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What is Le Chatelier's principle?

Class 11 ChemistryNCERT chapter: Equilibriumइसे हिंदी में पढ़ें

Exam-ready answer

Le Chatelier's principle states that a change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner as to reduce or counteract the effect of the change. It applies to all physical and chemical equilibria and predicts how concentration, pressure and temperature changes shift the equilibrium.

Explanation

At equilibrium the forward and backward reactions go on at equal rates. If we disturb it, the system adjusts. Concentration: adding a reactant or removing a product makes the reaction go forward to use up the added substance, until Qc again equals Kc. Removing a reactant or adding a product shifts it backward. This is why products such as ammonia are removed continuously in industry to keep the reaction moving forward.

Pressure matters only for reactions with gases where the number of gaseous moles changes. Increasing the pressure (by reducing the volume) shifts the equilibrium towards the side with fewer gaseous moles, because that reduces the pressure. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), 4 moles of gas become 2, so high pressure favours ammonia. For H₂(g) + I₂(g) ⇌ 2HI(g), moles are equal on both sides, so pressure has no effect.

Temperature is the only factor that changes the value of the equilibrium constant. For an exothermic reaction like ammonia synthesis, raising the temperature shifts the equilibrium backward and lowers K; for an endothermic reaction it shifts it forward. A catalyst speeds up the forward and backward reactions equally, so equilibrium is reached faster but its composition does not change. Adding an inert gas at constant volume also leaves the equilibrium unchanged.

Points to remember

  1. Le Chatelier's principle: a system at equilibrium shifts in the direction that counteracts the change imposed on it.
  2. Adding reactant or removing product shifts equilibrium forward; the reverse change shifts it backward.
  3. Increase in pressure shifts equilibrium towards fewer moles of gas; no effect if gaseous moles are equal (Δn_g = 0).
  4. Raising temperature favours the endothermic direction; temperature is the only factor that changes K.
  5. A catalyst does not shift equilibrium; it only helps reach it faster. Inert gas at constant volume has no effect.

Exam tip

Quote the principle in the textbook wording, then explain each factor with a balanced equation. For pressure questions, count the gaseous moles on each side and write Δn_g; for temperature, state whether the reaction is exothermic or endothermic first.

Questions students also ask

How does Le Chatelier's principle help in the Haber process?
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is exothermic and reduces the number of gas moles. So high pressure and removal of ammonia push it forward. Low temperature would favour ammonia, but the rate becomes too slow, so a moderate temperature with an iron catalyst is used as a compromise.
Does a catalyst change the equilibrium constant?
No. A catalyst lowers the activation energy for both the forward and the backward reactions by the same amount. Both rates increase equally, so equilibrium is reached sooner, but the equilibrium constant and the equilibrium composition stay the same.
What happens when water is added to the equilibrium Fe³⁺ + SCN⁻ ⇌ [Fe(SCN)]²⁺?
Dilution lowers the concentrations of all species. The equilibrium shifts towards the side with more dissolved particles, which is the reactant side, to counteract the dilution. Some blood-red [Fe(SCN)]²⁺ breaks up, so the colour of the solution becomes lighter than expected from dilution alone.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.