Exothermic and Endothermic Reactions
Energy Changes in Reactions
This GCSE chemistry lesson explores Exothermic and Endothermic Reactions. Students distinguish exothermic from endothermic changes, interpret reaction profiles and calculate or compare energy changes using bond energies. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.
Lesduur is: 55 minOnderdelen in deze les
Energy Changes in Reactions
This GCSE chemistry lesson explores Exothermic and Endothermic Reactions. Students distinguish exothermic from endothermic changes, interpret reaction profiles and calculate or compare energy changes using bond energies. It supports KS4 classroom teaching, independent study and exam revision using clear, subject-specific vocabulary.
Energy Changes in Reactions, KS4 Chemistry, GCSE Chemistry, Exothermic Reactions, Endothermic Reactions, Activation Energy, Reaction Profiles, Reaction Profile Diagrams, Bond Energy Calculations, Bond Breaking, Bond Making, Enthalpy Changes, Calorimetry, Thermal Decomposition, Combustion, MCQs, Open Questions, Polls, Emoji Polls, Exit Tickets
Learning goal
I can explain energy changes in chemical reactions and use correct scientific terms to identify whether a reaction is exothermic or endothermic.
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What is one everyday object, feeling, or event that you associate with the word 'HOT' or the word 'COLD'
Use this to surface prior ideas such as heat, temperature, energy, burning, cooling, exothermic, and endothermic. Pick up misconceptions too, such as students equating all reactions with heating.
Energy transfer in reactions
Chemical reactions involve energy transfer
Energy is needed to break bonds in reactants
Energy is released when new bonds form in products
The overall energy change depends on the balance between these two parts
Stress that bonds do not 'store' energy in a simple everyday sense; the key idea is that breaking bonds requires energy, while bond formation releases energy.
Breaking and making bonds
A reaction can be thought of in two stages:
Bond breaking takes in energy
Bond making gives out energy
If more energy is released than taken in, the reaction is exothermic. If more is taken in than released, it is endothermic.
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Chemical and physical changes
In a chemical change, new substances form and bonds are rearranged
In a physical change, the substance stays the same overall
Both can involve energy transfer
Chemical changes focus on energy changes from breaking and forming bonds
Useful contrast: melting ice is a physical change with energy transfer, but burning fuel is a chemical change with new substances formed.
What temperature change shows
A rise in temperature means the surroundings are getting warmer
This suggests energy has been released to the surroundings
A drop in temperature means the surroundings are getting cooler
This suggests energy has been taken in from the surroundings
Make clear that temperature change is an observation of the surroundings or reaction mixture, used as evidence for energy transfer.
Exothermic reactions
Exothermic reactions transfer energy to the surroundings
The temperature of the surroundings increases
The products have less energy than the reactants
More energy is released in bond making than is needed for bond breaking
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What happens when bonds are broken in reactants?
Energy is taken in
Energy is given out
New substances form
Temperature must rise
Checks the key mechanism from slides 2–5. Distractors target common confusions between bond breaking, bond making, and observed temperature change.
Energy transfer basics
In a chemical reaction, energy is needed to break bonds in reactants. Energy is released when new bonds form in products. If the surroundings get warmer, energy has been released to the surroundings.
All blanks echo exact lesson phrasing from slides 2–6 and reinforce the link between bond changes and temperature evidence.
Examples of exothermic change
Combustion reactions are exothermic
Neutralisation reactions are usually exothermic
In both cases, energy is released to the surroundings
Use simple examples such as fuels burning and acid reacting with alkali. Keep the focus on energy change rather than detailed equations unless needed by the class.
Why exothermic reactions warm up
Energy needed to break bonds → smaller amount than energy released when new bonds form → extra energy transferred to the surroundings as heating
This is a causal-chain slide. Link the observed temperature rise back to particle-level bond changes.
Which reaction is usually exothermic?
Thermal decomposition
Neutralisation
Melting ice
Boiling water
Uses the examples and chemical/physical contrast from slides 7–8. Melting and boiling are plausible distractors because they involve energy transfer but are physical changes.
