Chemical equilibrium is a dynamic state in which the forward and reverse reactions occur at equal rates. Although the composition remains constant at equilibrium, the system can still respond to changes in its surroundings.
This behavior is summarized by Le Chatelier's Principle:
When a system at equilibrium is stressed, it will respond in a manner that relieves the stress.
The stress may take many forms, including changes in concentration, pressure, volume, or temperature. The system then adjusts its composition until a new equilibrium state is established.
Le Chatelier's principle is not a separate law of chemistry. Rather, it is a convenient way of describing how systems move toward states that satisfy the thermodynamic conditions for equilibrium.
The effect of changing the concentration of a reactant or product can be understood using the reaction quotient, \(Q\).
For a general reaction
\[ aA+bB \rightleftharpoons cC+dD \]the reaction quotient is
\[ Q = \frac{[C]^c[D]^d} {[A]^a[B]^b} \]At equilibrium,
\[ Q = K \]where \(K\) is the equilibrium constant.
Suppose additional reactant \(A\) is added to the system. The denominator of the expression for \(Q\) increases, causing
\[ Q < K \]The system is no longer at equilibrium. To restore equilibrium, the reaction proceeds in the forward direction, consuming reactants and producing products until
\[ Q = K \]once again.
Similarly, adding a product increases the value of \(Q\), causing
\[ Q > K \]In response, the reaction shifts toward the reactants until the value of \(Q\) returns to the equilibrium value.
Thus, the "relief of the stress" described by Le Chatelier's principle is simply the system adjusting its composition until the reaction quotient again equals the equilibrium constant.
Consider the equilibrium
\[ N_2(g)+3H_2(g) \rightleftharpoons 2NH_3(g) \]Big picture: Le Chatelier's principle provides a simple way to predict how equilibrium systems respond to disturbances. For concentration changes, the response can be understood quantitatively using the reaction quotient: the system adjusts its composition until the condition \[ Q = K \] is restored.
Consider the reaction
\[ A+2B \rightleftharpoons 2C \]Initially,
\[ [A]=0.50\ \text{M} \qquad [B]=0.75\ \text{M} \qquad [C]=0 \]If
\[ K_c=1.2 \]determine the equilibrium concentrations. Then suppose that, after equilibrium is established, \(C\) is removed so that its concentration is reduced to half of its equilibrium value. Determine the new equilibrium concentrations.
| \(A\) | \(B\) | \(C\) | |
|---|---|---|---|
| Initial | 0.50 | 0.75 | 0 |
| Change | \(-x\) | \(-2x\) | \(+2x\) |
| Equilibrium | \(0.50-x\) | \(0.75-2x\) | \(2x\) |
Solving gives
\[ x=0.1477 \]Therefore,
\[ [A]=0.352\ \text{M} \] \[ [B]=0.455\ \text{M} \] \[ [C]=0.295\ \text{M} \]The concentration of \(C\) is reduced to half its equilibrium value:
\[ [C]=\frac{0.295}{2}=0.1477\ \text{M} \]Immediately after removal, the concentrations are
\[ [A]=0.352\ \text{M} \qquad [B]=0.455\ \text{M} \qquad [C]=0.1477\ \text{M} \]Since product has been removed, the reaction shifts toward products.
| \(A\) | \(B\) | \(C\) | |
|---|---|---|---|
| Initial after removal | 0.352 | 0.455 | 0.1477 |
| Change | \(-y\) | \(-2y\) | \(+2y\) |
| New equilibrium | \(0.352-y\) | \(0.455-2y\) | \(0.1477+2y\) |
Solving gives
\[ y=0.0404 \]Therefore, the new equilibrium concentrations are
\[ \boxed{[A]=0.312\ \text{M}} \] \[ \boxed{[B]=0.374\ \text{M}} \] \[ \boxed{[C]=0.229\ \text{M}} \]Physical interpretation: Removing \(C\) lowers the reaction quotient \(Q\), making \(Q < K\). The system responds by shifting toward products until \(Q=K\) is restored.
Consider the reaction
\[ A+2B \rightleftharpoons 2C \]The system is initially at equilibrium. A stress is then applied. Predict the direction of the shift, then use the reaction quotient to check your reasoning.
| Quantity | Value |
|---|---|
| \([A]_{\text{eq}}\) | |
| \([B]_{\text{eq}}\) | |
| \([C]_{\text{eq}}\) | |
| \(K_c\) | |
| Stress applied |
First prediction: Which direction will the reaction shift?
Big picture: Le Chatelier's principle is a simple way of predicting how equilibrium systems respond to disturbances. For concentration changes, the underlying thermodynamic explanation is that a stress changes the value of \(Q\), and the system responds by shifting its composition until the equilibrium condition \(Q = K\) is restored.