What is the difference between an exothermic and an endothermic reaction?

An exothermic reaction transfers energy to the surroundings, so the temperature rises and the products have less stored chemical energy than the reactants. Combustion and neutralisation are common examples. An endothermic reaction takes in energy from the surroundings, so the temperature falls and the products have more stored chemical energy than the reactants. On an energy level diagram, the products are below the reactants for an exothermic change and above them for an endothermic change.

How do the four factors change the speed of a reaction?

All four are explained by colliding particles. Increasing the concentration of a solution, or the pressure of a gas, packs more particles into the same space, so there are more collisions per second. Raising the temperature gives particles more energy, so they collide more often and harder. Increasing the surface area of a solid exposes more particles to collide with. A catalyst provides an easier path so more collisions are successful, and it is not used up.

What is the difference between defining redox by oxygen and by electrons?

The oxygen definition is the simpler one: oxidation is the gain of oxygen and reduction is the loss of oxygen. The electron definition is more powerful: oxidation is the loss of electrons and reduction is the gain of electrons. A helpful memory aid is OIL RIG, oxidation is loss, reduction is gain (of electrons). In any redox reaction one substance is oxidised while another is reduced, so the two always happen together.

What is a catalyst and is it used up in the reaction?

A catalyst is a substance that speeds up a chemical reaction without being used up in the reaction. It provides an alternative route that needs less energy, so a larger proportion of collisions are successful. Because it is not consumed, the same small amount can be used over and over, which is why catalysts are so valuable in industry.

How is Energetics, Rates and Redox assessed in N(A)-Level Science 5107?

It is examined in the two Chemistry papers. Paper 3 (multiple choice, 20 marks) tests recognising exothermic and endothermic changes, the effect of each rate factor, and identifying oxidation and reduction. Paper 4 (structured, 30 marks) asks you to interpret or sketch an energy level diagram, explain a rate change using colliding particles, and identify what is oxidised and reduced in a given equation. There is no separate practical paper.

SingaporeChemistry

Energetics, Rates and Redox (Singapore N(A)-Level Science Chemistry 5107): exothermic and endothermic reactions, the factors that change the speed of a reaction, and oxidation and reduction in terms of oxygen and electrons

A Singapore N(A)-Level Science Chemistry (SEAB 5107) overview of Energetics, Rates and Redox. Exothermic and endothermic reactions and their energy level diagrams, how concentration, temperature, surface area and catalysts change reaction speed through colliding particles, and oxidation and reduction defined by oxygen and by electron transfer, with links to every dot point.

Generated by Claude Opus 4.85 min readSEAB-5107Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is really about
Exothermic and endothermic reactions
The speed of a reaction
Oxidation and reduction
How this topic is examined
Check your knowledge

What this topic is really about

Energetics, Rates and Redox brings together three ideas about how reactions happen: how much energy they move, how fast they go, and how electrons are transferred. They sit together because they are all about the dynamics of a reaction rather than just its products. This guide ties the three dot points together and links to each one for the worked answers and practice.

The complete set of dot-point pages for this topic lives at /sg-n-level/chemistry/syllabus/energetics-rates-and-redox.

Exothermic and endothermic reactions

Exothermic and endothermic reactions describes whether energy is given out or taken in. Exothermic reactions release energy and warm the surroundings; endothermic reactions absorb energy and cool them. On an energy level diagram, the products sit below the reactants for an exothermic change and above them for an endothermic one. The overall change comes from the balance between energy taken in to break bonds and energy given out when new bonds form.

The speed of a reaction

Speed of reaction explains the four factors and ties them all to colliding particles: a reaction goes faster when particles collide more often and with enough energy. Higher concentration or pressure, higher temperature, and greater surface area all increase the rate, and a catalyst speeds it up without being used up.

Oxidation and reduction

Oxidation and reduction gives the two definitions. By oxygen, oxidation is gain of oxygen and reduction is loss of oxygen. By electrons, oxidation is loss of electrons and reduction is gain. The two always happen together in a redox reaction, and you should be able to name simple oxidising and reducing agents.

How this topic is examined

Match observation to energy diagram. A temperature rise means exothermic and products below reactants; a fall means endothermic and products above.
Always finish a rate answer with collisions. Name the factor, then explain it as more frequent or more energetic successful collisions.
Use OIL RIG. Oxidation is loss, reduction is gain of electrons; state which substance is oxidised and which is reduced.

Check your knowledge

A mix of recall, reasoning and explanation questions covering Energetics, Rates and Redox. Attempt them under timed conditions, then check against the solutions.

State whether combustion is exothermic or endothermic, and what happens to the temperature of the surroundings. (2 marks)
On an energy level diagram for an exothermic reaction, are the products above or below the reactants? (1 mark)
Explain, in terms of colliding particles, why increasing the temperature speeds up a reaction. (2 marks)
Define oxidation and reduction in terms of electrons. (2 marks)
In the reaction $\text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O}$ , state which substance is oxidised and which is reduced, using the oxygen definition. (2 marks)

Sources & how we know this

Singapore-Cambridge GCE Normal (Academic) Level Science (Physics, Chemistry) and Science (Chemistry, Biology) (Syllabus 5107) — Singapore Examinations and Assessment Board (2026)
Cambridge Assessment International Education, working with SEAB on the Singapore-Cambridge GCE N(A)-Level — Cambridge Assessment International Education (2026)