Chemical Equilibria / Chemical Systems

Chemical Components

Chemical components are the minimum number of constituents needed to describe the composition of a chemical system. For a system with Nspe species and Nreac independent reactions, the number of components is:

Ncomp = NspeNreac.

For aqueous solutions, H2O(l) is in excess, and it must be selected as a component. But, because its concentration (and activity) is approximately constant, water does not need to be included in the component list for the calculations.

Any species with fixed activity should be chosen as a component. For example: CO2(g) at fixed partial pressure; H+ if the pH is used as an axis variable; a solid phase present in large excess, etc.

Finally, species not capable of further dissociation are commonly selected as components: Ca2+, CO32−, PO43−, etc. The total concentration for these components is usually available.

Make sure that

are involved in each reaction!