Enthalpy Change & Activation Energy
Section: 5. Chemical Energetics | Syllabus: Cambridge AS Level Physics 9702
What is Energy Change in Reactions? All chemical reactions involve energy changes . Energy is required to break bonds and is released when new bonds are formed. The overall energy change determines whether a reaction is exothermic or endothermic.
Exothermic Reactions Exothermic reactions release energy to the surroundings, usually as heat. The temperature of the surroundings increases. Examples of exothermic reactions: Combustion - burning fuels (e.g., methane + oxygen → carbon dioxide + water) Neutralization - acid + alkali → salt + water Oxidation - rusting of iron, respiration Displacement reactions - reactive metal + less reactive metal salt Observable sign: Temperature rises (feels warm/hot) Energy diagram: Products have LESS energy than reactants Endothermic Reactions Endothermic reactions absorb energy from the surroundings, usually as heat.
The temperature of the surroundings decreases. Examples of endothermic reactions: Thermal decomposition - e.g., calcium carbonate → calcium oxide + carbon dioxide Photosynthesis - plants absorb light energy to make glucose Electrolysis - decomposition using electrical energy Dissolving some salts - e.g., ammonium nitrate in water Observable sign: Temperature falls (feels cold) Energy diagram: Products have MORE energy than reactants What is Enthalpy Change (ΔH)?
Enthalpy change (ΔH) is the heat energy change in a reaction at constant pressure. It is measured in kilojoules per mole (kJ/mol). For exothermic reactions: ΔH is NEGATIVE (energy is released) Example: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = -890 kJ/mol For endothermic reactions: ΔH is POSITIVE (energy is absorbed) Example: CaCO₃(s) → CaO(s) + CO₂(g) ΔH = +178 kJ/mol Energy Profile Diagrams - Exothermic An energy profile diagram shows the energy changes during a reaction: Feature Exothermic Reaction Starting energy Reactants start at higher energy level Ending energy Products end at lower energy level Energy released Difference = ΔH (negative value) Curve shape Goes up to activation energy, then DOWN to products Visual summary: Reactants are HIGHER than products (energy goes DOWN = released) Energy Profile Diagrams - Endothermic Feature Endothermic Reaction Starting energy Reactants start at lower energy level Ending energy Products end at higher energy level Energy absorbed Difference = ΔH (positive value) Curve shape Goes up to activation energy, stays UP at products Visual summary: Reactants are LOWER than products (energy goes UP = absorbed) What is Activation Energy (Ea)?
Activation energy (Ea) is the minimum energy that colliding particles must have for a reaction to occur. Think of it as an "energy barrier" that must be overcome: Reactants need enough energy to break existing bonds Only collisions with energy ≥ Ea will result in a reaction Lower Ea = easier for reaction to happen = faster reaction Higher Ea = harder for reaction to happen = slower reaction Activation Energy on Energy Profile Diagrams On an energy profile diagram, activation energy is shown as: Ea = Energy difference from reactants to the peak (highest point) Key points: ALL reactions (exothermic and endothermic) have an activation energy The peak represents the transition state where bonds are breaking and forming Even exothermic reactions need initial energy input to start Example: Wood needs a match (activation energy) before it burns (exothermic) Comparing Exothermic and Endothermic Diagrams Aspect Exothermic Endothermic Energy level Products LOWER than reactants Products HIGHER than reactants ΔH sign Negative (-) Positive (+) Energy flow Released to surroundings Absorbed from surroundings Temperature change Increases (gets hot) Decreases (gets cold) Activation energy Present (to peak) Present (to peak) Effect of Catalysts on Activation Energy A catalyst provides an alternative reaction pathway with a LOWER activation energy.
Effect on energy profile diagram: The peak is LOWER with a catalyst Ea decreases (smaller energy barrier) ΔH remains THE SAME (same reactants, same products) More particles now have enough energy to react Reaction rate increases Important: Catalysts speed up reactions by lowering Ea, but do NOT change the overall energy change (ΔH) of the reaction.
Real-Life Applications Exothermic reactions used for: Hand warmers - contain iron powder that oxidizes (releases heat) Self-heating cans - for coffee/soup (calcium oxide + water) Fuels - combustion releases energy for heating and transport Endothermic reactions used for: Sports injury packs - ammonium nitrate dissolves in water (absorbs heat = gets cold) Thermal decomposition - making quicklime (CaO) in industry Key Points Summary Exothermic: ΔH negative, products lower energy, temperature rises Endothermic: ΔH positive, products higher energy, temperature falls Activation energy (Ea): minimum energy needed for reaction to occur Energy profile diagrams show energy changes during reactions Peak = transition state, height from reactants = Ea Catalysts lower Ea b…
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