How does the effect of a catalyst on equilibrium position differ from its effect on reaction rate?

A catalyst has no effect on the equilibrium position because it speeds up the forward and reverse reactions equally. However, it significantly increases the reaction rate, allowing the system to reach equilibrium much faster.

When comparing pressure and concentration, which one can affect a system without changing the volume?

Concentration can be changed by adding or removing a substance at constant volume. Pressure changes in gaseous systems are typically achieved by changing the volume of the container, which simultaneously changes the concentrations of all gaseous components.

How does the response to a temperature increase differ between exothermic and endothermic reactions?

In an endothermic reaction, a temperature increase shifts the equilibrium to the right (products) to absorb heat. In an exothermic reaction, it shifts to the left (reactants) to remove the added heat.

What is the common error when predicting the effect of pressure on a reaction involving solids and gases?

Students often count the moles of all substances. Only the coefficients of gaseous species should be counted, as solids and liquids have negligible volume changes and do not contribute to pressure-based shifts.

Why is it incorrect to say that adding an inert gas at constant volume shifts the equilibrium?

Although the total pressure increases, the partial pressures (and thus concentrations) of the reacting gases remain unchanged. Since the collision frequency between reacting particles stays the same, the equilibrium position does not shift.

What mistake is made when assuming that 'shifting to the right' always means the reaction goes to completion?

A shift to the right only means the concentration of products increases relative to the reactants. The system remains in dynamic equilibrium, meaning some reactants will always remain present.

Define Le Chatelier's Principle.

It is a principle stating that if a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the system will shift its equilibrium position to counteract the effect of the disturbance.

What is meant by the 'position of equilibrium'?

It refers to the relative amounts of reactants and products present in the mixture at equilibrium. A position 'to the right' indicates a higher concentration of products than reactants.

What does a negative $\Delta H$ value indicate about a reaction's thermal behavior?

A negative $\Delta H$ indicates an exothermic reaction, where energy is released to the surroundings. In the context of equilibrium, heat can be treated as a product of the reaction.

Why does increasing the concentration of a reactant shift the equilibrium to the right?

Increasing reactant concentration increases the frequency of effective collisions for the forward reaction. This temporarily makes the forward rate higher than the reverse rate until a new equilibrium is established with more products.

Le Chatelier’s Principle | OCR AS-Level Chemistry

1. Definition & Core Concepts

Le Chatelier’s Principle states that if a system at dynamic equilibrium is subjected to a change in conditions, the position of the equilibrium will shift to minimize or counteract that change.
Dynamic Equilibrium occurs in reversible reactions when the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products.
The Position of Equilibrium refers to the relative proportions of reactants and products; a 'shift to the right' increases product yield, while a 'shift to the left' increases reactant concentration.

2. The Effect of Concentration Changes

Increasing Concentration: If the concentration of a reactant is increased, the system responds by consuming the excess reactant, shifting the equilibrium to the right (product side).
Decreasing Concentration: If a product is continuously removed from the system, the equilibrium shifts to the right to replace the lost product, which is a common technique used in industry to maximize yield.
Mechanism: These shifts occur because the change in concentration temporarily unbalances the reaction rates; the system adjusts until the rates of the forward and reverse reactions are equal once again.

Diagram showing a shift in equilibrium toward products when reactants are added.

3. Pressure and Volume in Gaseous Systems

Pressure Sensitivity: Changes in pressure only significantly affect equilibrium systems involving at least one gaseous component.
Increasing Pressure: When the total pressure is increased (or volume decreased), the equilibrium shifts toward the side with the fewer moles of gas to reduce the pressure.
Decreasing Pressure: If pressure is decreased, the system shifts toward the side with the greater number of gas moles to increase the pressure back toward its original state.
Equal Moles: If the number of moles of gas is identical on both sides of the equation (e.g., $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$ ), changing the pressure has no effect on the position of equilibrium.

4. Temperature and Enthalpy

Endothermic Reactions: In an endothermic reaction ( $\Delta H > 0$ ), heat acts like a reactant. Increasing the temperature shifts the equilibrium to the right to absorb the added thermal energy.
Exothermic Reactions: In an exothermic reaction ( $\Delta H < 0$ ), heat acts like a product. Increasing the temperature shifts the equilibrium to the left to remove the excess heat.
Equilibrium Constant ( $K$ ): Unlike concentration or pressure changes, temperature changes actually alter the value of the equilibrium constant itself, not just the position of the equilibrium.

5. The Role of Catalysts

No Shift in Position: A catalyst increases the rate of both the forward and reverse reactions equally, meaning it has no effect on the final position of equilibrium or the yield of products.
Time to Equilibrium: The primary function of a catalyst in an equilibrium system is to allow the system to reach its equilibrium state much faster than it would otherwise.
Industrial Importance: Catalysts are vital in industry because they allow reactions to occur at lower temperatures while still maintaining a viable production rate, saving energy costs.

6. Key Distinctions: Yield vs. Rate

It is critical to distinguish between the yield (how much product is formed at equilibrium) and the rate (how fast the reaction reaches that equilibrium).

Factor	Effect on Yield	Effect on Rate
Increase Temp	Depends on $\Delta H$	Always Increases
Increase Pressure	Depends on Gas Moles	Always Increases
Add Catalyst	No Change	Always Increases
Increase Conc.	Increases (if reactant)	Always Increases

For exothermic reactions, a 'compromise temperature' is often used: low enough to ensure a high yield, but high enough to maintain a commercially viable reaction rate.

7. Exam Strategy & Common Pitfalls

Check Physical States: Always verify the states of matter in the equation; solids ( $s$ ) and pure liquids ( $l$ ) are ignored when determining the effect of pressure changes.
Count the Moles: When analyzing pressure, sum the coefficients of gaseous species on each side separately. Do not include coefficients of non-gaseous species.
Identify Enthalpy Signs: Ensure you know whether the forward reaction is exothermic (negative $\Delta H$ ) or endothermic (positive $\Delta H$ ) before predicting temperature effects.
Inert Gases: Adding an inert gas at constant volume increases total pressure but does not change the partial pressures of the reactants/products, thus it does not shift the equilibrium.

Le Chatelier’s Principle

Summary

1. Definition & Core Concepts

2. The Effect of Concentration Changes

3. Pressure and Volume in Gaseous Systems

4. Temperature and Enthalpy

5. The Role of Catalysts

6. Key Distinctions: Yield vs. Rate

7. Exam Strategy & Common Pitfalls

Le Chatelier’s Principle

Summary

1. Definition & Core Concepts

2. The Effect of Concentration Changes

3. Pressure and Volume in Gaseous Systems

4. Temperature and Enthalpy

5. The Role of Catalysts

6. Key Distinctions: Yield vs. Rate

7. Exam Strategy & Common Pitfalls