Writing half reactions for redox reactions

Question

For this particular redox reaction, KI + H2 ➡️ K + HI we can balance it by writing out the half reactions:

KI ➡️ K + HI

H2 ➡️ H20?

My question is for the H2 half reaction above, would we balance it with water since K and HI on the product side is involved in the first half reaction? I know how to balance the rest of reaction after this but this part confuses me. Thanks!

Answer 1

In theory, you can balance any redox reaction using half reactions.

KI + H2 ==> K + HI overal reaction

The iodide (I^-) is simply a spectator and can be ignored. It's oxidation number remains constant)

K⁺ + e- ==> K ... reduction half reaction [x2]

H₂ ===> 2H⁺ + 2e- ... oxidation half reaction

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2K⁺ + 2e- ==> 2K

H₂ ====> 2H⁺ + 2e-

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2K⁺ + H₂ ==> 2K + 2H⁺ ... balanced redox equation (without spectator iodide)

2KI + H₂ ==> 2K + 2HI ... balanced redox equation (with spectator iodide)