Question

Why can't we have half-reactions by themselves?

Answer 1

Half reactions are said to have cell potentials that... well, are potential differences. You can't have a difference between `x` and nothing else. You need a `y`.

So, each half-cell potential was itself measured with respect to either:

• the Standard Hydrogen Electrode (SHE),
• the Saturated Calomel Electrode (SCE),

or some other reference electrode. The SHE is quite common, and it was declared to have an arbitrary absolute cell potential of `"0 V"`; it is easier to be in reference to a voltage of `"0 V"` than an absolute cell potential of `"4.44 V"`.