Exothermic and endothermic reactions
Every reaction transfers energy to or from its surroundings. If the surroundings warm up, the reaction is exothermic; if they cool down, it is endothermic. Reaction profiles and bond energies explain why.
Part 1 of 3: Learn it
In short
- Exothermic: energy is given out, the temperature of the surroundings rises.
- Endothermic: energy is taken in, the temperature of the surroundings falls.
- Breaking bonds takes in energy; making bonds gives it out.
Where this is in your specification
Spec points: AQA 4.5.1.1 to 4.5.1.3 and required practical 4 (Combined Trilogy 5.5.1), Edexcel Topic 7, OCR C3.1
| Board | Topic: Energy changes |
|---|---|
| AQA 8462 | 4.5 (8462); 5.5 (8464) |
| Edexcel 1CH0 | Topic 7 (1CH0 Topic 9 for fuel cells) |
| OCR J248 | C3.1 |
| Eduqas C410 | 8 |
| Cambridge IGCSE 0620 | 5.1 |
| National 5 C813 75 | 2(c) Energy from fuels |
Examples of each
| Exothermic | Endothermic |
|---|---|
| combustion of fuels | thermal decomposition, such as of calcium carbonate |
| neutralisation of acids and alkalis | citric acid with sodium hydrogencarbonate |
| many oxidation reactions | photosynthesis |
| uses: hand warmers, self-heating cans | uses: instant cold packs for sports injuries |
Reaction profiles
A reaction profile plots energy against the progress of the reaction. In an exothermic reaction the products end lower than the reactants; in an endothermic one they end higher. The hump in between is the activation energy: the minimum energy that particles need when they collide for the reaction to happen.
The temperature change practical
- Put a measured volume of one reactant in a polystyrene cup with a lid, standing in a beaker so it does not tip.
- Record the starting temperature, add the second reactant, stir, and record the highest (or lowest) temperature.
- Repeat while changing one variable, such as the volume of alkali or the mass of metal powder.
The cup and lid cut down the heat lost to (or gained from) the room, which would otherwise make the measured change too small.
Bond energies (Higher)
Total the energy taken in to split all the reactant bonds, then total the energy given out as the product bonds form. A negative answer means more energy came out than went in: the reaction is exothermic.
Is breaking bonds exothermic or endothermic?
Show the answer
Endothermic: energy must be taken in to break a bond.
Part 2 of 3: See it worked
Worked examples
Example 1
In a reaction in a polystyrene cup, the temperature starts at 21.0 °C and rises to 28.5 °C. What is the temperature change, and is the reaction exothermic or endothermic?
- Change = 28.5 − 21.0 = 7.5 °C
- The temperature rose, so energy was given out to the surroundings
Answer: A rise of 7.5 °C: the reaction is exothermic.
Example 2
Use bond energies to find the energy change for H₂ + Cl₂ → 2HCl. (H-H 436, Cl-Cl 242, H-Cl 431, all kJ/mol) (Higher)
- Bonds broken: 436 + 242 = 678 kJ/mol
- Bonds made: 2 × 431 = 862 kJ/mol
- Energy change = 678 − 862
Answer: −184 kJ/mol, so the reaction is exothermic.
Common mistakes
- Saying bond breaking releases energy. Breaking bonds always takes energy in.
- Drawing an exothermic profile with the products higher than the reactants.
- Doing bonds made minus bonds broken, which flips the sign.
- Forgetting to multiply a bond energy by the number of those bonds, such as the 2 in 2HCl.
Give one example of an endothermic reaction.
Show the answer
Thermal decomposition of calcium carbonate (or citric acid with sodium hydrogencarbonate, or photosynthesis).
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Is breaking bonds exothermic or endothermic?
Endothermic: energy must be taken in to break a bond.
Give one example of an endothermic reaction.
Thermal decomposition of calcium carbonate (or citric acid with sodium hydrogencarbonate, or photosynthesis).
On the reaction profile of an endothermic reaction, where are the products compared with the reactants?
Higher, because energy has been taken in from the surroundings.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Estos apuntes están en inglés porque siguen los programas de examen del Reino Unido.
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