AS & A-Level Chemistry 05 — Equilibria, acids and reaction rates

Public

Independent Deckloop AS Chemistry study material aligned with Cambridge International 9701 (2025–2027). Deck 5 of 18: Equilibria, acids and reaction rates. Original explanations, worked applications and practice. Not affiliated with or endorsed by Cambridge International Education.

Chemistry EN A-Level

71 cards

Study this deck on Deckloop

Preview Cards

A sample of cards from this deck.

Example Explainer

A sample of the AI explainer you can generate for cards in this deck.

Reversible Reactions and Le Chatelier's Principle

Many chemical reactions are reversible, meaning they can proceed in both the forward and reverse directions. In a closed system—where no matter can escape or enter—a reversible reaction may reach a state of dynamic equilibrium. At dynamic equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. Consequently, the macroscopic concentrations of all reactants and products remain constant, even though reactions are continuously occurring at the molecular level. Le Chatelier's principle states that if a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise this change. For instance, increasing the temperature favours the endothermic direction to absorb the excess heat. Compressing a gaseous system at constant temperature favours the side with fewer moles of gas to reduce the pressure. Increasing reactant concentration drives the equilibrium towards the products to consume the added reactant. A catalyst increases the rates of both the forward and reverse reactions equally; it allows equilibrium to be reached faster but does not alter the final position of equilibrium.

Key points

Reversible reaction: A reaction that can occur in both the forward and reverse directions.
Dynamic equilibrium: The forward and reverse reaction rates are equal, keeping reactant and product concentrations constant.
Closed system: Necessary for equilibrium, as no matter is permitted to enter or leave.
Le Chatelier's principle: If a change is made to a system at dynamic equilibrium, the position of equilibrium moves to minimise this change.
Temperature: Increasing temperature shifts equilibrium in the endothermic direction.

Worked example

Question

Predict and explain the effect of increasing the pressure by compression at constant temperature on the equilibrium position of the gas-phase reaction:

2 \mathrm{SO_{2}} (\mathrm{g}) + \mathrm{O_{2}} (\mathrm{g}) \rightleftharpoons 2 \mathrm{SO_{3}} (\mathrm{g}) \Delta H = - 197 \mathrm{kJ} \mathrm{mol}^{-1}

Solution

1. Count the moles of gas on each side of the balanced equation.

2. The reactants have

2 + 1 = 3\,\text{moles of}

gas. The products have

2\,\text{moles of}

gas.

3. According to Le Chatelier's principle, increasing pressure shifts the equilibrium to minimise the change, meaning it will favour the side with fewer moles of gas.

The equilibrium position shifts to the right (towards the products) because the right side has fewer moles of gas

(2\,\text{moles})

compared to the left side

(3\,\text{moles})

, thereby reducing the pressure.

Common pitfalls

Believing that a catalyst increases the yield of products. A catalyst only increases the rate at which equilibrium is achieved; the final equilibrium position and yield remain completely unchanged.
Thinking that dynamic equilibrium means the concentrations of reactants and products are equal. In reality, concentrations are constant, but they are rarely equal to one another.

Prerequisites

Study Moles, formulas and chemical calculations first.
Study Bonding, shape and states of matter first.
Study Enthalpy, chemical energy and redox first.