Non-Standard Conditions

The half equations are reversible reactions. The E^θ values depend on the position of equilibrium. Any factor that alters the position of equilibrium will alter the electrode potential in the direction predicted by Le Chatelier's principle. In the following equilibriumMnO₄^-(aq) + 8H⁺ + 5e^- ⇌ Mn²⁺(aq) + 4H₂O(l)The standard electrode potential E^θ is +1.51V. If the concentration of manganate (VII) ions or hydrogen ions is increased above 1.0 moldm^-3 the position of equilibrium will move to the right to counteract the change according to Le Chatelier's principle. Fewer electrons will be released and the electrode potential will become more positive than the standard value.A change in pressure will affect gases only. The position of equilibrium will shift to the side with the fewest moles. The standard electrode potential for the redox half-equationO₂(g) + 2 H₂O(l) + 4e^- ⇌ 4OH^- E^θ = +0.40VThis assumes the concentrations are at 1.0 moldm^-3 and the gases are at 1 atm of pressure (100kPa). Oxygen makes up about 20% of the air so the partial pressure of oxygen is 0.2 atm. The reduced pressure shifts the equilibrium to the left. The E^θ value will be less positive than +0.4V. A change in temperature will depend on whether the reaction is endothermic or exothermic.