A phenomenon or action that can take place in two directions. It is applied to mechanical, thermal, chemical, and biological processes. For example, the Carnot cycle is a reversible thermodynamic cycle. A reversible process is a process in which the process variables that define the equilibrium thermodynamics can be made to change in such a way that the process can be made to return to its original thermodynamic equilibrium. All natural processes are irreversible. A reversible reaction is a chemical or physical phenomenon that can take place in two directions. Strictly, all chemical reactions are reversible. For many chemical reactions, the equilibrium conversion is effectively complete such as the combustion of fuel and therefore considered to be irreversible. For example, the reversible reaction:
(p. 328) where k1 and k2 are the forwards and reverse reaction constants. The rate equation for component A assuming a constant volume reactor is therefore:
At equilibrium:
such that the equilibrium constant, K, for the reaction is: