Definition
Exothermic reaction: releases energy to the surroundings (temperature rises) — products have less energy than reactants.
Endothermic reaction: absorbs energy from the surroundings (temperature falls) — products have more energy than reactants.
Rule
Breaking a bond always requires energy (endothermic); forming a bond always releases energy (exothermic) — a reaction's overall energy change depends on the balance between bonds broken and bonds formed.
Example 1 — enthalpy from bond energies
Average bond energies: O–O , O=O kJ/mol. Estimate the enthalpy change for converting O–O single bonds into one O=O double bond (illustrative pattern).
- Energy to break O–O bonds (endothermic, ): kJ.
- Energy released forming one O=O bond (exothermic, ): kJ.
- .
Answer: kJ/mol (exothermic)
Definition
Activation energy is the minimum energy needed for reactant particles to reach the transition state and react — even an exothermic reaction needs this initial energy input to get started.
Example 2 — why some reactions don't happen at room temperature
Explain why doesn't happen under normal room conditions, even though nitrogen and oxygen surround us constantly.
- The reaction has a very high activation energy — the triple bond in is extremely strong and hard to break.
- At room temperature, essentially no colliding molecules carry enough energy to reach the transition state.
Answer: The activation energy barrier is too high at normal temperatures
Remember
This is exactly why food needs cooking: heat supplies the activation energy that lets otherwise-slow chemical changes (breaking down starches, proteins) happen quickly enough to matter, while some foods' reactions have a low enough barrier to proceed even without added heat.
A reaction profile plots energy against the progress of a reaction: it rises to a peak (the transition state, at the height of the activation energy) before falling to the products' energy level. A faster reaction (at the same temperature) has a lower activation energy peak.
Quick Recap
- ●Exothermic: releases energy, products lower energy. Endothermic: absorbs energy, products higher energy
- ●Breaking bonds: always endothermic. Forming bonds: always exothermic
- ●Activation energy: minimum energy needed to reach the transition state and react
- ●A high activation energy can stop even an exothermic reaction from happening at room temperature
- ●Lower activation energy peak on a reaction profile faster reaction
