Exothermic and Endothermic Reactions

Exothermic reactions

Release energy to the surroundings — temperature rises.

• Examples: combustion, neutralisation, oxidation, most displacement, hand warmers.

Endothermic reactions

Take in energy from the surroundings — temperature falls.

• Examples: thermal decomposition, citric acid + sodium hydrogen carbonate, sports cold packs, photosynthesis.

Reaction profiles

• Exothermic: products are lower than reactants (energy released).
• Endothermic: products are higher than reactants (energy absorbed).
• The "hump" is the activation energy — the minimum energy needed to start the reaction.

Bond energy calculations

energy change = (energy to BREAK bonds) − (energy RELEASED making bonds)

• Breaking bonds = endothermic (needs energy).
• Making bonds = exothermic (releases energy).
• Negative answer → overall exothermic; positive → endothermic.

Worked idea

If 1500 kJ is needed to break bonds and 1800 kJ is released forming bonds:

1500 − 1800 = −300 kJ  → exothermic

Exam tip

"Temperature went up" = exothermic; "went down" = endothermic. For bond energies: break − make; a negative result means exothermic.