Question

Assertion: Addition of inert gases at equilibrium at constant pressure will support the dissociation of \[PCl_5\] at a constant temperature.
Reason: The addition of inert gas at constant volume will not affect the equilibrium.
A. Both assertion and Reason are correct and reason is the correct explanation for assertion.
B. Both assertion and reason are correct, but reason is not the correct explanation for assertion.
C. Assertion is correct, but reason is incorrect.
D. Assertion is incorrect but the reason is correct.

Answer 1

Hint: In order to understand this question, one should know about Le Chateller’s principle. According to Le Chatelier's principle. When any system at equilibrium is subjected to change in concentration, volume, pressure or temperature, then the system shifts towards ne equilibrium and shifting will take place in such a manner so as to minimise the effect of change.

Complete step by step solution:
The given reaction, \[PCl_5 \leftrightharpoons PCl_2+Cl_2\]
On addition of insert gas at equilibrium at constant pressure, moles will increase and thus volume will also increase, thus overall pressure of at equilibrium will remain constant.
Now as volume of container is increasing, pressure of reaction will tend to decrease (as pressures merely proportional to volume), thus in order to increase the pressure of reaction, it will also increase, thus overall pressure at equilibrium will remain constant.
Now as volume of container is increasing, pressure of reaction will tend to decrease (as pressure is inversely proportional to volume), thus in order to increase the pressure of reaction, it will shift in that direction where gaseous stoichiometry is more.
Now number of moles on reactant side \[=1\]
Number of moles on product side \[=2\]
Thus, in addition, inert gas at equilibrium at constant pressure will shift the reaction in forward direction.
And thus, it will support the dissociation of \[PCl_5\].
Therefore, the assertion statement is correct.
Now, in addition to inert gas at constant volume, the number of moles of reaction mixture will not change (as inert gas doesn’t react with other reactant molecules). Now since volume and number of moles are constant, pressure will also remain constant, and number change will take place in equilibrium. Thus, addition of inert as at constant volume will not affect the equilibrium.
Therefore, reason statement is correct
Hence, correct option is (b) as both assertion and reason are correct, but reason is not the correct explanation for assertion.
Note: On decreasing volume of container, pressure will increase and in order to decrease the pressure, reaction will shift in the direction having lesser gaseous stoichiometry.