sketching voltaic cells (1 Viewer)

[zzephyr]

for example,
Sketch the voltaic cell (electrochemical cell) that could be constructed using a strip of Ni, a strip of Mg, a Mg(NO3)2 solution and a Ni(NO3)2 solution.

What determines which is a cathode and which is an anode? I understand anodes are oxidised and cathodes are reduced so the cathode is Ni^2+ +2e^- ---->Ni(s) and the anode is Mg(s)----->Mg^2+ +2e^-
but how come it isnt something like Mg^2+ +2e^------>Mg(s) where Mg is now reduced

 

[Crisium]

The more reactive metal will be oxidised so in this case it will be Magnesium.

If you refer to your standard reduction potentials sheet with the list of equations the higher the element/compound is on the table, the more likely it is to be oxidised. Since magnesium is higher on the table than nickel it will be oxidised and the electrons released during oxidation will flow into the external wire and will reduce the nickel ions onto the nickel electrode (You can actually see a build-up or deposit on the electrode).

Think of it in relation to displacement reactions. A displacement reaction will only occur if you add in a more reactive metal into another metal ion solution, but if you add in a less reactive metal then no reaction will occur.
I hope this makes sense.

Things to look out for with galvanic cells:

* The standard reduction potential table has equations set out in the form of a reduction reaction. When you need to find the EMF of the oxidation reaction you have to swap the sign of the EMF if that equation were to be the reduction equation. So in this case for magnesium, rather than being -2.36V you would have to change the sign to +2.36V. Also there are some equations where there are different valencies and it will require you to balance the equation so that the overall cell equation has no electrons in it. When you multiply by this number IT DOES NOT EFFECT THE EMF.

* Be careful when writing down the EMF for an equation because if you refer to the lower parts of the standard reduction table you will see that some elements/compounds come in different states (i.e. Liquid, aqueous solution, solid, gas) or have different valencies

 

[zzephyr]

Ah so its as easy as that, thanks for your help!