IB Chemistry›Reactivity 1›R1.1›R1.1.1
R1.1.1

Enthalpy & Enthalpy Changes

Exothermic vs endothermic. The direction of energy flow in chemical reactions.

Bond-Breaking & Bond-Forming

Bond breaking = endothermic (energy absorbed to overcome electrostatic attraction)

Bond forming = exothermic (energy released as new attractions form)

The net ΔH = energy to break reactant bonds + energy released forming product bonds.

Exothermic vs Endothermic

Exothermic Endothermic
Energy flow System → Surroundings Surroundings → System
ΔH sign Negative (ΔH < 0) Positive (ΔH > 0)
Temperature Rises Falls
Product stability Products more stable Reactants more stable
Examples Combustion, neutralisation Thermal decomposition, photosynthesis

Energy Profile Diagrams

Energy Profile Diagrams

Energy profile diagrams for exothermic and endothermic reactions Exothermic (ΔH < 0) Reaction Coordinate Potential Energy Reactants Products ΔH Ea Endothermic (ΔH > 0) Reaction Coordinate Potential Energy Reactants Products Ea ΔH Activation Energy (Ea) Enthalpy Change (ΔH)

📐 Reading the Diagram

  • ΔH = net vertical difference between reactants and products
  • Ea = vertical distance from reactants to the transition state (peak)
  • Exothermic: products sit lower than reactants
  • Endothermic: products sit higher than reactants

⚠️ Examiner Trap

Do not confuse ΔH with Ea on the diagram. ΔH is the difference between start and end (reactants → products). Ea is the height of the activation barrier above the reactants.

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