with Zinc and Copper sulphate solutions, all connected to a voltmeter. When I increase the concentrations of the solutions at the same time by the same amount, the voltage doesn't change. 

Now I know this can be explained mathematically using the Nernst equation, E=Eo−(0.0592/n)log Q where Q is the quotient of the concentration of products over reactants; since the quotient always equals one, E always = Eo. I understand that, but I don't understand how to explain it using motions of charges.

Maybe it's better to ask: how come the following reasoning doesn't work: the higher the concentrations of the solutions, the faster the half reactions occur and the Zinc gets oxidized and electrons travel through wires to the copper electrode, i.e. greater p.d.? It just seems confusing to explain scientifically (not mathematically) how the p.d. can be the same when the two solution concentrations are the same, but lower, than when they are also the same, but higher.

Hopefully this made some sense, thank you. Oh and could you please try to respond in somewhat simpler terms, I take chemistry at a standard level (i.e. no changes in standard electrode potentials).

[Borek]

the higher the concentrations of the solutions, the faster the half reactions occur

Speed of the reaction doesn't control the potential, it controls the current.

(Yeah, this is quite handwavy, but you asked for the dumbed down approach).

[IBguy]

the higher the concentrations of the solutions, the faster the half reactions occur

Speed of the reaction doesn't control the potential, it controls the current.

(Yeah, this is quite handwavy, but you asked for the dumbed down approach).

Thanks, that makes sense logically. But could you now explain it in more scientific terms and particularly why current Is dependent but potential is not?

[Borek]

More scientifically it is all in the Nernst equation (which reflects thermodynamics of the system) and Faraday's law (which is basically a conversion between moles of electrons exchanged in the redox reactions and the definition of a coulomb - unit of charge).

[AdiDex]

the higher the concentrations of the solutions, the faster the half reactions occur and the Zinc gets oxidized

Firstly Your statement is not appropriate.
As You increase the concentration of Zn²⁺ in half cell where reaction is like -

Zn  Zn²⁺ + 2e^-

While you are asking for P.D. then always think about Thermodynamics .
Try to apply le chatelier's principle in both of the half cell you will get your answer.

By the way i have a question how current vary with concentration ??
Until unless we don't know the Kinetics of cell ?? it can be zero order too !!
i think Current should vary upon internal and external resistance otherwise we can't able to handle our gadgets with any other source of power having same Potential difference but having different concentration Eg. take example of portable charger and Home charger.