🔥 Energy Changes
Secondary School Chemistry • MalawiHub
1. Introduction
Chemical reactions involve energy changes. Energy may be released to the surroundings or absorbed from the surroundings.
Learning Objectives
- Distinguish exothermic and endothermic reactions.
- Explain activation energy.
- Interpret energy profile diagrams.
- Relate bond breaking and bond forming to energy changes.
- Describe simple energy-change experiments.
2. Exothermic Reactions
An exothermic reaction releases energy to the surroundings, usually as heat.
The surroundings become warmer and the temperature usually increases.
Examples include:
- Combustion of fuels.
- Many neutralisation reactions.
- Respiration.
CH₄ + 2O₂ → CO₂ + 2H₂O + energy
3. Endothermic Reactions
An endothermic reaction absorbs energy from the surroundings.
The surroundings may become colder.
Examples include:
- Thermal decomposition.
- Some dissolving processes.
- Photosynthesis.
CaCO₃ → CaO + CO₂
This decomposition requires heating.
4. Activation Energy
Activation energy is the minimum energy required for particles to react successfully.
Even exothermic reactions require activation energy to start.
Catalysts
A catalyst provides an alternative reaction pathway with a lower activation energy. It is not permanently consumed.
5. Energy Profile Diagrams
Energy profile diagrams show how the energy of a reacting system changes as a reaction proceeds. The peak represents the activated complex. The difference between the reactant and product energy levels gives the overall enthalpy change.
Real graph: Exothermic vs Endothermic energy profiles
Reading the graph: red shows an exothermic pathway where products are lower in energy; blue shows an endothermic pathway where products are higher in energy. Both require activation energy.
| Reaction | Energy of products | Surroundings |
|---|---|---|
| Exothermic | Lower | Gain energy |
| Endothermic | Higher | Lose energy |
6. Bond Breaking and Bond Making
Breaking chemical bonds requires energy.
Forming chemical bonds releases energy.
The overall energy change depends on the balance between these two processes.
Energy absorbed to break bonds − energy released when bonds form = overall energy change.
7. Measuring Energy Changes
A simple experiment can measure the temperature change during a reaction.
The approximate heat change can be calculated using:
q = mcΔT
- q = heat energy
- m = mass of solution
- c = specific heat capacity
- ΔT = temperature change
If 100 g of solution has a temperature rise of 5°C and c = 4.2 J g⁻¹ °C⁻¹:
q = 100 × 4.2 × 5
q = 2100 J
8. MANEB Examination Focus
- Define exothermic and endothermic reactions.
- Give examples of each.
- Explain activation energy.
- Explain the effect of a catalyst.
- Interpret energy profile diagrams.
- Use q = mcΔT in simple calculations.
- Explain why bond breaking requires energy.
A reaction causes the temperature of a solution to rise from 20°C to 27°C. Is the reaction exothermic or endothermic? Explain.
The reaction is exothermic because energy has been released to the surroundings, causing the temperature to increase.
9. Practice Exercises
- Define an exothermic reaction.
- Define an endothermic reaction.
- Give two examples of exothermic processes.
- What is activation energy?
- What does a catalyst do?
- Why does bond breaking require energy?
- Calculate q when m = 50 g, c = 4.2 J g⁻¹ °C⁻¹ and ΔT = 10°C.
Answers
- A reaction that releases energy to the surroundings.
- A reaction that absorbs energy from the surroundings.
- Combustion and many neutralisation reactions.
- The minimum energy needed for a reaction to occur.
- It lowers the activation energy by providing an alternative pathway.
- Energy is needed to overcome the forces holding atoms together.
- q = 50 × 4.2 × 10 = 2100 J.
10. Common Mistakes
- Do not say every reaction that needs heating is necessarily endothermic.
- Remember that catalysts lower activation energy, not the overall energy change.
- Bond breaking absorbs energy; bond formation releases energy.
- Always include units in calculations.
11. Exam Tip
For temperature-change questions, first decide whether the temperature of the surroundings increased or decreased. This helps identify the energy change.