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5. Chemical potential, simple mixtures, chemical reactions andequilibria (ii) Mixtures
The GibbsDuhem equation Since the total free energy of a system is give by: and hence by differentiation we get: But, as discussed in the previous section, for a system at constant temperature and pressure we have: and hence since G is a state function, then the two expressions must be equal, which means that:………………….(GibbsDuhem equation)The significance of the GibbsDuhem equation is that the chemical potential of a component in a mixture cannot change independently of the chemical potential of the other components. For example, in a binary mixture we have: i.e., if one chemical potential increases, the other must decrease. Note: There is also the a similar 'GibbsDuhem equation for the partial molar volumes' which is given by: where the partial molar volume, defined as: which signifies the volume of component J in a mixture. 
