Because all of the half-reactions in the table are written as reductions, it is necessary to reverse one (to make an oxidation) before combining them to give an overall balanced equation for an electrochemical process. When the half-reaction is reversed, the sign on the E^o value changes. The decision of which one to reverse can be made in a number of ways:

• spontaneous reactions result in a positive voltage
• the half-reaction containing the stronger reducing agent must be reversed

The second method only works if you can determine which is the stronger reducing agent. Our table is labeled with the relative strengths of the oxidizing and reducing agents. Most are not. However, the alkali metals are very strong reducing agents and the halogens are very strong oxidizing agents. Locating these on any table indicates not only the relative placements of the agents but also the arrangement of reactants for spontaneous processes in that particular table.