Applications and conventions in chemical thermodynamics

To apply the preceding concepts of chemical thermodynamics to chemical reaction systems (and to understand how thermodynamic variables such as free energy vary with concentrations of species), we have to develop a formalism for the dependence of free energies and chemical potential on the number of particles in a system. We develop expressions for the change in Helmholtz and Gibbs free energies in chemical reactions based on the definition of A and G in terms of Q and Z. The quantities Q and Z are called the partition functions for the NVT and NPT systems, respectively. 

Consider the case where we have an open system consisting of a single protein molecule in solution. This system could consist, for example, of a biological molecule in a bath of water held at constant temperature. If this molecule adopts a number of conformational states, its NVT and NPT partition functions are the familiar quantities  

If our system consists of two identical and independent macromolecules, then the two-molecule canonical partition function is expressed 

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