Why does an exothermic reaction warm the surroundings?
Expected elements: an exothermic reaction warms the surroundings because bond making releases more energy than bond breaking needs, so the extra energy is transferred out as heating.
Endothermic reactions
Endothermic reactions take in energy from the surroundings
The temperature of the surroundings decreases
The products have more energy than the reactants
More energy is needed for bond breaking than is released by bond making
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Example of endothermic change
Thermal decomposition is an endothermic reaction
Energy must be supplied for the reaction to continue
The surroundings can become cooler if energy is taken in
Thermal decomposition is a useful example because students can see that heating is needed to keep the reaction going.
Why endothermic reactions cool down
Energy needed to break bonds → larger amount than energy released when new bonds form → extra energy taken in from the surroundings
Emphasise that 'cooling down' is evidence that energy is entering the reaction from outside.
If the temperature falls, the reaction is most likely
Exothermic
Neutral
Physical
Endothermic
Checks whether students can use temperature evidence from slides 9–11 rather than just recall a definition.
Endothermic change
An endothermic reaction takes in energy from the surroundings. The products have more energy than the reactants. In thermal decomposition, energy must be supplied for the reaction to continue.
Reinforces the load-bearing terms from slides 9–11 and the named example of thermal decomposition.
Energy level diagrams
An energy level diagram compares the energy of reactants and products
Higher on the diagram means more energy
Lower on the diagram means less energy
The difference between the two levels shows the overall energy change
Students often confuse amount of substance with energy height on the graph. Clarify that the vertical axis represents energy only.
Exothermic and endothermic diagrams
Exothermic
Reactants at a lower energy level
Products at a higher energy level
Overall energy change is positive in direction: energy enters the reaction
Endothermic
Reactants at a higher energy level
Products at a lower energy level
Overall energy change is negative in direction: energy leaves the reaction
Use careful language: students at this level do not need advanced sign conventions in detail, but they should understand the direction of energy transfer.
Activation energy
Activation energy is the minimum energy needed to start a reaction
Even exothermic reactions need some energy at the start
This energy allows bonds in the reactants to begin breaking
A common misconception is that exothermic reactions need no energy input at all. Point out ignition as a familiar example.
Reaction profiles
A reaction profile shows how energy changes during a reaction
The peak shows the activation energy
The final level compared with the start shows whether the reaction is exothermic or endothermic
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Reading a reaction profile
Start level = energy of the reactants
Peak = activation energy barrier
End level = energy of the products
End lower than start → exothermic
End higher than start → endothermic
Keep linking graph features to the core language of reactants, products, activation energy, and overall energy change.
On a reaction profile, what does the peak show?
Product energy
Overall change
Reactant energy
Activation energy
Tests graph reading from slides 12–16 and targets confusion between start, peak, and end of the profile.
Reading diagrams and profiles
An energy level diagram compares the energy of reactants and products. On a reaction profile, the peak shows the activation energy. If the end level is lower than the start, the reaction is exothermic.
Summarises the final subtopic and checks whether students can connect diagram features to reaction type.
How can a profile show exothermic or endothermic change?
Expected elements: exothermic profile ends lower than it starts; endothermic ends higher; peak is activation energy in both cases.
How confident do you feel with exothermic and endothermic reactions?
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Glossary
energy transfer - movement of energy from one place or system to another
chemical reaction - a process in which reactants change into new substances
bond breaking - separating atoms joined in a compound, which requires energy
bond making - forming new bonds between atoms, which releases energy
exothermic - describing a reaction that releases energy to the surroundings
endothermic - describing a reaction that takes in energy from the surroundings
surroundings - everything outside the reacting chemicals
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Glossary
temperature change - an increase or decrease in temperature that gives evidence of energy transfer
combustion - burning in oxygen, usually with release of energy
neutralisation - reaction of an acid with a base or alkali to form salt and water
thermal decomposition - breakdown of a compound by heating
energy level diagram - a diagram comparing the energy of reactants and products
activation energy - the minimum energy needed to start a reaction
reaction profile - a graph showing energy changes as a reaction happens
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