[T]

Hello,

The standard reduction/electrode potential is found by measuring the ionisation of a material relative to the hydrogen electrode. The 2 half cells are connected by a voltmeter.

If my example was a hydrogen half cell connected to a magnesium half cell by a voltmeter, what would be the redox equation for both cells?

From what I learnt, since both the hydrogen and magnesium half cell are connected by a voltmeter, the reactions are separate, so they would both be oxidised to some extent. (From http://www.chemguide.co.uk/physical/redoxeqia/introduction.html#top)

Mg  Mg²⁺ + 2e^-
H₂  2H⁺ + 2e^-

But there doesn't seem to be an reduction equation from chemguide explanation. Could someone explain what the reduction equation for each of the half cell is if they are connected by a voltmeter?

Thanks

[Borek]

I think you misread their explanation. Magnesium reaction is not

Mg Mg²⁺ + 2e^-

but

Mg Mg²⁺ + 2e^-

which means that when you have two reactions one will go one way, and the other another way (that is, one will be reduction, the other oxidation).

[T]

I see, but how do they measure the voltage difference between the electrodes? Since there will be no "build up of electrons"?
Thanks

[Borek]

Perfect voltmeter has an infinite internal resistance, so there will be build up of charges. And the charge (which is directly related to the electrode potential) is different for different half cells.

[T]

So when a Mg atom from the electrode oxidizes , a Mg ion from the solution will take its place instead by reducing. This causes a build up of charge, and for each half cell the charge is diffrent. Is this a correct interpretation?

Thanks

[T]

I am pretty sure my explanation before is wrong so I will redefine my problem/question.

Where does electrons from the magnesium half cell go? Since it can't go to the hydrogen half cell because voltmeters have infinite internal resistance.

Perfect voltmeter has an infinite internal resistance, so there will be build up of charges. And the charge (which is directly related to the electrode potential) is different for different half cells.

How are there build up of charges, what does it mean?

[Borek]

Let's say each half cell consists of a solution (with some ions) and a dipped electrode.

When the circuit is open electrode gets charged, and the charge it collects reflects the potential of the half cell present in the solution.

When there are two charged objects, and each is charged differently, there is a potential difference between them.

To measure this potential difference, we connect these objects and measure the current that flows. The higher the resistance of the connector, the better. Perfect voltmeter has an infinite resistance, but every real one has some finite resistance (although the better the voltmeter, the higher its internal resistance is).

Electrons from the hydrogen half cell don't have to go anywhere (but the electrode) for the potential difference to be present.

Once you connect the electrodes through a finite resistance, electrons start to flow. In a working battery charge on the electrodes is replenished by the chemical reaction.

[T]

I still don't really understand but I think this part of the theory probably won't be needed in the olympiad. As long as I remember the 2 equations, and how the potential difference is found.

Thanks Borek