Temperature Dependence
The van 't Hoff equation.
shows that when the reaction is exothermic (ΔH is negative), then K decreases with increasing temperature, in accordance with Le Chatelier's principle. It permits calculation of the reaction equilibrium constant at temperature T2 if the reaction constant at T1 is known and the standard reaction enthalpy can be assumed to be independent of temperature even though each standard enthalpy change is defined at a different temperature. However, this assumption is valid only for small temperature differences T2 − T1. In fact standard thermodynamic arguments can be used to show that
where Cp is the heat capacity at constant pressure. The equilibrium constant is related to the standard Gibbs energy change of reaction as
where ΔG is the standard Gibbs free energy change of reaction, R is the gas constant, and T the absolute temperature.
If the equilibrium constant has been determined and the standard reaction enthalpy has also been determined, by calorimetry, for example, this equation allows the standard entropy change for the reaction to be derived.
Read more about this topic: Keq
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