We have seen that all the observable properties of the system become constant at equilibrium. It may lead us to think that the reaction stops altogether at equilibrium and that equilibrium is static in nature. 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 the equilibrium state is a dynamic balance between the forward and the backward reaction. This can be illustrated by considering the reaction between hydrogen and iodine to form hydrogen iodide.
Fig. 21.2 Reaction between H2 and I2 in a closed vessel.
When hydrogen and iodine are taken in a closed vessel (Fig. 21.2) maintained at 717 K, hydrogen molecules combine with iodine molecules to form hydrogen iodide.
H2(g) + I2(g) à H2(g) + I2(g)
The molecules of HI formed, also begin to dissociate to form H2 and I2.
2HI(g) à H2(g) + I2(g)
As the reaction progresses, the concentrations of H2 and decrease and hence, rate of the forward reaction slows down. On the other hand, concentration of hydrogen iodide increases and therefore 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 rates of forward and backward reactions have been shown in Fig. 21.3. 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 or ward and backward reactions. Thus, the equilibrium is dynamic in 1Ullure.
Fig. 21.3. Attainment of equi11brium state
(A) Variation of concentrations with time.
(B) Variation of rate of forward and backward 1actions the time
The difference between static equilibrium and dynamic equilibrium can be understood by the following analogy:
Consider two children sitting on a see-saw. At the balance point no movement of the children or the see-saw occurs. This is analogous to static equilibrium.
Fig. 21.4.State equilibrium.
Now consider a child is ascending an escalator at the same rate as the escalator is descending. At this point, the child and the escalator are moving at the same rate in opposite directions and there is no change in the position of the child. This is analogous to dynamic equilibrium.
Fig. 24 .5. Dynamic equilibrium
The dynamic nature of chemical equilibrium can be demonstrated by the synthesis of ammonia by Haber’s process. Haber started with known concentrations of nitrogen and dihydrogen maintained at high temperature and pressure. He determined the concentrations of ammonia formed and, nitrogen and hydrogen left unreacted at regular intervals. He observed that after a certain time the composition of the reaction mixture became constant (Fig. 21.6) even though some of the reactants were still present. This constancy in composition indicates that the reaction has attained equilibrium.
Fig. 21.6. Variation of concentrations of N2, H2 and NH3 with time starting with 1 M N2 and 3 M H2 tor the reaction
In order to demonstrate the dynamic nature of equilibrium, synthesis of ammonia was carried out with exactly the same starting conditions of partial pressures and temperature but using D2 (deuterium) in place of The reaction was allowed to attain equilibrium. At equilibrium, the composition of the mixture was found to be same as in case of the reaction between N2 and H2the only difference being that D2 and ND3 were present in place of H2 and NH3. The two reaction mixtures, the one containing H2, N2 and NH3, and the other containing D2, N2 and ND3 were mixed together and left for some time. It was observed that the concentration of ammonia remains constant with the passage of time, indicating that the equilibrium remains undisturbed. However, when the reaction mixture was analyzed with the help of a mass spectrometer it was observed that all deuterium containing forms of ammonia (NH3, NH2D, NHD2 and ND3) and hydrogen (H2, HD and D2) were present. It indicated that the forward as well as backward reactions are continuing even at equilibrium and that the equilibrium is dynamic in nature. If the reaction had stopped at equilibrium then there would have been no mixing of isotopes in this way. The formation of products such as HD, NH2D, NHD2 demonstrated that the equilibrium is dynamic in nature.
OTHER CHARACTERISTICS OF CHEMICAL EQUILIBRIUM
It has already been discussed that chemical equilibrium is dynamic in nature. In addition to this, other characteristics of chemical equilibrium are:
1. The observable properties of the system such as pressure, colour, concentration, etc., become constant at equilibrium and remain unchanged thereafter. For example, for the reaction between hydrogen and iodine to form hydrogen iodide, the colour of the reaction mixture becomes constant at equilibrium.
2. The equilibrium can be approached from either direction. This can be proved with the help of a very simple experiment of the equilibrium between dinitrogen tetraoxide, N2O4 (colourless gas) and nitrogen dioxide, NO2 (a reddish brown gas).
It is observed that at ordinary temperature, say 298 K, two gases exist as equilibrium mixture having pale brown colour. At very low temperature (273 K), N2O4 is stable and exists as pure N2O4 (colourless gas). When temperature is increased, it decomposes to NO2. At 373 K it decomposes almost completely to NO2 which is dark brown in colour.
Let us take two identical bulbs A and B. Fill both of them with nitrogen dioxide and seal them. Place the bulb A in an ice bath and the bulb B in boiling water as shown in Fig. 21.7.
Fig.21.7. Study of the equilibrium
The gas in bulb A becomes almost colourless and consists of mostly N2O4 molecules. On the other hand, the gas in bulb B becomes reddish brown and consists of mainly NO2 molecules.
Now transfer both the bulbs into a bath maintained at 298 K and observe the change in colour. The colour of the gas in bulb A starts changing into brown colour indicating the gradual conversion of N2O4 into NO2
On the other hand, the reddish brown colour in bulb B begins to fade and becomes pale brown indicating the gradual formation of N2O4. This may be represented as:
After some time, when both the bulbs attain the temperature of the bath, the colour in the two bulbs becomes identical and no further change in colour occurs. This constancy of colour indicates that equilibrium has been attained in both cases and both contain mixture of NO2 and Np4 of the same composition.
This experiment clearly demonstrates that the equilibrium can be approached from either direction.
Similarly, let us consider the reaction between hydrogen and iodine to form hydrogen iodide.
Suppose 1 mole each of H2 and I2 is taken in a closed container and heated to 730 K. The forward reaction starts. As a result concentrations of H2 and I2 start decreasing while that of HI starts increasing. Finally, all the concentrations become constant and the reaction attains equilibrium. The colour of the reaction mixture becomes constant at this stage. Now take 2 moles of HI in the closed container of the same size and heat to 730 K. The backward reaction starts taking place. As a result, concentration of HI starts decreasing while that of H2 and I2 start increasing and fine any become constant when the equilibrium is attained.
Fig. 21.8. Chemical equilibrium can be attained from either direction.
If we compare the intensity of purple colour in the two cases we find that it is same. It indicates that same equilibrium is attained in both cases, in one case starting with H2 and I2 and in the other case starting with HI. It shows that equilibrium can be attained from either direction.
3. The equilibrium can be attained only if the system is closed. For the establishment of equilibrium, it is necessary that the system should be close one. If the system is not closed, the products may escape from the container and, therefore, backward reaction may not take place.
4. A catalyst does not alter the equilibrium point. In a reversible reaction, catalyst increases the rate of forward as well as backward reaction to the same extent. Therefore, equilibrium point is not altered. However, the equilibrium is attained earlier.