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ChemistryEdu Logo Chemical Equilibrium | Reaction Quotient#

Reaction Quotient (Qc)#

  • Reaction quotient helps in predicting the direction of reaction. It helps to decide whether the reaction is moving in forward direction or in backward direction.
  • Qc has a tendency to become equal to Kc.

Let us consider a chemical reaction:

\[n_1A + n_2B ⇌ n_3C + n_4D\]

At any time t, we can define reaction quotient as:

\[Q_c = {[C]^{n_3} [D]^{n4} \over [A]^{n_1} [B]^{n_2}}\]

Case 1: If Qc > Kc

Qc tends to become equal to Kc. For this, its value should decrease. Hence, the concentration of C and D will decrease and that of A and B will increase.

This indicates that reaction will shift in backward direction.

Case 2: If Qc < Kc

Qc tends to become equal to Kc. For this, its value should increase. Hence, the concentration of C and D will increase and that of A and B will decrease.

This indicates that reaction will shift in forward direction.

Case 3: If Qc = Kc

Reaction has already reached equilibrium.

Question#

At 1400 K, Kc = 2.5 x 10-3 for the reaction:
CH4(g) + 2H2Sg ⇌ CS2(g) + 4H2(g)
A 10 L reaction vessel at 1400 K contains 2 moles of CH4, 3 moles of CS2, 3 moles of H2 and 4 moles of H2S. Is the reaction mixture at equilibrium? If not, in which direction does the reaction proceed to reach equilibrium?

\[Q_c = {[CS_2][H_2]^4 \over [CH_4][H_2S]^2}\]
\[Q_c = {({3 \over 10}) \times ({3 \over 10})^4 \over ({2\over 5})\times ({4\over 10})^2} = 7.5 \times 10^{-2}\]

Here Qc > Kc. Hence, the reaction will proceed in backward direction to achieve equilibrium.