Page 171 - Chemical equilibria Volume 4
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Determination of the Values Associated with Reactions – Equilibrium Calculations 147
equilibria between neutral molecules or ions. All these types of equilibrium
may be encountered in the same system.
To calculate a system is to give, in specific conditions – i.e. of
temperature, pressure, concentrations of certain species, and initial
conditions – the nature and composition of the phases in equilibrium.
The calculation requires a clear definition of the system, and the result
obtained is valid only in the strict context of that definition.
4.10.1. Definition of the system
The system is defined by the exhaustive list of the compounds and phases
which the operator takes into consideration, and the choice of constraints
s/he imposes, such as the temperature, the pressure or the domains of
temperature, pressure, the concentrations or the initial quantities.
This definition is highly important, as no computation method can show
the existence of a compound in a specific phase which the operator has not
chosen in the definition list.
The conditions of evolution imposed on the system must be defined:
– isothermal or adiabatic;
– isobaric or isochoric (constant volume);
– closed or open system with a specific chemical potential (or partial
pressure) imposed for a component.
The definition of the system also specifies a list of the independent
reactions taken into account, including inter-phase reactions. It is not
necessary for those reactions to actually take place, because we are dealing
here with equilibrium calculations. Yet it must be borne in mind that such
equilibria impose specific relations between the molar Gibbs energies of
their components. Certain automated computation methods, which are
specialized for certain types of systems, reveal only those equilibria.
4.10.2. Output mode – graphical representation
The results of the calculation can be presented in one of two forms:
