5.1 Exothermic and endothermic reactions
- Syllabus
- 0620–2026–2027
- Topic
- 5.1
- Level
- —
Learning objectives
5.1.1An exothermic reaction transfers• State: an exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings5.1.2An endothermic reaction takes in• State: an endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings5.1.3Interpret reaction pathway diagrams• Interpret reaction pathway diagrams showing exothermic and endothermic reactions5.1.4Transfer of thermal energy during a• State: the transfer of thermal energy during a reaction is called the enthalpy change, ∆H, of the reaction. ∆H is negative for exothermic reactions and positive for endothermic reactions5.1.5Activation energy, Ea, as the minimum• Define activation energy, Ea, as the minimum energy that colliding particles must have to react5.1.6And label reaction pathway diagrams• Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include: (a) reactants (b) products (c) enthalpy change of the reaction, ∆H (d) activation energy, Ea5.1.7Bond breaking is an endothermic• State: bond breaking is an endothermic process and bond making is an exothermic process and explain the enthalpy change of a reaction in terms of bond breaking and bond making5.1.8Enthalpy change of a reaction using• Calculate the enthalpy change of a reaction using bond energies
Recognise an exothermic reaction
In an exothermic reaction, the reacting chemicals transfer thermal energy to the surroundings. The surroundings therefore warm up and their measured temperature rises.
| Part | Energy change | Observable result |
|---|---|---|
| reacting chemicals | lose energy | products end at lower energy than reactants |
| surroundings | gain thermal energy | temperature rises |
Combustion and many neutralisation reactions are exothermic. Identify the direction of energy transfer first: reaction → surroundings.
The temperature reading describes the surroundings, such as the solution or reaction vessel. Do not say that the reaction itself 'contains heat'.
Recognise an endothermic reaction
In an endothermic reaction, the reacting chemicals take in thermal energy from the surroundings. The surroundings therefore cool down and their measured temperature falls.
| Part | Energy change | Observable result |
|---|---|---|
| reacting chemicals | gain energy | products end at higher energy than reactants |
| surroundings | lose thermal energy | temperature falls |
Thermal decomposition and some dissolving processes are endothermic. Identify the direction of transfer as surroundings → reaction.
Endothermic does not mean 'no reaction' or simply 'cold'. It describes energy transferred into the reacting system from its surroundings.
Interpret a reaction pathway diagram
A reaction pathway diagram plots energy on the vertical axis against progress of reaction on the horizontal axis. The curve starts at the reactant energy level, rises to a peak, then ends at the product energy level.
| Feature | What it tells you |
|---|---|
| products below reactants | exothermic reaction |
| products above reactants | endothermic reaction |
| reactant level to peak | activation energy, Ea |
| reactant level to product level | enthalpy change, ΔH |
Read the two horizontal energy levels before looking at the curve: compare products with reactants, classify the reaction, then read Ea and ΔH from their vertical differences.
The horizontal axis is reaction progress, not time. A taller peak does not by itself decide whether the reaction is exothermic or endothermic.
Link enthalpy change to energy transfer
The thermal-energy transfer during a reaction is its enthalpy change, ΔH. On a pathway diagram, ΔH is the vertical energy difference from the reactants to the products.
| Reaction | Product energy | Energy transfer | Sign of ΔH |
|---|---|---|---|
| exothermic | lower than reactants | to surroundings | negative |
| endothermic | higher than reactants | from surroundings | positive |
Use ΔH = energy of products − energy of reactants. A drop gives a negative value; a rise gives a positive value.
ΔH is not activation energy. ΔH compares products with reactants; Ea compares the peak with reactants.
Define activation energy
Activation energy, Ea, is the minimum energy that colliding particles must have for a reaction to occur.
On a reaction pathway diagram, Ea is the vertical energy difference from the reactant energy level to the top of the curve. The peak represents the energy barrier that must be overcome.
Particles can collide without reacting. A successful collision requires enough energy to meet or exceed Ea, as well as a suitable collision arrangement.
Ea is required for both exothermic and endothermic reactions. Exothermic reactions do not start with zero activation energy.
Draw and label a reaction pathway diagram
- Label the vertical axis energy and the horizontal axis progress of reaction. 2. Draw and label the reactant level. 3. Draw a smooth curve rising to one peak. 4. End at and label the product level. 5. Add vertical arrows for Ea and ΔH.
| Label | Correct placement |
|---|---|
| reactants | starting horizontal level |
| products | finishing horizontal level |
| Ea | reactant level to peak |
| ΔH | reactant level to product level |
For an exothermic reaction, place products below reactants and show ΔH downward. For an endothermic reaction, place products above reactants and show ΔH upward.
Keep Ea anchored at the reactant level, not the product level. Include arrowheads and labels so the two energy differences cannot be confused.
Explain ΔH using bond breaking and bond making
| Process | Energy transfer | Type |
|---|---|---|
| breaking bonds in reactants | energy is taken in | endothermic |
| making bonds in products | energy is released | exothermic |
Compare the two totals. If bond making releases more energy than bond breaking takes in, the reaction is exothermic and ΔH is negative. If bond breaking takes in more, the reaction is endothermic and ΔH is positive.
A complete explanation names both processes, states their energy directions, compares their magnitudes, and then links the difference to the sign of ΔH.
Breaking a bond never releases energy. Energy may be released by the reaction overall only because forming new bonds releases a larger amount.
Calculate ΔH from bond energies
ΔH = Σ(bond energies of bonds broken) − Σ(bond energies of bonds formed). Bond energies are usually given in kJ/mol, so the calculated ΔH is reported in kJ/mol for the reaction as written.
Count every bond in the reactants and multiply by its bond energy. Add these to obtain energy taken in. Then count every bond formed in the products and add their energies. Subtract formed from broken, keeping the sign.
For H₂ + Cl₂ → 2HCl, using H–H = 436, Cl–Cl = 243 and H–Cl = 432 kJ/mol: bonds broken = 436 + 243 = 679; bonds formed = 2 × 432 = 864; ΔH = 679 − 864 = −185 kJ/mol. The negative result means exothermic.
Use the equation coefficients when counting bonds, and subtract the energy released in bond formation. Reversing the subtraction reverses the sign and the reaction classification.