[Big-Daddy]

Problem:
An acidified sample is passed through the reductor, collected, and titrated with a strong oxidizing titrant of known concentration (for example KMnO4). The most common version is the so-called Jones-reductor that contains amalgamated zinc granules.

Give the reactions that take place, and the corresponding equilibrium constants, when the following solutions are passed through a Jones-reductor:
0.01 mol/dm3 CuCl₂
0.01 mol/dm3 CrCl₃
0.01 mol/dm3 NH₄VO₃ (pH =1)

Data provided:
E°(Zn²⁺/Zn) = –0.76
E°(Cu²⁺/Cu) = 0.34
E°(Cu²⁺/Cu⁺) = 0.16
E°(Cr³⁺/Cr) = –0.74
E°(Cu²⁺/Cu⁺) = 0.16
E°(Cr²⁺/Cr) = –0.90
E°(VO₂⁺/VO²⁺) = 1.00
E°(V³⁺/V²⁺) = –0.255
E°(V²⁺/V) = –1.13

Comment:
If I could work out what reaction takes place, there is sufficient data for me to easily work out the equilibrium constants for those reactions. So how do I work out which reactions take place?

[sjb]

What determines if a reaction takes place?

[Big-Daddy]

The Gibbs' free energies, so in this case the E° values would be used to give an indication. So is the process to go through the stages of reduction one by one, and whenever a certain reduction has negative E°, it won't occur (assume E° and E take the same sign), but all previous reductions will occur?

In other words, which reaction occurs has no dependence on E° of Zn²⁺ couple?