Shift of Equilibrium

Le Chatelier's Principle

Le Chatelier’s principle says that if concentration, pressure, or temperature is disturbed, equilibrium shifts to counteract that change. Adding reactant drives the forward reaction; raising pressure favors the side with fewer gas moles. Only temperature changes K itself, and a catalyst shortens the time to equilibrium without moving its position. Here you change concentration and pressure to see which way the equilibrium moves.

Nature Tries to Restore Balance

💡 Analogy: Seesaw Balance

①Equilibrium = a level seesaw

②Add weight on one side? → the other side rises

③Nature moves to 'offset' external changes

④Why a soda fizzes when opened: pressure↓ → CO₂ released

⑤This is 'Le Chatelier's principle'

Concentration Change and Shift

Reactant [A] change0

Concentration Effect

[reactant] ↑ → forward reaction → [product] ↑

Shifts toward the side consuming the increased species

🔍 Why Does It Shift That Way?

①[A] up → reaction quotient Q falls below K

②Q < K → forward reaction speeds up, products grow

③[A] down → Q > K → reverse reaction

④Key: reactions always move toward Q = K

Pressure Change and Shift

Pressure P (atm)2

Pressure Effect

P ↑ → shifts toward fewer moles of gas

Reduces moles to offset the pressure increase

💡 Effect of Temperature

①T ↑ → shifts in the endothermic direction (absorbs heat to lower T)

②T ↓ → shifts toward exothermic

③Key: temperature changes the value of K itself!

④Concentration/pressure changes leave K unchanged; only temperature changes K

Catalysts and Equilibrium

🔍 Catalysts Do NOT Shift Equilibrium

①Catalyst: speeds both forward and reverse equally

②Only shortens time to equilibrium

③K is unchanged — equilibrium position is unchanged

④Adding inert gas (constant volume): total P↑ but partial pressures unchanged → no shift

Worked Examples

Example 1

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g) at equilibrium, which way does it shift if pressure is increased?

Le Chatelier: pressure↑ → shifts toward fewer gas moles (to offset the increase).

Reactant side 4 mol (1+3) vs product side 2 mol → shifts toward the fewer-mole product side (forward).

▸ Forward (toward NH₃)

Pressure↑ → toward fewer total gas moles. If both sides have equal moles, pressure changes do not shift equilibrium.

Example 2

For an equilibrium whose forward reaction is exothermic (ΔH<0), what are the shift direction and the change in K when temperature is raised?

Temperature↑ → shifts toward the heat-absorbing (endothermic = reverse) direction.

Shifting reverse reduces products, so K decreases (only temperature changes K).

▸ Shifts reverse, K decreases

Only temperature changes K. For an exothermic reaction, temp↑ → reverse → K↓; temp↓ → forward → K↑.

Summary

Le Chatelier's Principle

External change → equilibrium shifts to offset it

System's response to changes in concentration, pressure, temperature

CSAT-style

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

A catalyst speeds up forward and reverse equally, only shortening the time to reach equilibrium.

The equilibrium position and K stay the same, so ④ does not shift it.

🎯 Exam Points

①Conc ↑ → shifts to consume that species

②Pressure ↑ → shifts to fewer gas moles

③Temp ↑ → shifts toward endothermic (K changes!)

④Catalyst: equilibrium position unchanged, only time shortened

⑤Inert gas (constant V): no shift

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Chemical Equilibrium

Reaction Rate

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