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

An exothermic reaction gives out heat energy to the surroundings, so the temperature rises and the products have less stored energy than the reactants. An endothermic reaction takes in heat from the surroundings, so the temperature falls and the products have more stored energy than the reactants. The overall energy change comes from comparing the energy needed to break bonds in the reactants with the energy released when new bonds form in the products: if more is released than absorbed, the reaction is exothermic.

How does collision theory explain the factors that affect reaction rate?

Collision theory says a reaction happens when particles collide with enough energy (the activation energy) and the correct orientation. Anything that increases the frequency or energy of effective collisions increases the rate. Higher concentration or pressure packs more particles into the same volume, so collisions are more frequent. A larger surface area exposes more particles to collide. Higher temperature makes particles move faster, so collisions are both more frequent and more energetic. A catalyst lowers the activation energy so more collisions are successful.

How does a catalyst speed up a reaction?

A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions have enough energy to react and the rate increases. It is not used up in the reaction, so a small amount can be reused many times, and it does not change the products or the position of equilibrium. Enzymes are biological catalysts that speed up reactions in living things and work best within a narrow temperature and pH range.

What are oxidation and reduction in terms of electrons?

Oxidation is the loss of electrons, and reduction is the gain of electrons; the mnemonic OIL RIG captures this (Oxidation Is Loss, Reduction Is Gain). The two always happen together in a redox reaction, because the electrons lost by one species are gained by another. Oxidation can also be defined as gain of oxygen and reduction as loss of oxygen, which is the simpler view used for reactions like the reduction of metal oxides by carbon.

How is Energetics, Rates and Redox assessed in O-Level Chemistry 6092?

It is examined across all three papers. Paper 1 (multiple choice, 40 marks, 30 percent) tests energy profiles, rate factors and the identification of oxidising and reducing agents. Paper 2 (structured and free response, 80 marks, 50 percent) rewards explaining rate changes with collision theory, interpreting rate graphs and writing redox in terms of electron transfer. Paper 3 (practical, 40 marks, 20 percent) often involves measuring a rate, for example by timing gas evolution or following a temperature change.

SingaporeChemistry

Energetics, Rates of Reaction and Redox (Singapore O-Level Chemistry 6092): exothermic and endothermic energy changes, collision theory and the factors affecting rate, catalysts and enzymes, and oxidation and reduction by oxygen and electron transfer

A Singapore O-Level Chemistry (SEAB 6092) overview of Energetics, Rates of Reaction and Redox. Exothermic and endothermic reactions and energy profiles, collision theory and the factors that change reaction rate, catalysts and enzymes, and redox defined by oxygen and by electron transfer, with links to every dot point.

Generated by Claude Opus 4.86 min readSEAB-6092Updated 2026-06-13

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

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What this topic is really about
Energetics: heat in and heat out
Rates: collision theory and catalysts
Redox: oxygen and electron transfer
How this topic is examined
Check your knowledge

What this topic is really about

This topic asks three different questions about a reaction. Energetics asks whether energy is given out or taken in. Rates ask how fast the reaction goes and what controls its speed. Redox asks which species is gaining and which is losing electrons or oxygen. The unifying idea is energy and electrons in motion: bonds breaking and forming change the energy, collisions with enough energy drive the rate, and electron transfer defines the chemistry. This guide draws the four dot points together and links to each one.

The complete set of dot-point pages for this topic, each with worked examples and questions, lives at /sg-o-level/chemistry/syllabus/energetics-rates-and-redox.

Energetics: heat in and heat out

Exothermic and endothermic reactions distinguishes reactions that release heat (temperature rises, products lower in energy) from those that absorb it (temperature falls, products higher in energy). The energy profile diagram shows this as a downhill or uphill step, with an activation energy hump in between.

The deeper explanation is bond breaking and bond forming. Breaking bonds takes in energy; forming bonds gives it out. If forming the product bonds releases more than breaking the reactant bonds absorbs, the reaction is exothermic overall.

Rates: collision theory and catalysts

Speed of reaction and collision theory explains rate through effective collisions: concentration, pressure, surface area and temperature all change how often or how energetically particles collide. Catalysts and reaction speed adds the catalyst, which lowers the activation energy without being used up, and enzymes as biological catalysts. The same page covers reading a rate graph of product against time, where a steeper slope means a faster rate and a flat line means the reaction has finished.

Redox: oxygen and electron transfer

Redox and oxidation states defines oxidation as gain of oxygen or loss of electrons and reduction as loss of oxygen or gain of electrons, with OIL RIG as the memory aid. An oxidising agent accepts electrons (and is itself reduced); a reducing agent donates electrons (and is itself oxidised). You should know the colour-change tests that detect them, such as orange acidified potassium dichromate(VI) turning green when reduced, and purple potassium manganate(VII) being decolourised.

How this topic is examined

Use the right energetics language. State heat in or out, the temperature change of the surroundings, and the sign of the energy change.
Always finish a rate answer with collision theory. Name the factor, then say how it changes the frequency or energy of effective collisions.
Define redox both ways. Be ready to explain oxidation and reduction by oxygen and by electron transfer, and to name the oxidising and reducing agents.

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.

Define an exothermic reaction and state what happens to the temperature of the surroundings. (2 marks)
Explain, using bond breaking and bond forming, why a reaction is exothermic overall. (2 marks)
Explain, using collision theory, why increasing the temperature increases the rate of a reaction. (2 marks)
State two properties of a catalyst. (2 marks)
In the reaction of carbon with copper(II) oxide, identify what is oxidised and what is reduced, and name the reducing agent. (3 marks)

Solutions

Q1: An exothermic reaction is one that gives out heat energy to the surroundings; the temperature of the surroundings rises.
Q2: Breaking the bonds in the reactants absorbs energy and forming the bonds in the products releases energy. If more energy is released forming bonds than is absorbed breaking them, there is a net release of energy, so the reaction is exothermic.
Q3: Raising the temperature gives the particles more kinetic energy, so they move faster and collide more often, and a greater proportion of collisions have at least the activation energy. More frequent and more energetic effective collisions per second increase the rate.
Q4: A catalyst speeds up a reaction by lowering the activation energy, and it is not used up in the reaction (so it can be reused). It also does not change the products.
Q5: Carbon is oxidised (it gains oxygen to form carbon dioxide). Copper(II) oxide is reduced (it loses oxygen to form copper). The reducing agent is carbon, because it removes the oxygen and is itself oxidised.

Sources & how we know this

Singapore-Cambridge GCE O-Level Chemistry (Syllabus 6092) — Singapore Examinations and Assessment Board (2026)
Cambridge Assessment International Education, working with SEAB on the Singapore-Cambridge GCE O-Level — Cambridge Assessment International Education (2026)