Nernst Equation

in one direction. This is especially true in direct current (DC) situations, in which electrons go from one place to another in an electrochemical cell.

According to Faraday’s law of electrolysis, the amount of substance made at each electrode in an electrochemical cell is directly proportional to the quantity of charge that flows through the cell. Because substances have different oxidation/reduction number increases or decreases in terms of the electrons per atom or ion that are gained or lost, the amount of substances produced will not be produced in the same molar amounts. When these ratios are factored into the equation, the law will be correct in all situations.

NERNST EQUATION

As you have determined, the energy of a chemical system is what drives the electrons to move such that the driving force gives rise to the cell potential of the galvanic cell. This energy is also highly related to the chemical equilibrium of the reaction. The Nernst equation helps tie this all together.

Remember that energy can take on many forms. It can be mechanical, radiation (photons), chemical energy, nuclear energy, thermal energy, and electrical energy. These can change back and forth so that, as you have seen, chemical energy can be used to create electrical energy. It is not always a perfect transformation, which is why batteries will heat up when the chemical reaction is going to make electrical energy.

According to the Nernst equation, we must assume that the energy of the cell is equal to the potential energy of reduction minus the potential energy of oxidation. In order to drive the reaction forward, the change in energy must be a negative number.

The change in energy or delta G (which means Gibbs free energy) is equal to the negative product of the number of electrons transferred, Faraday’s constant (which is 96,500 calories per mole), and the potential difference across the cell. Figure 63 shows this equation:

Figure 63.

The general Nernst equation relates to the concentrations of the different reactants and end products. It also depends on the temperature in Kelvin. According to the standard conditions of 25 degrees Celsius (298 degrees kelvin), the Nernst equation becomes what is depicted in figure 64:

Figure 64.

Interestingly, the Nernst equation indicates that a battery can be built by simply using the same reactants and products but having a higher concentration in one side of the cell and a lower concentration of reactants and products on the opposite side of the cell. This is called a “concentration cell”. In the equation, it predicts whether or not a reaction will or will not happen, based primarily on the change in potential energy over the course of the reaction.