Dynamic nature of chemical equilibrium and its characteristics

The state of equilibrium is a state in which the measurable properties of the system do not undergo any noticeable change under a particular set of conditions.

We have seen that all observable properties of a system become constant at equilibrium. It may lead us to think that the reaction stops altogether at equilibrium. But this is not true. Actually at equilibrium the rate of forward reaction becomes equal to the rate of backward reaction so that there is no net change in the concentration of various species. In other words we can say that equilibrium state is a dynamic balance between the forward and backward reaction. This can be illustrated by considering the reaction between hydrogen and iodine to form hydrogen iodide.

hydrogen iodide formed

When hydrogen and iodine are taken in a closed vessel maintained at 717k hydrogen molecules combine with iodine to form hydrogen iodide.

The molecules of HI formed also begin to dissociate to form H2 and I2.

reaction rate time graph

As the reaction progresses, the concentration of H2 and I2 decrease and hence rate of forward reaction slows down. On the other hand, concentration of hydrogen iodide increases and therefore the rate of backward reaction increases. A stage is reached when the rate of backward reaction becomes equal to the rate of forward reaction and the system attains equilibrium. The variation of the rate of forward reaction and backward reaction are shown in the graph. After this no change in concentration occurs provided the temperature of the reaction mixture is kept constant. Thus at equilibrium the reaction does not stop but the system acquires constant observable properties because of the equal rates of forward and backward reactions. Thus the equilibrium is dynamic in nature. The above reaction in equilibrium may be represented by the following reversible equation:

The dynamic nature of chemical equilibrium can be demonstrated by adding small amount of radioactive iodine (I2*) to the above reaction in the state of equilibrium. It is noticed after some time that a mixture contains radioactive hydrogen iodide (HI*). It indicates that the reaction is going on even at equilibrium.

* indicates that the sample is radioactive.

The equilibrium achieved in physical processes such as dissolution of salt, evaporation of water etc. is called physical equilibrium whereas the equilibrium achieved in chemical processes such as reaction between H2 and I2 is called chemical equilibrium.

Main Characteristics of Chemical Equilibrium

  • The equilibrium can be attained only if the system is closed.
  • A catalyst does not alter the equilibrium point.
  • The equilibrium can be approached from either direction.
  • The observable properties such as concentration, density of colour of the system become constant at equilibrium and remain unchanged thereafter.

Dissociation of Electrolytes

Sodium chloride an ionic compound has the sodium ion, Na+ and chloride ion Cl- held together by strong electrostatic force of attraction in the solid state. When dissolved in water, the ions become free to move.

The process of breaking of NaCl into free Na+ and Cl- ions is dissociation. NaCl is completely dissociated into Na+ and Cl- and is called a strong electrolyte.

Similarly a polar covalent compound like HCl is also dissociated into H+ and Cl- ions when added to water. Thus dilute hydrochloric acid too is a strong electrolyte.

Substances like CH3COOH which when added to water, dissociate partially as shown below:

In this case, all the molecules of CH3COOH do not dissociate into H3O+ and CH3COO- ions. There is equilibrium between the ions formed and the undissociated molecules. Such substances are called weak electrolytes. Other weak electrolytes are NH4OH, oxalic acid etc.

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