O-Level Pure Chemistry: Energy Changes — Exothermic vs. Endothermic Reactions

Introduction to Energy Changes

In every chemical reaction, energy is either absorbed from or released to the surroundings. This is known as an energy change. In the Singapore O-Level Pure Chemistry syllabus (Syllabus 6092), understanding the distinction between exothermic and endothermic reactions is crucial for scoring in both Paper 1 (MCQ) and Paper 2 (Structured Questions).

Exothermic Reactions: Releasing Energy

An exothermic reaction is one that releases heat energy to the surroundings. As a result, the temperature of the surroundings rises.

Key Examples: Combustion of fuels, neutralisation (acid + base), respiration, and the rusting of iron.
Energy Level: The total energy of the products is less than the total energy of the reactants.
Enthalpy Change (ΔH): Always negative (-ΔH).

Endothermic Reactions: Absorbing Energy

An endothermic reaction is one that absorbs heat energy from the surroundings. This causes the temperature of the surroundings to drop.

Key Examples: Photosynthesis, thermal decomposition (e.g., heating calcium carbonate), and dissolving certain salts like ammonium nitrate in water.
Energy Level: The total energy of the products is more than the total energy of the reactants.
Enthalpy Change (ΔH): Always positive (+ΔH).

Bond Breaking and Bond Forming

Students often get confused about why energy is absorbed or released. Remember this simple rule:

Bond Breaking: Absorbs energy (Endothermic). Think of it like snapping a pencil — you need to put effort in.
Bond Forming: Releases energy (Exothermic).

The overall ΔH of a reaction is calculated by: Energy taken in to break bonds - Energy given out when bonds are formed.

Energy Profile Diagrams: Must-Know for Exams

In your exams, you will frequently be asked to draw or interpret energy profile diagrams. Look out for these three components:

Activation Energy (Ea): The minimum energy that reacting particles must possess to react. It is always a positive value, represented by the arrow from reactants to the peak of the curve.
Enthalpy Change (ΔH): The difference in energy between reactants and products.
Reactants vs. Products: In exothermic diagrams, products are lower than reactants. In endothermic diagrams, products are higher.

Common Exam Pitfalls

Confusing Temperature with Energy: In an exothermic reaction, the system loses energy, but the thermometer (part of the surroundings) shows a temperature increase.
Mislabeling Arrows: ΔH and Ea arrows must be single-headed or double-headed according to specific SEAB marking schemes. Check your latest school notes for the preferred style!

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