E05 Reactants and products present at equilibrium

Aim: To show that reactants and products are present in the equilibrium mixture

Accurate observation of the two reaction mixtures depictedon illustration E03 would show that after a time no further change in colour could be detected. Simple qualitative tests for Fe3+, I-, Fe2+ and iodine would show that all these species are present in the mixtures. These tests are summarized in the table below and depicted in illustration E05 in which the actual colours of the reagents and the colour changes obtained are shown.


  Using these tests it can be shown that both the reactants and products are present in the equilibrium mixture. As no further colour change is observed, it can be assumed that the reaction does not go to completion and hence that the concentrations of the species present will not change. Thus reactants and products are present in the equilibrium mixture whether the starting mixture contains reactants or products!

To exclude the possible presence in the reactants of traces of the products or vice versa, blank experiments using the same tests are necessary on solutions of FeCl3, FeCl2, KI and iodine.

Reactions in which, under the same conditions of temperature and pressure, equilibrium mixtures are obtained starting from reactants or products, are known as reversible reactions.

Reversibility is a characteristic of equilibrium reactions.
Whether or not specific reactions are reversible, depends largely on the thermodynamic characteristics of the reaction. Thus the formation of water from hydrogen and oxygen is an irreversible reaction, the decomposition of water, e.g. by electrolysis, requiring the constant supply of energy. Furthermore, some reactions are irreversible at one temperature and reversible at other temperatures.