State that an exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings

Chemical Energetics – Exothermic & Endothermic Reactions 🚀

What is an Exothermic Reaction?

An exothermic reaction releases heat to its surroundings, causing the surroundings to warm up. Think of a campfire: the fire gives off heat that warms the air around you. In chemistry, the reaction’s products have lower energy than the reactants, so the excess energy is released as heat.

Key Points

  • Heat is transferred from the reaction to the surroundings.

  • Temperature of the surroundings increases.

  • ΔH (enthalpy change) is negative for exothermic reactions.

  • Energy is conserved – the energy lost by the reaction equals the energy gained by the surroundings.

Analogy: The Rolling Ball

Imagine a ball rolling down a hill. It loses potential energy, which turns into kinetic energy and a little heat that warms the ground. The ball’s energy is lower at the bottom – just like products in an exothermic reaction.

Examples

  • Combustion of methane: \$\ce{CH4 + 2O2 -> CO2 + 2H2O}\$ (ΔH = –890 kJ/mol)

  • Neutralisation of acid and base: \$\ce{HCl + NaOH -> NaCl + H2O}\$ (ΔH ≈ –57 kJ/mol)

  • Formation of water from hydrogen and oxygen: \$\ce{2H2 + O2 -> 2H2O}\$ (ΔH = –483 kJ/mol)

Comparison Table

Reaction TypeΔH (kJ/mol)Temperature Change
ExothermicNegative (ΔH < 0)↑ (surroundings warm)
EndothermicPositive (ΔH > 0)↓ (surroundings cool)

Exam Tip

Remember: In multiple‑choice questions, look for the sign of ΔH. A negative ΔH indicates an exothermic reaction. Also, if the question mentions an increase in temperature of the surroundings, the reaction is exothermic. 📚

Summary

An exothermic reaction transfers thermal energy to the surroundings, raising their temperature. The key sign is a negative ΔH. Keep this in mind when answering exam questions about heat flow.

Quick Quiz

  1. Which of the following reactions is exothermic?

    1. \$\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O}\$ (ΔH = –2800 kJ/mol)

    2. \$\ce{NH4Cl -> NH3 + HCl}\$ (ΔH = +15 kJ/mol)

    3. \$\ce{Fe + S -> FeS}\$ (ΔH = –200 kJ/mol)

  2. What happens to the temperature of the surroundings when an exothermic reaction occurs?

    1. It increases.

    2. It decreases.

    3. It stays the same.

Answers: 1a, 1c, 2a. Good luck! 🎓