Question

For the gas phase reaction, $C_2 H_4 + H_2 \rightleftharpoons C_2 H_6$ \hspace15mm $( \triangle H = - 32.7 \, kcal )$ carried out in a vessel, the equilibrium concentration of $C_2H_4$ can be increased by

• A. increasing the temperature
• B. decreasing the pressure
• C. removing some $H_2$
• D. adding some $C_2 H_6$

Answer 1

Answer: (D)

adding some $C_2 H_6$

Answer 2

$C_2 H_4 + H_2 \rightleftharpoons C_2 H_6, \, \triangle H = - 32.7$ kcal
The above reaction is exothermic, increasing temperature will favour backward reaction, will increase the amount of $C_2H_4$ .
Decreasing pressure will favour reaction in direction containing more molecules (reactant side in the present case). Therefore, decreasing pressure will increase amount of $C_2 H_4$ .
Removing $H_2$, which is a reactant, will favour reaction in backward direction, more $C_2 H_4$ will be formed.
Adding $C_2 H_6$ will favour backward reaction and some of the $C_2 H_6$ will be dehydrogenated to $C_2 H_4$ .