225 Electrochemistry
A science of chemical changes and electric charges that is used to interconvert chemical energy and electrical energy.
Redox half-reactions. Many reactions move electric charge: electrons or ions pass from one substance to another. Oxidation is the electron-losing step. Reduction is the electron-gaining step. This principle is used to write a redox reaction as two half-reactions.
The separated-electrode cell. Electrochemistry separates those half-reactions so electrons travel through an external wire. This principle is used to turn a chemical change into a current, or a current into a chemical change.
The galvanic cell and electrical work. A galvanic cell turns a spontaneous reaction into electrical work. A galvanic cell is an electrochemical cell that produces current from a reaction that runs on its own. This principle is used to describe batteries.
The electrical work of a cell is
\[ w_{\mathrm{el}} = -nFE \]
where
- \(w_{\mathrm{el}}\) is the electrical work done on the system.
- \(n\) is the number of moles of electrons transferred.
- \(F\) is the Faraday constant.
- \(E\) is the cell potential.
The Nernst equation. The cell potential is fixed by the standard potential and by the composition. This principle is used to compute the voltage of a cell that is not in its standard state.
The Nernst equation is
\[ E = E^{\circ} - \dfrac{RT}{nF}\ln Q \]
where
- \(E\) is the cell potential.
- \(E^{\circ}\) is the standard cell potential.
- \(R\) is the gas constant.
- \(T\) is the absolute temperature.
- \(n\) is the number of moles of electrons transferred.
- \(F\) is the Faraday constant.
- \(Q\) is the reaction quotient.
The relation of \(E^{\circ}\) to \(\Delta_{r}G^{\circ}\). The standard cell potential is proportional to the standard Gibbs energy of the cell reaction. This principle is used to obtain equilibrium constants from measured voltages.
The standard Gibbs energy of the cell reaction is
\[ \Delta_{r}G^{\circ} = -nFE^{\circ} \]
where
- \(\Delta_{r}G^{\circ}\) is the standard Gibbs energy of reaction.
- \(n\) is the number of moles of electrons transferred.
- \(F\) is the Faraday constant.
- \(E^{\circ}\) is the standard cell potential.
The electrolytic cell. An electrolytic cell uses an outside current to force a reaction that would not run on its own. This principle is used to describe electrolysis and electroplating.