[sinthreck]

An electrolysis cell contains a mixture of the following salts in solution in the cell around the cathode:
0.2 M CdCl 2, 1.0 x 10 -5 M CoCl 2, 5 M FeCl 2, and 1.0 x 10 -5 M NiCl 2.

Given the following standard reduction potentials, in which order are the metals deposited at the cathode as the voltage of the electrolysis cell is slowly increased?

Ni²⁺ ( aq) + 2e - → Ni ( s)      E°= -0.275 V
Co²⁺ ( aq) + 2e - → Co ( s)    E°= -0.280 V
Cd²⁺ ( aq) + 2e - → Cd ( s)    E°= -0.400 V
Fe²⁺ ( aq) + 2e - → Fe ( s)     E°= -0.440 V
ANS: Fe, Ni, Co, Cd

My understanding is that the reaction which is most easily reduced will plate out first, however, I suppose it also depends on concentration.

My next thought is to use the nerst equation but what concentration do I put for the anode?

[FreeTheBee]

If the reaction on the anode is the same in all cases, do you really need to know any details about it to make a comparison?

[sinthreck]

thanks for that.

Only thing is, I got a slightly different answer to their's

Fe, Cd, Ni, Co

[Borek]

Cd & Ni order is highly temperature dependent, it can be either Fe, Cd, Ni, Co or Fe, Ni, Cd, Co (that is assuming we know concentrations with a high precision). But in general you are right and they seem to be wrong.

Or at least you are wrong in a good company

[FreeTheBee]

If I use the numbers you wrote I get the same order as you. According to my tables the standard potential for nickel should 0.257V though. But with that number I don't get the "correct" order either.
The numbers are so close that in practice they will have little predictive power, but I guess that is not so relevant for the exercise 

[sinthreck]

thanks guys!

Exam in 4 hrs time.

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