Question

To increase yield of HI in the following equilibrium,
\[{{H}_{2}}(g)+{{I}_{2}}(g)\overset{{}}{\leftrightarrows}2HI(g) \\
  \Delta {{n}_{g}}=0,\text{ }\Delta H=+ve \\ \]
Which factors are favorable?
(A) high T, low P
(B) low T, high P
(C) low T, no effect of P
(D) addition of ${{I}_{2}}$

Answer 1

Hint: To determine how to increase the yield of products, Le Chatelier's principle can be used. Le Chatelier's principle or the equilibrium law states that when a dynamic equilibrium is disturbed, the equilibrium will be restored by shifting the position of the equilibrium in an opposite direction.

Complete step-by-step answer:
We know, according to the Le Chatelier's principle, that when there is a disturbance in a dynamic equilibrium, like change in temperature, concentration or pressure, the equilibrium will push to counteract the change.
So, by manipulating the conditions like the temperature, concentration or pressure of a reaction, we can make the equilibrium favor the products, and hence increase the yield.
 In the question,
\[
{{H}_{2}}(g)+{{I}_{2}}(g)\overset{{}}{\leftrightarrows}2HI(g) \\
  \Delta {{n}_{g}}=0,\text{ }\Delta H=+ve \\
\]
We can see that the equilibrium is in the gaseous phase. Also, it is given to us that there is no change in the number of gaseous molecules $(\Delta {{n}_{g}}=0)$ and also that the equilibrium is endothermic in nature \[(\Delta H=+ve)\], which means absorption of heat occurs.
Now, let us see how change in different factors will affect the given equilibrium.
- Change in concentration: When there is an increase in the concentration of either of the reactants, ${{I}_{2\text{ }}}or\text{ }{{H}_{2}}$, the equilibrium will shift towards the products, and the yield of HI will increase.
- Change in temperature: Since the equilibrium is endothermic in nature, increase in temperature will shift the equilibrium forward and the yield of the product will increase.
- Change in pressure: Since there is no change in the number of gas molecules, change in pressure will have no effect on the equilibrium of the reaction.
So, to increase yield of HI in the given equilibrium, option (D) addition of ${{I}_{2}}$ will favor the equilibrium.

Note: It should be noted that when there is change in the conditions of the equilibrium, like phase state of the equilibrium, enthalpy of the equilibrium, or the change in number of molecules of reactants and products, the effect of change of temperature, pressure and concentration will also differ.