KEY2CHEM

Le Chatelier's Principle: Comparing $$Q$$ and $$K$$

The magnitudes of reaction quotient ($$Q$$) and equilibrium constant ($$K$$) can be compared to determine the net direction of a reaction. The comparison of $$Q$$ and $$K$$ can also be linked to Le Chatlelier’s Principle. For the reaction $$\require{mhchem}\ce{A(aq) + B(aq) <=> C(aq) + D(aq)}$$, adding additional $$A$$ shifts the equilibrium position to the right (toward products). The magnitude of $$Q$$ ($$Q = \frac{[C][D]}{[A][B]}$$) decreases when $$[A]$$increases, and when $$Q < K$$, the reaction is thermodynamically favorable to the right (toward formation of products).

Example 1.

The reaction $$\require{mhchem}\ce{A <=> B}$$ is at equilibrium. What is true immediately after the concentration of $$B$$ is increased?

A. $$Q = K$$

B. $$Q < K$$

C. $$Q > K$$

Solution

C. $$Q > K$$

When $$[B]$$ increases, $$Q > K$$ and the equilibrium position will shift to the left (toward reactant).

Example 2.

A stress to a chemical system shifts the equilibrium position to the right. Immediately before the shift, $$Q$$ ___ $$K$$.

A. $$<$$

B. $$=$$

C. $$>$$

Solution

A. $$<$$

When $$Q < K$$, the equilibrium position will shift to the right (is thermodynamically favorable toward product formation).

Example 3.

A chemical system at equilibrium is exposed to a stress. After the system re-equilibrates, which is correct?

A. $$Q = K$$

B. $$Q < K$$

C. $$Q > K$$

Solution

A. $$Q = K$$

At equilibrium, $$Q = K$$. When a system is disturbed from equilibrium, $$Q \neq K$$, and the equilibrium position adjusts to re-achieve equilibrium.