Question: 80. For the reaction 2 \mathrm{~A} \rightleftharpoons \mathrm{~B}+\mathrm{C}, \mathrm{K}_{\mathrm{c}}=4 \times 10^{-3}. At a given time, the composition of reaction mixture is: [\mathrm{A}]=[\mathrm{B}]=[\mathrm{C}]=2 \times 10^{-3} \mathrm{M}.
Then, which of the following is correct?
(1) Reaction is at equilibrium.
(2) Reaction has a tendency to go in forward direction.
(3) Reaction has a tendency to go in backward direction
(4) Reaction has gone to completion in forward direction.
Answer: Option (3)
Explanation:
For the reaction 2\mathrm{A} \rightleftharpoons \mathrm{B} + \mathrm{C},
the reaction quotient \mathrm{Q}_{\mathrm{c}} is given by
\mathrm{Q}_{\mathrm{c}}=\dfrac{[\mathrm{B}][\mathrm{C}]}{[\mathrm{A}]^{2}}Substituting the given concentrations,
\mathrm{Q}_{\mathrm{c}}=\dfrac{(2 \times 10^{-3})(2 \times 10^{-3})}{(2 \times 10^{-3})^{2}} \mathrm{Q}_{\mathrm{c}}=\dfrac{4 \times 10^{-6}}{4 \times 10^{-6}}=1The equilibrium constant is given as \mathrm{K}_{\mathrm{c}}=4 \times 10^{-3}.
Since \mathrm{Q}_{\mathrm{c}} > \mathrm{K}_{\mathrm{c}}, the reaction mixture contains more products than required at equilibrium.
Therefore, the reaction will shift in the backward direction to attain equilibrium.
Hence, the reaction has a tendency to go in backward direction.