Chemical Equilibria in Gases, Solids, and Solutions

The key thermodynamic concept for describing equilibria in chemical processes is the Gibbs free energy. The Gibbs free energy is defined as 

Fundamental Concepts in Heterogeneous Catalysis, First Edition. Jens K. N0rskov, Felix Studt, Frank Abild-Pedersen and Thomas Bligaard. 

The practical utilization of the concept of Gibbs free energy is therefore that equilibrium of the system requires that any possible (element-conserving) chemical reaction that the system could undertake must satisfy the relation 

The entropy per atom or molecule of an ideal gas or a dilute solution is given by expressions such as 

The pressure (p) and concentration (C) dependence basically stems from the fact that the entropy is proportional to the logarithm of the number of accessible states, Q, as 

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