Chemistry – Chemical energetics - Exothermic and endothermic reactions | e-Consult

Bond breaking is endothermic because energy is required to overcome the attractive forces holding atoms together in a bond. This energy input is absorbed from the surroundings, leading to an increase in the surroundings' thermal energy (and therefore a decrease in the temperature of the reaction mixture). Think of it like needing to apply force to stretch a rubber band – you have to put energy into it.

Bond making is exothermic because when new bonds form, energy is released. The atoms are moving into a more stable, lower-energy state. This released energy is given back to the surroundings, causing an increase in the surroundings' thermal energy (and therefore an increase in the temperature of the reaction mixture). This is like the rubber band snapping back – energy is released when it returns to its original shape.

Example: Formation of water (H₂O) from hydrogen (H₂) and oxygen (O₂).

The reaction is: 2H₂(g) + O₂(g) → 2H₂O(g)

To break the H-H bonds in H₂ and the O=O bonds in O₂ requires energy (endothermic). However, when the O-H bonds form in H₂O, energy is released (exothermic). Since the energy released during bond formation is greater than the energy required to break the initial bonds, the overall reaction is exothermic. The enthalpy change (ΔH) is positive, indicating energy is released to the surroundings.