Chemical Energetics – Exothermic & Endothermic Reactions
Objective
The transfer of thermal energy during a reaction is called the enthalpy change, ΔH, of the reaction. ΔH is negative for exothermic reactions and positive for endothermic reactions.
What is Enthalpy Change (ΔH)?
Think of a chemical reaction like a heat‑bank. When the bank releases heat, the reaction is exothermic (ΔH < 0). When the bank absorbs heat, the reaction is endothermic (ΔH > 0).
Exothermic Reactions 🔥
- Heat is released into the surroundings.
- ΔH is negative: ΔH < 0.
- Common example: Combustion of methane – CH₄ + 2O₂ → CO₂ + 2H₂O + heat.
- Analogy: A campfire warms your hands – the fire gives off heat.
Endothermic Reactions ❄️
- Heat is absorbed from the surroundings.
- ΔH is positive: ΔH > 0.
- Common example: Photosynthesis – 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.
- Analogy: A sponge soaking up water – the sponge takes in heat.
ΔH in a Table
| Reaction Type | Heat Flow | ΔH Sign |
|---|
| Exothermic | Heat released | ΔH < 0 |
| Endothermic | Heat absorbed | ΔH > 0 |
Exam Tips 📚
- Remember: ΔH < 0 → exothermic, ΔH > 0 → endothermic.
- Use the heat flow direction: “heat released” = exothermic, “heat absorbed” = endothermic.
- When given a reaction, quickly sketch the energy diagram: a drop in energy = exothermic, a rise = endothermic.
- Practice with real‑world examples: burning wood, dissolving ammonium nitrate, photosynthesis.
- Check units: ΔH is usually in kJ mol⁻¹.
Quick Quiz
Fill in the blanks: If a reaction releases 50 kJ mol⁻¹ of heat, then ΔH = and it is a _ reaction. (Answer: -50 kJ mol⁻¹, exothermic)