Question

Chemical equilibrium has an important role to play in a number of industrial processes. The aim of every industry is to get maximum yield of the product in minimum time. Le Chatelier principle helps to know the suitable conditions to achieve the goal. 

Now answer the following questions: 

(i) Lime is prepared from limestone by heating in lime kilns that are well ventilated. Why? 

(ii) How does Le Chatelier principle help the industry to know the suitable conditions for the manufacture of ammonia, a large amount of which is needed for making fertilizers? 

(iii) Methanol is manufactured from carbon monoxide and hydrogen. Predict the most suitable conditions for its manufacture. 

Answer 1

(i) The reaction involved is

ΔH= +178 kJ mol^-1 

Well ventilated kiln is used so that CO₂ is able to escape out and forward reaction is favored. Further, as the reaction is endothermic, therefore, formation of CaO is favored by high temperature (at least 900°C).

(ii) The reaction involved is 

 ΔH= -92 kJ mol^-1 

As reaction is accompanied by decrease in the number of moles (from 4 to 2), high pressure would favor formation of ammonia. But very high pressure means more dangerous and more costly engineering. Hence, an optimum pressure of about 200 atm is used. 

Further, as reaction is exothermic, low temperature will favor the formation of NH₃. But too low temperature makes the reaction very slow. Hence an optimum temperature of 750 K is employed because it is better to have lower yield than wasting too much time unnecessarily. Further, to speed up the reaction, finely divided iron is added as catalyst with molybdenum as promoter.

iii. The reaction involved is

CO(g) + 2H₂(g) ⇄ CH₃OH(g) , ΔH= -90 kJ mol^-1 

Theoretically, the reaction is favored by high pressure and low temperature. In practice, a high pressure is used to give an acceptable yield but a moderate temperature along with a catalyst is employed to get an economic production rate. 

These conditions are: 250°C, Cu-ZnO-Al₂O₃ as catalyst and a pressure of 50-100 atm.