Energy & Chemical Change
Exothermic and endothermic reactions, activation energy
Exothermic and Endothermic Reactions
Chemical reactions involve energy changes.
• Exothermic reactions release energy (heat) to the surroundings — the temperature increases. Example: combustion of wood, neutralisation reactions.
• Endothermic reactions absorb energy from the surroundings — the temperature decreases. Example: photosynthesis, dissolving ammonium nitrate in water.
Activation Energy
Activation energy (Eₐ) is the minimum energy needed to start a chemical reaction. Even exothermic reactions need activation energy to begin — like striking a match. A catalyst lowers the activation energy, making the reaction faster without being consumed.
Example
Energy Profile Diagrams
Exothermic reaction:
Reactants start HIGH → activation energy hump → Products end LOW
ΔH is negative (energy is released)
Endothermic reaction:
Reactants start LOW → activation energy hump → Products end HIGH
ΔH is positive (energy is absorbed)
ΔH = Energy of products − Energy of reactants
Note
Real-World Applications
South Africa's Sasol converts coal to liquid fuels using exothermic and endothermic reactions (Fischer-Tropsch process). Cold packs used in sports medicine rely on endothermic reactions. Hand warmers use exothermic reactions (iron oxidation).
Key Vocabulary
ExothermicA reaction that releases energy to the surroundings
EndothermicA reaction that absorbs energy from the surroundings
Activation energyThe minimum energy required to start a chemical reaction
CatalystA substance that speeds up a reaction without being consumed
ΔHThe change in energy during a reaction (enthalpy change)
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Exothermic
Speed: