7.9.A.1 Le Châtelier’s Principle: Predicting System Response to Stresses:

1. Introduction to Le Châtelier’s Principle:

Le Châtelier’s Principle is that if a system in equilibrium is disrupted by changes in pressure, temperature, or concentration, the system will shift to oppose the change and restore equilibrium.

– Concentration: Adding a reactant or product shifts the equilibrium to consume it.
– Temperature: Increasing temperature shifts the equilibrium towards the endothermic side; decreasing the temperature favors the exothermic side.
– Pressure (for gases): Adding pressure shifts the equilibrium to the side with fewer gas molecules; taking away pressure favors the side with more gas molecules.

This principle is applied to predict how systems respond to external changes.

2. Chemical Equilibrium:

Chemical Equilibrium is a state in a reversible reaction where the rate of forward reaction is equal to the rate of backward reaction and the concentration of reactants and products remains constant over time.

i. Dynamic Equilibrium:
– Dynamic means the reactions are still occurring, but there is no net change in the concentration of reactants or products.
– Both forward and reverse reactions occur but at an equal rate, so the proportion of the product to reactants remains constant.

ii. Equilibrium Constant (K):
The equilibrium constant (K) is the ratio of concentration of products to reactants at equilibrium, each raised to their coefficients in the balanced chemical equation.

For a general reaction:
aA + bB ⇌ cC + dD

The equilibrium constant (K) is:

$$K=\frac{[C{]}^c[D{]}^d}{[A{]}^a[B{]}^b}$$

– If (K > 1), products are favored at equilibrium.
– If (K < 1), reactants are favored at equilibrium.
– If (K = 1), the reactant concentration and product concentration are roughly equal.

3. Types of Stresses:

i. Addition or Removal of Chemical Species:
– Add reactant: Shifts equilibrium toward products.
– Add product: Shifts equilibrium toward reactants.
– Remove reactant: Shifts equilibrium toward reactants.
– Remove product: Shifts equilibrium toward products.

ii. Change in Temperature:
– Increase temperature: Shifts equilibrium toward the endothermic direction.
– Lower temperature: Shifted reversible reaction to opposite of increasing temperature.

iii. Volume/Pressure change (only for gases):
– Increased pressure: The equilibrium is pushed in the direction of fewer gas molecules.
– Decreased pressure: The equilibrium shifted towards greater molecules of gases.

iv. Reaction system dilution:
– Dilution: Adjusts the equilibrium to gain increased concentration of a reactant or product as per the reaction.

These changes serve to restore balance as per Le Châtelier’s Principle.

4. Applications of Le Châtelier’s Principle:

i. Industrial Processes:
– Haber Process: Pressure is raised to shift equilibrium towards product for the production of ammonia (NH3), and temperature is regulated to optimize reaction rate and yield.

ii. Biological Systems:
– Oxygen Transport: Hemoglobin will bind oxygen under high-oxygen conditions (in lungs) and release it under low-oxygen conditions (in tissues).
– Blood Buffering: Balance between bicarbonate and carbonic acid gives feedback to maintain the blood’s pH stabilized according to the CO2 difference.

5. Mathematical Analysis:

i. Set up ICE table:
– Initial (I): Initial concentrations.
– Change (C): Changes as the reaction occurs.
– Equilibrium (E): Equilibrium concentrations.

ii. Example:
N2+3H2⇌2NH3

$$\begin{array}{|c|c|c|c|} \hline & N_2 & H_2 & NH_3 \\ \hline \text{Initial} & 1.00 & 3.00 & 0 \\ \hline \text{Change} & -x & -3x & +2x \\ \hline \text{Equilibrium} & 1.00 – x & 3.00 – 3x & 2x \\ \hline \end{array}$$

iii. Solve for (x) using the equilibrium constant (K).

iv. Shifts:
– Add reactants or remove products: Shift right.
– Add products or remove reactants: Shift left.

6. Limitations:

i. Non-reversible reactions: Only holds for reversible reactions.
ii. Extreme conditions: Does not hold under very high or low temperatures.
iii. Non-ideal systems: Deviations can exist in real-world situations.
iv. Large changes: Very large concentration/pressure changes can lead reactions to completion.
v. Kinetics: Does not consider slow rates of reaction.

The principle is best applied for reversible reactions at moderate conditions.