An example ammonia synthesis

It is convenient to indicate the progress of a chemical process with a variable called the advancement. The reaction equation N2 + 3 H2 2 NH3 for the synthesis of ammonia synthesis will serve to illustrate this concept. Let the system be a gaseous mixture of N2, H2, and NH3. 

If the system is open and the intensive properties remain uniform throughout the gas mixture, there are five independent variables. We can choose them to be T, p, and the amounts of the three substances. We can write the total differential of the enthalpy, for instance, as 

T/termodynam/cs andC/)9m/stry, second edition, version 3 2011 by Howard DeVoe. Latest version uuw.chem.umd.edu/thermobook 

The notation m stands for the set of amounts of all substances in the mixture, and the quanrities //n2 H2 and //nhs are partial molar enthalpies. For example, //n2 is defined 

If the system is closed, the amounts of the three substances can still change because of the reaction Ni + 3 Hi 2 NH3, and the number of independent variables is reduced from five to three. We can choose them ioh T, p, and a variable called advancement. 

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