Exothermic vs Endothermic Reactions
Exothermic reaction
An exothermic reaction releases energy, usually as heat, to its surroundings. The chemical bonds in the products store less energy than the bonds in the reactants, so the excess energy flows out and the surroundings get warmer. By convention its enthalpy change ΔH is negative.
Example: Burning natural gas in a stove: methane reacts with oxygen to form carbon dioxide and water, releasing heat and light that warm the room.
Endothermic reaction
An endothermic reaction absorbs energy, usually as heat, from its surroundings. The products store more energy in their bonds than the reactants did, so energy must flow in for the reaction to proceed, and the surroundings get colder. By convention its enthalpy change ΔH is positive.
Example: Photosynthesis: plants absorb sunlight energy to convert carbon dioxide and water into glucose and oxygen, storing energy in the sugar's bonds.
Key differences
| Aspect | Exothermic reaction | Endothermic reaction |
|---|---|---|
| Sign of enthalpy change | ΔH is negative — energy leaves the chemical system. | ΔH is positive — energy enters the chemical system. |
| Temperature of surroundings | Surroundings warm up (a hand warmer feels hot). | Surroundings cool down (a cold pack feels cold). |
| Energy diagram shape | Reactants start higher than products; the curve drops downhill overall. | Reactants start lower than products; the curve climbs uphill overall. |
| Everyday examples | Combustion, rusting of iron, cellular respiration, setting concrete. | Melting ice, baking bread (heat input), photosynthesis, dissolving some salts. |
| Spontaneity | Often spontaneous, but not always — some exothermic reactions still need a spark to start. | Often nonspontaneous without continuous energy input, but some proceed if entropy increases enough. |
The sign of ΔH describes the chemical system, not the surroundings — a negative ΔH means heat flows OUT of the system and the surroundings warm up. Students also mix up exothermic with 'fast': an exothermic reaction can still be extremely slow (rusting releases heat but takes years). Rate depends on activation energy, not on whether the reaction releases or absorbs heat. Finally, phase changes like melting follow the same convention: melting is endothermic (ΔH positive) because heat is absorbed.