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An exothermic reaction releases energy to the surroundings, as in combustion, and activation energy = peak − reactants. Free notes and 10 questions answered.
Chemical Reactions and Energy is part of Key ideas in GCSE AQA Higher Chemistry.
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An exothermic reaction releases energy to the surroundings, as in combustion, and activation energy = peak − reactants. Free notes and 10 questions answered.
Use this lesson to stay inside Key ideas while connecting the detail back to the wider Topic 11: Key ideas topic.
Read the core explanations from Chemical Reactions and Energy before testing yourself from memory.
Wood doesn't burst into flames the moment it touches oxygen, even though burning wood releases lots of energy. There's a barrier stopping reactions from happening instantly.
Think of it like a hill between two valleys. Even if rolling down the other side releases energy, you need enough energy to get over the hill first. This energy barrier is called activation energy.
This explains why reactions happen at different rates. Higher activation energy means fewer particles have enough energy to react, so the reaction is slower. Lower activation energy means more particles can react, so it's faster.
Here's a fundamental rule of the universe: energy is conserved in chemical reactions. You can't create energy from nothing, and you can't make it disappear completely.
When bonds break, energy is absorbed (endothermic process). When bonds form, energy is released (exothermic process). The overall energy change depends on which process needs more energy.
Chemical reactions either release energy to the surroundings (exothermic) or absorb energy from the surroundings (endothermic). You can tell which type by measuring temperature changes.
Examples: Combustion (burning) is exothermic - it heats up the surroundings. Thermal decomposition is usually endothermic - it needs continuous heating to keep going.
Energy profile diagrams show the energy changes during a reaction. They're like a map of the energy journey from reactants to products.
Worked example: Reading an Energy Profile Diagram
A reaction has reactants at 50 kJ/mol and products at 20 kJ/mol. The peak of the energy profile is at 80 kJ/mol. Is this exothermic or endothermic?
Answer: Exothermic reaction (releases 30 kJ/mol)
The activation energy is always measured from the reactants to the peak of the curve. It's the same whether the reaction is exothermic or endothermic.
Exam questions on energy changes follow predictable patterns. Here's how to tackle them systematically and avoid common traps.
Exam precision note: In Chemical Reactions and Energy - Key ideas - Key ideas, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Bond-energy questions and energy-profile questions use related but different pieces of information. In a bond-energy calculation, you total the energy needed to break the reactant bonds and subtract the energy released when the product bonds form. In an energy-profile diagram, you read the reactant and product energy levels directly from the graph. Keeping those contexts separate prevents a very common GCSE mistake.
Worked bond-energy example: for H2 + Cl2 -> 2HCl, energy is absorbed to break the H-H and Cl-Cl bonds, then energy is released when two H-Cl bonds form. If the energy released when the H-Cl bonds form is greater than the energy absorbed to break the starting bonds, the overall reaction is exothermic. This is why bond-energy calculations are written as bonds broken minus bonds formed.
A reaction can still be endothermic even though bond formation releases energy, because the energy needed to break the starting bonds can be greater than the energy released when new bonds form. That is the overall comparison the question is asking you to make. The words exothermic and endothermic describe the net energy change, not whether individual bond-making or bond-breaking steps happen.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
An exothermic reaction releases energy to the surroundings, so the temperature increases, as in combustion. An endothermic reaction absorbs energy from the surroundings, so the temperature decreases, as in thermal decomposition, which needs continuous heating to keep going.
Combustion (burning) is an example of an exothermic reaction because it releases energy and heats up the surroundings. Thermal decomposition is usually endothermic, because it absorbs energy and needs continuous heating to keep going.
An energy profile diagram shows the energy changes during a reaction, like a map of the energy journey from reactants to products. The energy levels of reactants and products are read directly from the graph, and the peak shows the activation energy.
Activation energy is measured from the reactants to the peak of the energy profile: activation energy = peak − reactants. For reactants at 50 kJ/mol and a peak at 80 kJ/mol, the activation energy is 80 − 50 = 30 kJ/mol.
Reactions happen at different rates because they have different activation energy barriers. Higher activation energy means fewer particles have enough energy to react when they collide, so the reaction is slower, while lower activation energy means more particles can react, so it is faster.
Energy is conserved in chemical reactions: it cannot be created or destroyed, only transferred between different forms. When bonds break energy is absorbed, and when bonds form energy is released, and the overall energy change depends on which is greater.
A reaction is exothermic if the temperature of the surroundings increases, because energy is released, and endothermic if the temperature decreases, because energy is absorbed. On an energy profile, products lower than reactants show an exothermic reaction.
The energy change from an energy profile is products minus reactants. With reactants at 50 kJ/mol and products at 20 kJ/mol, the energy change is 20 - 50 = -30 kJ/mol, and the negative value means energy is released, so the reaction is exothermic.
Wood does not burst into flames when it touches oxygen because there is an activation energy barrier, even though burning releases lots of energy. Like a hill between two valleys, particles need enough energy to get over the barrier before the reaction can happen.
Activation energy is measured the same way for exothermic and endothermic reactions: from the reactants to the peak of the curve. For reactants at 50 kJ/mol and a peak at 80 kJ/mol, the activation energy is 80 - 50 = 30 kJ/mol.