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Factors Affecting Equilibrium & Le Chatelier's Principle | chemca

Chemical Equilibrium

Factors Affecting Equilibrium

Understanding Le Chatelier's Principle and the difference between shifting equilibrium and changing the constant.

By chemca Team • Updated Jan 2026

When a system at equilibrium is subjected to a change in conditions (concentration, pressure, or temperature), the system adjusts itself to counteract the change and establish a new equilibrium. This qualitative rule is known as Le Chatelier's Principle.

Crucial Distinction: Most factors (Concentration, Pressure, Volume, Catalyst) shift the position of equilibrium (forward/backward) but do NOT change the numerical value of the Equilibrium Constant ($K$).

Temperature is the ONLY factor that changes the value of $K$.

1. Effect of Concentration

Rule

If the concentration of a substance is increased, the equilibrium shifts in the direction that consumes that substance.

Add Reactant: Shift Forward $\rightarrow$ (To consume reactant).
Remove Product: Shift Forward $\rightarrow$ (To replace product).
Add Product: Shift Backward $\leftarrow$.

2. Effect of Pressure & Volume

This applies only to gaseous reactions where the number of moles changes ($\Delta n_g \neq 0$). Remember: $P \propto 1/V$.

Rule

Increase Pressure (Decrease Volume): Equilibrium shifts towards the side with fewer moles of gas.

Decrease Pressure (Increase Volume): Equilibrium shifts towards the side with more moles of gas.

Example: $N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$
Reactants: 4 moles | Products: 2 moles
High Pressure shifts this forward (towards fewer moles).

3. Effect of Temperature

This depends on the enthalpy of reaction ($\Delta H$).

Exothermic ($\Delta H < 0$)

Heat is a product: $A \rightleftharpoons B + \text{Heat}$

Increase T: Shift Backward $\leftarrow$ ($K$ decreases).
Decrease T: Shift Forward $\rightarrow$ ($K$ increases).

Endothermic ($\Delta H > 0$)

Heat is a reactant: $A + \text{Heat} \rightleftharpoons B$

Increase T: Shift Forward $\rightarrow$ ($K$ increases).
Decrease T: Shift Backward $\leftarrow$ ($K$ decreases).

4. Effect of Catalyst

Rule

A catalyst increases the rate of both forward and backward reactions equally.

It does NOT shift the equilibrium position.
It does NOT change the value of $K$.
It only helps the system attain equilibrium faster.

5. Effect of Inert Gas Addition

At Constant Volume

Total pressure increases, but partial pressures of reactants/products remain unchanged.

Result: No Effect.

At Constant Pressure

Volume increases to maintain pressure, effectively diluting the mixture.

Result: Shifts towards more moles of gas.

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