When a reversible reaction is carried out in a closed vessel it does not go to completion. In a reversible reaction both forward as well as backward reaction are feasible, therefore, when such a reaction is carried in a closed vessel then both the reactions occur simultaneously and before whole of the reactants are converted into products a state of equilibrium is established. In the state of equilibrium, the reaction appears to be stopped even though the reactants are still present. Therefore, reversible reactions do not proceed to completion. On the other hand, irreversible reactions are feasible only in one direction and hence, go to completion.
Let us illustrate the state of equilibrium in some reversible processes and see how it is recognised.
DECOMPOSITION OF CALCIUM CARBONATE
Calcium carbonate when heated to 800°C in a closed evacuated vessel, starts decomposing to yield calcium oxide and carbon dioxide gas. Carbon dioxide builds up pressure within the vessel which can be recorded on a manometer, as shown in Fig. 21.1. The pressure goes on increasing as the reaction proceeds and finally becomes constant and remains so as long as the temperature remains constant. It appears as if the reaction has come to a stop although CaCO3 is still present. This indicates that the system has attained the equilibrium state.
Fig. 21.1. Attainment of equilibrium in the thermal decomposition of CaCO3
REACTION BETWEEN HYDROGEN AND IODINE
When hydrogen and iodine are heated in a closed vessel at 444oc, they react to form hydrogen iodide. The colour of the reaction mixture is deep violet in the beginning due to the presence of iodine. As the reaction progresses the intensity of colour decreases due to the formation of hydrogen iodide which is colourless.
After some time, it is observed that intensity of colour becomes constant indicating that the reaction has stopped although hydrogen and iodine are still present. Actually, the system acquires a state of equilibrium.
The attainment of equilibrium in a reaction is recognised by constancy of certain observable properties. For example, in the case of dissociation of calcium carbonate, we recognised the attainment of equilibrium by observing that pressure of carbon dioxide had become constant. In this case, attainment of equilibrium has been recognised by observing constancy of pressure. In case of reaction between hydrogen and iodine the attainment of equilibrium is recognised by observing constancy of intensity of colour of the reaction mixture.
Sometimes, concentration of one of the reactants or products is measured to recognise the state of equilibrium. Thus, state of equilibrium is the state in which the measurable properties of the system do not undergo any noticeable change under a particular set of conditions.