Exothermic and endothermic reactions
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
Exothermic Reactions
All chemical reactions involve energy changes. In an exothermic reaction, thermal energy is transferred to the surroundings, so the temperature of the surroundings increases.
- Combustion - burning fuels, e.g. methane + oxygen → carbon dioxide + water
- Neutralisation - acid + alkali → salt + water
- Oxidation - rusting of iron, respiration
Endothermic Reactions
In an endothermic reaction, thermal energy is taken in from the surroundings, so the temperature of the surroundings decreases.
- Thermal decomposition - e.g. calcium carbonate → calcium oxide + carbon dioxide
- Dissolving some salts - e.g. ammonium nitrate in water
Worked Example: Identifying Endothermic Changes
- Question: which row describes the changes during an endothermic reaction?
- A - thermal energy released to surroundings; temperature of surroundings decreases
- B - thermal energy released to surroundings; temperature of surroundings increases
- C - thermal energy taken in from surroundings; temperature of surroundings decreases
- D - thermal energy taken in from surroundings; temperature of surroundings increases
- Answer: C. An endothermic reaction absorbs thermal energy from the surroundings, so the surroundings lose thermal energy and their temperature decreases. Rows A and B describe exothermic changes (energy released); row D is impossible - if energy leaves the surroundings, their temperature cannot rise.
Reaction Pathway (Energy Profile) Diagrams (Supplement)
A reaction pathway diagram plots energy against the progress of the reaction. A fully labelled diagram must show all four of: the reactants, the products, the overall energy change of the reaction, and the activation energy (Ea).
| Feature | Exothermic | Endothermic |
|---|---|---|
| Products vs. reactants | products are LOWER in energy | products are HIGHER in energy |
| Energy-change arrow | points DOWN, from reactants to products | points UP, from reactants to products |
| Curve shape | rises to a peak, then falls BELOW the reactant line | rises to a peak, then settles ABOVE the reactant line |
Both diagrams must label the reactant and product energy levels, the Ea arrow (reactants to peak), and the overall energy-change arrow (reactants to products)
Worked Example: Labelling a Reaction Pathway Diagram
- Question: a reaction pathway diagram shows the reactant energy level above the product energy level, with a peak between them. Label the diagram to show the energy change and Ea, and state whether the reaction is exothermic or endothermic. [3]
- Answer: draw the upward arrow from the reactant line to the top of the peak and label it Ea [1]; draw the downward arrow from the reactant line to the product line and label it as the overall energy change [1]; since the products are at a lower energy than the reactants, the reaction is exothermic [1]
Activation Energy, Ea (Supplement)
Activation energy, Ea, is the minimum energy that colliding particles must have in order to react.
- On a reaction pathway diagram, Ea is the energy difference from the reactants up to the peak (the highest point)
- Every reaction has an activation energy - even exothermic ones need an initial energy input to get started (e.g. wood needs a lit match before it will burn)
- Only collisions with energy ≥ Ea result in a successful reaction
Common Mistakes
- ❌ Thinking every reaction with no visible flame must be endothermic
- ✓ classify by the temperature change, not by appearance - neutralisation releases no flame but is still exothermic
- ❌ Saying an exothermic reaction has no activation energy
- ✓ all reactions, exothermic or endothermic, need an initial energy input (Ea) to break bonds and start the reaction
Bond Breaking and Bond Forming (Supplement)
During a reaction, bonds in the reactants are broken and new bonds in the products are formed.
- Bond breaking is endothermic - energy must be put IN to overcome the attractive forces holding atoms together and separate them
- Bond making is exothermic - when atoms bond together, they reach a more stable, lower-energy state, and the excess energy is released OUT to the surroundings
| Process | Energy change | Type |
|---|---|---|
| Bond breaking | energy IN (absorbed) | endothermic |
| Bond forming | energy OUT (released) | exothermic |
Common Mistakes
- ❌ Forgetting that bond breaking is endothermic
- ✓ breaking bonds is always energy IN, regardless of whether the overall reaction is exothermic or endothermic
- ❌ Mixing up which process is exothermic
- ✓ bond breaking always needs energy (endothermic); bond forming always releases energy (exothermic) - whether the overall reaction is exothermic or endothermic depends on which effect is bigger
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