3.3.5Movement and Interactions
Energy Changes and Reactions
5 min read 5 parts Lesson 38 of 59 1 quiz
Section 1 of 5Exothermic and endothermic reactions
| Feature | Exothermic Reaction | Endothermic Reaction |
|---|---|---|
| Heat energy transfer | Releases thermal energy to surroundings | Absorbs thermal energy from surroundings |
| Temperature change | Temperature of surroundings increases (rises) | Temperature of surroundings decreases (falls) |
| Enthalpy change (ΔH) | Negative (ΔH < 0) | Positive (ΔH > 0) |
| Energy of products | Products have lower energy than reactants | Products have higher energy than reactants |
| Examples | Combustion, neutralisation, hand warmers | Thermal decomposition, photosynthesis, cold packs |
Section 2 of 5Reaction profiles
- Reaction profile diagrams show the relative energy levels of reactants and products during a reaction:
- The hump between reactants and products represents the activation energy (Ea): the energy required to break bonds.
- For an exothermic reaction, the product line is below the reactant line (ΔH is negative, arrow points downwards).
- For an endothermic reaction, the product line is above the reactant line (ΔH is positive, arrow points upwards).
- A catalyst lowers the height of the energy hump (Ea) without changing the energy levels of reactants or products (ΔH unchanged).
Section 3 of 5Bond energy calculations
1Bond breaking vs bond forming
- Bond breaking is an endothermic process (requires energy, ΔH > 0).
- Bond making is an exothermic process (releases energy, ΔH < 0).
- "MEXO BENDO": Making is EXOthermic, Breaking is ENDOthermic.
2Overall energy change formula
ΔH = Σ(Energy required to break bonds in reactants) - Σ(Energy released making bonds in products)
- If energy released forming new bonds > energy absorbed breaking old bonds → reaction is exothermic (ΔH is negative).
- If energy absorbed breaking old bonds > energy released forming new bonds → reaction is endothermic (ΔH is positive).
Section 4 of 5Key terms
| Term | Meaning |
|---|---|
| Exothermic | A reaction that gives out energy to the surroundings, causing a rise in temperature. |
| Endothermic | A reaction that takes in energy from the surroundings, causing a drop in temperature. |
| Enthalpy change (ΔH) | The overall difference in chemical potential energy between reactants and products. |
| Bond energy | The amount of energy required to break one mole of a specific covalent bond. |
Section 5 of 5Exam technique
Avoid these mistakes
Confusing bond breaking with bond making — remember that breaking chemical bonds always consumes energy (endothermic).
Drawing the ΔH arrow backwards in reaction profiles — ΔH always starts from the reactant level and points towards the product level.