Chemical equilibrium: Class 10 Chemistry

Key points

In an irreversible reaction the reactants change completely into products and the reaction goes in one direction only, as when magnesium burns in air. In a reversible reaction the products can react to form the reactants again, so the reaction can go in both directions. A reversible reaction is shown with the sign ⇌, for example N₂ + 3H₂ ⇌ 2NH₃.

Equilibrium. In a closed container the forward reaction slows down as the reactants are used up, and the backward reaction speeds up as products build up. A state is reached in which the rate of the forward reaction equals the rate of the backward reaction. This is dynamic equilibrium. The reactions have not stopped, but the concentrations of all substances stay constant.

Features of a system at equilibrium:

• It can be reached only in a closed system, where nothing enters or leaves.
• The concentrations of reactants and products stay constant, but they are not usually equal.
• The rate of the forward reaction equals the rate of the backward reaction.
• It can be reached from either side, starting from the reactants or from the products.
• A catalyst makes equilibrium reached faster but does not change the position of equilibrium.

Law of mass action. The rate of a chemical reaction is directly proportional to the product of the active masses (molar concentrations) of the reacting substances. For a reaction aA + bB ⇌ cC + dD, the equilibrium constant is Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ, where the square brackets mean the concentration in mol/dm³ at equilibrium. The concentrations of pure solids and pure liquids are not written in the expression.

A large value of Kc means that the products are favoured at equilibrium, and a small value means that the reactants are favoured. Kc changes only with temperature. It does not change when the concentrations or the pressure are changed, or when a catalyst is added.

Le Chatelier's principle. If a change is made to a system in equilibrium, the system shifts in the direction that opposes the change. This is used to predict how the position of equilibrium moves.

Applying the principle:

• Adding a reactant, or removing a product, shifts the equilibrium to the right (towards the products).
• Removing a reactant, or adding a product, shifts it to the left.
• Increasing the pressure of a gas mixture shifts it towards the side with fewer gas molecules. Changing the pressure has no effect if both sides have the same number of gas molecules.
• Increasing the temperature shifts it in the direction of the endothermic reaction, and lowering the temperature shifts it in the direction of the exothermic reaction.

The Haber process makes ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), and the forward reaction is exothermic. Four molecules of gas become two, so a high pressure favours ammonia. A low temperature would favour ammonia, but it would make the reaction too slow, so a moderate temperature of about 450 °C is used with an iron catalyst. The ammonia is removed as a liquid, which shifts the equilibrium to the right so that more is made.

Study tip: For a Le Chatelier question, state the change, state that the system opposes it, and then say which way the equilibrium shifts and what happens to the amount of product.

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Common mistakes

• Saying that the concentrations of reactants and products are equal at equilibrium. They are constant, not equal.
• Writing Kc as reactants over products. The products go on the top.
• Forgetting to raise each concentration to the power of its coefficient.
• Saying that a catalyst shifts the position of equilibrium. It speeds up both reactions equally.

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