Equilibrium Involving Chemical Systems

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In reversible reactions chemical reactions take place in both the forward and backward directions. For example, the reaction between gaseous hydrogen and iodine vapours to give gaseous hydrogen iodide is a reversible reaction and may be expressed as:

Chemical reactions in which the products formed do not combine to give back the reactants are known as irreversible reactions. For example, potassium chlorate decomposes on heating to form potassium chloride and oxygen:

In this reaction, the products cannot combine to give back potassium chlorate. In such irreversible reactions, the reactions occur only in the forward direction. These reactions go to completion and no starting materials are left.

However, reversible reactions do not go to completion. When these reactions are performed in closed containers, a stage is reached at which the reaction appears to have been stopped although some starting materials are left. This state is called chemical equilibrium. As in physical equilibrium, this state may be recognised, by the constancy of some measurable properties.

For example, when solid calcium carbonate is heated in a closed vessel at 1073 K, it decomposes into solid calcium oxide and gaseous carbon dioxide. Due to the production of CO₂, pressure will be developed in the vessel, which can be recorded with help of a manometer. The pressure goes on increasing as the decomposition of calcium carbonate continues. After some time, the pressure becomes constant even though some CaCO₃ is still present. This means that the amount of CO₂ produced has become constant. The constancy of pressure tells us that the equilibrium has been reached. This may be represented as:

Fig: 7.3 - Decomposition of calcium carbonate

Similarly, when hydrogen gas and iodine vapours are heated in a closed vessel at a constant temperature, 717K, the two react to form hydrogen iodide. In the beginning, the colour of the reaction mixture is deep violet due to the presence of iodine. But as the reaction proceeds, the intensity of the colour decreases as more and more iodine reacts with hydrogen to form hydrogen iodide. After some time, the intensity of the colour in the reaction mixture becomes constant even though some reactants are still present. This indicates that the concentration of both the reactants and products have become constant and a state of equilibrium has been reached.

Fig: 7.4 - Reaction between hydrogen and iodine

Thus, chemical equilibrium may be defined as the state in which the measurable properties of the system (such as pressure, density, colour or concentration) do not undergo any further noticeable change under given set of conditions is said to be a state of equilibrium.

When a reaction is carried in a closed vessel at a constant temperature.

At the start of the experiment, the reactants A and B are present in definite concentration. As the reaction proceeds, the effective collisions among the reactant molecules result in the formation of the products.

The forward reaction may be written as:

The concentration of the reactants decreases while those of products increases with the passage of time. Since the product molecules are not allowed to leave the vessel (reaction is carried out in the closed container), they also react back among themselves to give back reactants. The backward reaction may be written as:

As the reaction progresses, the concentration of the reactants decrease and therefore, the rate of the forward reaction falls off with time. On the other hand, the rate of backward reaction increases because of the increase in concentration of the products. The two rates approach each other and finally a state is achieved at which the two rates become equal and this is the equilibrium state. Thus, at equilibrium

Rate of forward reaction = Rate of backward reaction

Fig: 7.5 - Equilibrium in a reversible reaction