Question
Differentiate between exothermic and endothermic reactions. Give five examples of each from daily life.
(NCERT Class 7, Chapter 6 — Physical and Chemical Changes)
Solution — Step by Step
Exothermic reaction: A chemical reaction that releases heat to the surroundings. The products have less energy than the reactants. (“Exo” = outside, “thermic” = heat)
Endothermic reaction: A chemical reaction that absorbs heat from the surroundings. The products have more energy than the reactants. (“Endo” = inside, “thermic” = heat)
| Example | What happens |
|---|---|
| Burning of fuels (wood, LPG, petrol) | Combustion releases a large amount of heat and light |
| Respiration | |
| Mixing water with quicklime | (the container becomes very hot) |
| Rusting of iron | Slow oxidation that releases heat gradually |
| Setting of cement/concrete | The hardening process releases heat over hours |
| Example | What happens |
|---|---|
| Photosynthesis | Plants absorb sunlight energy to make glucose from and |
| Melting of ice | Ice absorbs heat from surroundings to turn into water |
| Dissolving ammonium chloride in water | The solution becomes cold — it absorbs heat |
| Cooking food | We must continuously supply heat; the reaction won’t happen on its own |
| Evaporation of water | Water absorbs heat from surroundings to become vapour (that’s why sweating cools you down) |
| Feature | Exothermic | Endothermic |
|---|---|---|
| Heat | Released | Absorbed |
| Temperature of surroundings | Increases | Decreases |
| Energy of products vs reactants | Products have less energy | Products have more energy |
| Needs continuous heat supply? | No (self-sustaining after ignition) | Yes |
Why This Works
Every chemical reaction involves breaking old bonds and forming new ones. Breaking bonds requires energy (endothermic step), while forming bonds releases energy (exothermic step). If the energy released by forming new bonds is more than the energy needed to break old bonds, the overall reaction is exothermic — and vice versa.
Think of it like this: in an exothermic reaction, the products are more “stable” (lower energy) than the reactants, so the extra energy escapes as heat. In an endothermic reaction, the products are “higher energy” — they store the absorbed heat as chemical potential energy.
Alternative Method — The energy diagram approach
Draw an energy level diagram: reactants on the left, products on the right. If the products are lower than the reactants, the reaction is exothermic (energy goes out as heat). If products are higher, it’s endothermic (energy absorbed from surroundings). The height difference is the heat of reaction. This diagram makes MCQs on this topic very easy to answer.
Common Mistake
Students confuse physical changes (like melting, boiling) with chemical reactions. While melting ice is endothermic, it’s a physical change, not a chemical reaction. In CBSE exams, if the question specifically asks for “chemical reactions,” don’t list phase changes. If it asks for “exothermic/endothermic processes,” then phase changes are acceptable.