Final answer:
Hydrogen ions are not present in equations for half-reactions in basic solution because they would immediately react with excess hydroxide ions to produce water, effectively not participating as reactants or products.
Step-by-step explanation:
Hydrogen ions do not appear in equations for half-reactions occurring in basic solution because in such solutions, the concentration of hydrogen ions is essentially zero. In a basic solution, hydroxide ions ([OH-]) are in excess, and any hydrogen ions produced would immediately react with these hydroxide ions to form water. Thus, they should not appear as reactants or products in such situations.
When balancing redox reactions in basic conditions using the half-reaction method, steps are taken to ensure that oxygen and hydrogen atoms are balanced by adding water molecules and hydroxide ions instead of hydrogen ions.