Kncn h uaUb kb

where n is the number density of each molecule. The equilibrium constant is given by the thermodynamic expression

where R is the universal gas constant, and AG° the change in the molar Gibb's free energy of the system given by

with G° the free energy of formation of the species involved in the reaction in their standard state, usually at a pressure of 1 atm and room temperature of 298 K. If AG° is negative, K will be greater than unity and the forward reaction rate will be greater than the backward reaction rate, eqn (7.73).

The change in free energy is related to the change in enthalpy of the system, H, and the change in the entropy, S, according to

where the changes are evaluated from the enthalpy and entropy of formation for each reactant and product species, and from eqn (7.76) we have

Now since AH° and AS° are weakly temperature dependent, we have the van't Hoff equation dlnK AH °

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