Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: To determine the overall voltage of a particular voltaic cell, simply combine the voltages of the oxidation and reduction half reactions. Even if you need to take a multiple of a half reaction for the electrons to cancel, do not take the multiple of the E1/2. Use the values directly as is from - ball-ch14_s03_t01. Spontaneous redox reactions have positive overall voltages. If the voltage of the reaction as written is negative, it is not spontaneous in that direction. Rather, the reverse reaction is the spontaneous redox reaction. Answer: voltage
Combining the voltages of the oxidation and reduction half reactions helps to determine what?