What should be added to balance the oxygen in a redox half-reaction?

In a redox half-reaction, balancing oxygen atoms is crucial for ensuring that the mass and charge are conserved. When reviewing the options for balancing oxygen, adding water (H2O) is the method employed when there is a deficit of oxygen in the half-reaction.

Specifically, if there are fewer oxygen atoms on one side of the half-reaction compared to the other, adding water effectively increases the number of oxygen atoms. Each water molecule introduces one oxygen atom into the equation. Therefore, in reactions where oxygen is needed on the side with fewer oxygen atoms, water serves as the appropriate choice to achieve balance.

In contrast, other options do not serve the purpose effectively. For instance, adding H2 would not increase the number of oxygen atoms but would instead introduce hydrogen, complicating the balance. Likewise, OH- would only be applicable if the reaction was taking place in a basic medium and is used differently to balance oxygen and hydrogen when hydroxide ions are present. Finally, adding O2 directly would not work, as this would add additional oxygen molecules rather than balancing the existing oxygen already present in the half-reaction. Thus, using water is a strategic way to achieve the necessary balance in a redox half-reaction.