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.