Chem question help please

Hi,
Please could I have help on this question? Why can’t we use potassium as it as a more negative e cell value than chromium?
Question: https://ibb.co/20wVzH74
Ms: https://ibb.co/Kzy0RJ29
Thanks!

Potassium would reduce it further than Cr^2+ (aq) - you’d make Cr (s). Your reducing agent must have a more -ve reduction potential than the Cr^3+/Cr^2+ system, but it can’t be more -ve than the Cr^2+/Cr system.
Additionally, can you add something like potassium to an (acidic) aqueous solution without it exploding?

But one of the metals in the ms, managanese has an e value of -1.18 which is more negative than both systems?

Interesting. Some metals will not reduce Cr^3+ (aq) all the way to Cr (s) for kinetic reasons (e.g the activation energy being too high), but you couldn’t possibly be expected to know which ones that is true of.
It’s probably more that because there is an acid added as stated in the question and the solution is aqueous that you can’t get away with using an alkali metal or alkaline earth metal.
To be honest, it’s not a great exam question anyway.

Sorry why can’t you add an acid to an aqueous solution?

It’s not that you can’t add an acid to an aqueous solution - it’s that alkali metals and alkaline earth metals react violently with acids and water, e.g
2K + 2H2O —> 2KOH + H2
2K + H2SO4 —> K2SO4 + H2
Ba + 2H2O —> Ba(OH)2 + H2
Ba + H2SO4 —> BaSO4 + H2