Enthalpy Change & Activation Energy
Section: 5. Chemical Energetics | Syllabus: Cambridge IGCSE Chemistry (0620)
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
- Displacement reactions - a reactive metal displacing a less reactive metal from its salt
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
- Photosynthesis - plants absorb light energy to make glucose
- Electrolysis - decomposition using electrical energy
- 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.
Enthalpy Change (ΔH)
The transfer of thermal energy during a reaction is called the enthalpy change, ΔH, measured in kilojoules per mole (kJ/mol).
| Reaction type | Sign of ΔH | Example |
|---|---|---|
| Exothermic (releases energy) | Negative | CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = −890 kJ/mol |
| Endothermic (absorbs energy) | Positive | CaCO₃(s) → CaO(s) + CO₂(g) ΔH = +178 kJ/mol |
Reaction Pathway (Energy Profile) Diagrams
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 enthalpy change (ΔH), and the activation energy (Ea).
| Feature | Exothermic | Endothermic |
|---|---|---|
| Products vs. reactants | products are LOWER in energy | products are HIGHER in energy |
| ΔH arrow | points DOWN, from reactants to products | points UP, from reactants to products |
| ΔH sign | negative | positive |
| 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 ΔH 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 ΔH 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 ΔH [1]; since the products are at a lower energy than the reactants, the reaction is exothermic [1]
Activation Energy (Ea)
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)
- The peak represents the transition state, where old bonds are breaking and new bonds are forming
- 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
Worked Example: Defining Activation Energy
- Question: which statement describes activation energy?
- A - the maximum energy given out during a reaction
- B - the maximum energy taken in during a reaction
- C - the minimum energy colliding particles must have to react
- D - the minimum energy particles must have to make bonds
- Answer: C. Activation energy is a minimum energy threshold for a successful collision to lead to reaction - not a maximum, and not specifically about bond-making.
Effect of Catalysts on Activation Energy
A catalyst provides an alternative reaction pathway with a lower activation energy, so more particles have enough energy to react and the reaction speeds up.
- On a diagram, the catalysed pathway has a LOWER peak than the uncatalysed pathway
- ΔH stays exactly the same - the reactants and products are unchanged, only the pathway between them is different
Real-Life Applications
| Exothermic uses | Endothermic uses |
|---|---|
| Hand warmers (iron powder oxidises) | Instant cold packs (ammonium nitrate dissolving) |
| Self-heating cans (CaO + water) | Industrial production of quicklime (CaO) from limestone |
| Combustion of fuels for heating/transport | Photosynthesis |
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
- ❌ Thinking a catalyst changes ΔH
- ✓ a catalyst only lowers Ea (via an alternative pathway); ΔH depends only on the reactants and products, so it is unchanged
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