What are half-reactione?

Answer 1

Some chemical reactions require oxidation and reduction to occur. Sodium and Chlorine react ... the sodium atoms lose electrons and become sodium ions. That is one half of what is going on. That is a half-reaction. At the same time the chlorine atoms are absorbing those same electrons to become chloride ions. That is the other half of what is going on. That is also a half reaction.

Symbolically you can write these as 2 separate things:

Na --> Na+ + e-

Cl2 +2e- --> 2Cl-

notice because chlorine is diatomic, there are 2e- involved in that half reaction.

You should now reconcile this by balancing the number of electrons to be the same in both half reactions by multiplying the top half-reaction by 2.

2Na --> 2Na+ + 2e-

Cl2 +2e- --> 2Cl-