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Grade 11-12 (age 16-18)

high school Shift of Equilibrium

Le Chatelier's Principle

Add more reactant to a reaction at equilibrium, or change the pressure or temperature, and the equilibrium does not stay put; it moves to a new one. It moves in the direction that eases the change, which is Le Chatelier's principle: adding a reactant favors the forward reaction that uses it up, and raising the pressure favors the side with fewer gas molecules. Changing concentration or pressure leaves the equilibrium constant K unchanged at a given temperature; only a change in temperature alters K itself. A catalyst does not move the equilibrium; it only shortens the time to reach it. Changing the amount of reactant and the pressure in the diagrams shows which way the concentrations and molecule counts of the new equilibrium move.

Equilibrium Moves to Ease a Change

💡 Opening a Can of Soda
①At equilibrium the forward and reverse reactions are balanced against each other
②Change something from outside, and the system reacts in the direction that partly undoes the change until it finds a new equilibrium
③Adding a reactant favors the forward reaction that uses it; raising the pressure favors the side that reduces the number of gas molecules
④Opening a soda lowers the pressure, so the equilibrium shifts toward dissolved CO₂ escaping as gas, and the drink fizzes
⑤This tendency is summed up as Le Chatelier's principle

Changing a Concentration

0 mol/L
Concentration Change
[reactant] ↑ → shift toward the forward reaction → product ↑
The equilibrium shifts to use up a substance that was added or to replace one that was removed
🔍 Why It Shifts That Way
①At the first equilibrium [A] = [B] = 1 mol/L and K = 1
②Right after adding 0.5 mol/L of A, Q = [B] / [A] = 1 / 1.5 ≈ 0.67, smaller than K
③Since Q < K, the forward reaction turns some A into B until Q = K again at [A] = [B] = 1.25 mol/L
④Only half of the added 0.5 mol/L turns into B, so A is still higher than at the start: the shift reduces the change but does not cancel it
⑤Remove A and Q > K, so the reverse reaction runs

Changing the Pressure

2 atm
Effect of Pressure
P ↑ → shift toward the side with fewer gas molecules
Fewer molecules ease the rise in pressure; if both sides have the same number of gas molecules, nothing shifts
💡 Changing the Temperature
①Raising the temperature shifts the equilibrium toward the endothermic direction, which absorbs heat and eases the rise
②Lowering the temperature shifts it toward the exothermic direction
③Unlike changes in concentration or pressure, a temperature change alters the value of K itself
④The forward reaction of N₂O₄ ⇌ 2NO₂ is endothermic, so heating makes more brown NO₂ and the gas darkens

Catalysts and Equilibrium

🔍 Catalysts Do NOT Shift Equilibrium
①A catalyst speeds up the forward and reverse reactions by the same factor
②So it only shortens the time to reach equilibrium; the position of equilibrium and K stay the same
③Adding an unreactive gas such as He to a container of fixed volume raises the total pressure but leaves each gas's partial pressure, and so the equilibrium, unchanged

Worked Examples

Example 1
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g) at equilibrium, which way does the equilibrium shift when the pressure is raised?
1
Raising the pressure shifts the equilibrium toward fewer gas molecules.
2
The reactant side has 1 + 3 = 4 mol and the product side 2 mol, so it shifts toward the products, the forward reaction.
▸ Toward the forward reaction (more NH₃)
This is why the Haber process for making ammonia runs at high pressure. If both sides had the same number of gas molecules, changing the pressure would not move the equilibrium.
Example 2
For an equilibrium whose forward reaction is exothermic (ΔH < 0), which way does it shift when the temperature is raised, and what happens to K?
1
Raising the temperature shifts it toward absorbing heat, which is the endothermic reverse reaction.
2
At the new equilibrium there is less product and more reactant, so K becomes smaller. Temperature is the only condition that changes K.
▸ Shifts toward the reverse reaction; K decreases
For an exothermic reaction, heating lowers K and cooling raises it; for an endothermic reaction it is the other way round.

Summary

Le Chatelier's Principle
a change from outside → the equilibrium shifts to reduce that change
K stays the same for changes in concentration and pressure; only a temperature change alters K
CSAT-style problem, adapted
Which action does NOT shift the equilibrium position of N₂O₄(g) ⇌ 2NO₂(g)?
①Raise the temperature
②Reduce the volume to increase pressure
③Add more NO₂
④Add a catalyst
⑤Remove N₂O₄
▸ ④ Add a catalyst
1
A catalyst speeds up the forward and reverse reactions equally, only shortening the time to reach equilibrium.
2
The position of equilibrium and K do not change, so ④ does not shift the equilibrium; the others change temperature, pressure or concentration.
🎯 Exam Points
①Raising a concentration shifts the equilibrium toward using up that substance
②Raising the pressure shifts it toward fewer gas molecules
③Raising the temperature shifts it toward the endothermic side and changes K
④A catalyst does not move the equilibrium; it only shortens the time to reach it
⑤Adding an inert gas at constant volume leaves the equilibrium unchanged
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