Question

Which of the following conditions will favor maximum formation of the product in the reaction?
\[{{\text{A}}_2}({\text{g}}) + {{\text{B}}_2}({\text{g}}) \rightleftharpoons {{\text{X}}_2}({\text{g}}){\text{ }}{\Delta _{\text{r}}}{\text{H}} = - {\text{X kJ}}\]
A.High temperature and high pressure
B.Low temperature and high pressure
C.High temperature and low pressure
D.Low temperature and low pressure

Answer 1

Hint: The above question is based on Le chatelier’s principle. For an exothermic reaction, low temperature is always favored for product formation. High temperature will increase the product when the total number of mole of product is lesser than total number of moles of reactant.

Complete step by step answer:
Le Chaterlier’s principle states that:
If a system in equilibrium is subjected to any change such as change in concentration, temperature or pressure then the change occurs in such a way as to oppose the change imposed and to restore the equilibrium.
The above equilibrium reaction is exothermic in nature as the value of enthalpy change is negative. This means heat is released when reactants react to form products or I can say that heat will be absorbed in the reverse direction. When products react to forms reactant then the process will be endothermic. When the temperature of this system is increased then according to le chatelier's principle if will oppose the change in temperature and hence reaction will proceed in that direction where the temperature decreases or heat is absorbed. That is reaction will proceed in the reverse or backward direction and concentration of product will decrease.
So to increase the concentration of product we must keep the temperature low.
Now we know pressure is directly proportional to the number of moles. Number of moles of reactant (2) is more than the number of mole of product (1), so the pressure of the reactant side is more than the pressure on the product side. When a reactant reacts to form a product the pressure of the system will decrease in the above reaction. If we increase the pressure of the system then the reaction will move in that direction where pressure will decrease. Hence the reaction will move forward and concentration of the product will be more.
High pressure favors the above reaction.

The correct option is B.

Note:
The same type of equilibrium that is given in question exists in formation of ammonia from nitrogen and hydrogen gas using Haber’s process. Also, high temperature and pressure favors more yield of ammonia.