high school 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. Change concentration and pressure and watch 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] shift (relative ticks −3~3)0

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

exam-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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