text: All equilibrium constants vary with temperature according to the van 't Hoff equation
   is the gas constant and  is the absolute temperature. Thus, for exothermic reactions, the standard enthalpy change, , is negative and K decreases with temperature. For endothermic reactions,  is positive and K increases with temperature.
  The standard enthalpy change for a reaction is itself a function of temperature, according to Kirchhoff's law of thermochemistry:
  where  is the heat capacity change at constant pressure. In practice  may be taken to be constant over a small temperature range.