text: The standard pressure, , is used to define the reference state for the Van 't Hoff equation, which is 
  where  denotes the natural logarithm,  is the thermodynamic equilibrium constant, and  is the ideal gas constant. This equation is exact at any one temperature and all pressures, derived from the requirement that the Gibbs free energy of reaction be stationary in a state of chemical equilibrium. 
  In practice, the equation is often integrated between two temperatures under the assumption that the standard reaction enthalpy  is constant (and furthermore, this is also often assumed to be equal to its value at standard temperature). Since in reality  and the standard reaction entropy  do vary with temperature for most processes, the integrated equation is only approximate. Approximations are also made in practice to the activity coefficients within the equilibrium constant.
  A major use of the integrated equation is to estimate a new equilibrium constant at a new absolute temperature assuming a constant standard enthalpy change over the temperature range. To obtain the integrated equation, it is convenient to first rewrite the Van 't Hoff equation as
  The definite integral between temperatures  and  is then
  In this equation  is the equilibrium constant at absolute temperature , and   is the equilibrium constant at absolute temperature .