What is an Exothermic Reaction?
An exothermic reaction is a chemical process that releases energy to the surroundings, usually as heat, light, or sound. The products have less energy than the reactants, so energy flows outward.
Exothermic reactions release energy (ΔH < 0) to the surroundings. Common examples: combustion, neutralization of acids/bases, many synthesis reactions. The temperature of the surroundings increases.
Step-by-step worked examples
Is the combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) exothermic or endothermic?
Combustion always releases large amounts of heat and light energy. ΔH = −890 kJ/mol (negative). This is exothermic.
Neutralization: HCl + NaOH → NaCl + H₂O. ΔH = −57 kJ. Identify the reaction type.
ΔH is negative (−57 kJ). Negative ΔH means energy is released. This is exothermic.
Freezing water: H₂O(l) → H₂O(s). ΔH = −6 kJ/mol. Which type?
ΔH is negative (releases energy as heat to surroundings). The process releases energy. This is exothermic.
Flashcards
Quick quiz
Q1.Which is exothermic?
Q2.In an exothermic reaction, ΔH is
Q3.What happens to surroundings in exothermic?
Q4.Is freezing water exothermic or endothermic?
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Common mistakes
Exothermic reactions feel cold. — Correct: Exothermic reactions release heat → feel warm to touch.
All chemical reactions are exothermic. — Correct: Some release energy (exothermic), others absorb it (endothermic).
Exothermic means very fast. — Correct: Exothermic = energy released (thermodynamics). Speed is separate (kinetics).
Dissolving is always exothermic. — Correct: Most are (e.g. salt in water), but some are endothermic (e.g. NH₄NO₃ in water).
FAQ
What is an exothermic reaction definition?
A chemical reaction that releases energy (usually heat) to the surroundings. ΔH < 0.
Exothermic reaction examples?
Combustion (burning wood, candles), acid–base neutralization, rusting, freezing, crystallization, many synthesis reactions.
How to tell if a reaction is exothermic?
ΔH < 0 (negative enthalpy change), or the surroundings warm up, or energy is released as heat/light.
Difference between exothermic and endothermic?
Exothermic: ΔH < 0, releases heat, surroundings warm. Endothermic: ΔH > 0, absorbs heat, surroundings cool.