24.2.8—How the value of an electrode potential, E

For a reduction half-cell Ox + ze⁻ ⇌ Red, increasing [Ox] generally makes reduction more favourable and increasing [Red] makes it less favourable, at fixed temperature and other standard conditions.

The sign and size of the change depend on the reaction quotient. Qualitative reasoning should identify the species in the numerator and denominator before predicting the direction.

Increasing [Cu²⁺] raises the reduction potential of Cu²⁺/Cu; increasing [Cu(s)] is not meaningful because a pure solid has effectively constant activity.

Do not treat every concentration increase as raising E, and do not include pure solids in the concentration ratio.