Question

Why are half-reactions written to describe redox reactions?

Answer 1

Redox reactions have a lot going on at one time with electrons, H and OH and H2O being added to the left and right. Sometimes it is much easier to balance those, especially complicated ones, by separating them into oxidation half and reduction half and balancing each with a set of rules that exist. I can give you a link to those rules if you need them.