This Nernst equation calculator presents the fundamental formula of electrochemistry - the Nernst equation, also known as the cell potential equation. It is a formula that allows you to calculate the reduction potential of a half-cell or full cell reaction. If you don't know what is the reduction potential, don't worry - we will explain all terms, describe in detail how to calculate the cell potential, and finish with a Nernst equation example that best presents its application.
Reduction potential of a half-cell or full cell reaction is also called the redox potential or oxidation/reduction potential. It measures the tendency of molecules (or atoms, ions etc.) to acquire electrons and hence be reduced. This value is measured in volts (V) - the same units that are used by our Ohm's law calculator.
Why exactly is it called oxidation/reduction potential? Oxidation occurs when electrons are removed - for example when a free radical steals an electron from a cell. Reduction, on the other hand, means receiving or gaining electrons, for instance when an antioxidant donates an electron to a free radical.
What does it mean in terms of reduction potential? A solution with a higher potential will have a tendency to gain electrons (be reduced), and a solution with a lower potential - to lose electrons (be oxidized). Note that a high reduction potential doesn't mean that the reaction will occur - the reaction still requires some activation energy to be supplied.
It is difficult to measure the absolute potential of a solution. That's why the reduction potentials are usually defined relative to a reference electrode.
The standard reduction potential is the redox potential measured under standard conditions: 25°C, activity equal to 1 per ion, and pressure of 1 bar per gas participating in the reaction. The standard reduction potential is defined relative to a standard hydrogen electrode(SHE), which is arbitrarily given a potential of 0 volts.
The Nernst equation (cell potential equation) relates the reduction potential to the standard electrode potential, temperature, and activities of molecules. Activities can be substituted by concentrations for an approximate result. For a half-cell or full cell reaction,
E = E₀ - RT/zF * ln([red]/[ox])
We will use the Nernst equation calculator to find the reduction potential of a cell basing on the following reactions:
Mg → Mg2+ + 2e-, where E₀ = +2.38 V
Pb2+ + 2e- → Pb, where E₀ = -0.13 V
Pb2+(aq) + Mg(s) → Mg2+(aq) + Pb(s)
E₀(cell) = 2.38 V + -0.13 V = 2.25 V
[red]/[ox] = [Mg2+]/[Pb2+] = 0.020/0.200 = 0.1.