a) In which of the following reversible reactions does the change in pressure not influence equilibrium? What is the reason?

i) H₂(g) + I₂ (g)⇋ 2HI(g)

ii) N₂(g) + 3H₂(g) ⇋ 2NH₃(g)

b) What is the use of applying high pressure during formation of ammonia from nitrogen and hydrogen?

a) (i) H₂(g) + I₂(g) ⇋ 2HI(g)

Change in pressure means change in the number of molecules. If we increase the pressure then number of molecules per unit volume increases and the number of effective collisions also increases. So, to decrease this pressure, the reaction will proceed in that direction to attain equilibrium where the number of molecules are less. But in this reaction the number of reactant and product molecules are same so there will no change in equilibrium state. So, the change in pressure will not influence equilibrium.

b) In gaseous system, decrease in the number of molecules helps to decrease the pressure. According to Le Chatelier principle, If we increase the pressure then the number of molecules per unit volume increases which results increment in the number of effective collisions. So, to decrease the pressure, reaction will decrease reactant molecules and the reaction will proceed in that direction to attain equilibrium where the number of molecules is less. So, the above reaction will proceed in forward direction. So, in the above reaction, the reactant molecules will react and form product very easily because there are less molecules at product side. Hence the formation of ammonia will increase.