5.1 Exothermic and endothermic reactions

2026 Syllabus Objectives

Core:

  1. State that an exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings
  2. State that an endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings
  3. Interpret reaction pathway diagrams showing exothermic and endothermic reactions

Supplement: 4. State that the transfer of thermal energy during a reaction is called the enthalpy change, ΔH\Delta H, of the reaction. ΔH\Delta H is negative for exothermic reactions and positive for endothermic reactions 5. Define activation energy, EaE_a, as the minimum energy that colliding particles must have to react 6. Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include: (a) reactants (b) products (c) enthalpy change of the reaction, ΔH\Delta H (d) activation energy, EaE_a 7. State that bond breaking is an endothermic process and bond making is an exothermic process and explain the enthalpy change of a reaction in terms of bond breaking and bond making 8. Calculate the enthalpy change of a reaction using bond energies

Energy in Chemical Reactions 🔥

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Chemical reactions involve the transformation of chemical energy. Energy comes in many forms such as heat, light, sound, electricity, and most importantly in this context, chemical energy.

Chemical energy is the energy released or absorbed during chemical reactions. Some chemical reactions are capable of releasing vast amounts of energy, while others absorb energy from their surroundings.

Real-World Examples

  • Forest fires can release overpowering and devastating waves of heat
  • Natural gas (mainly methane) is burnt under controlled conditions to produce heat in homes and industry
  • Photosynthesis in plants uses energy from sunlight to convert carbon dioxide and water into carbohydrates

Exothermic and Endothermic Processes ⚡

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Exothermic Processes

Exothermic processes are processes that release heat energy (thermal energy) to the surroundings.

Key characteristics:

  • Transfer thermal energy to the surroundings
  • Lead to an increase in the temperature of the surroundings
  • The system (reacting chemicals) loses energy
  • The surroundings become hotter

Examples of exothermic processes:

  • Burning methane on a domestic cooker
  • Burning magnesium in air
  • Condensation of steam to water
  • Neutralization reactions
  • Oxidation reactions
  • Hand warmers used in wintertime

Endothermic Processes

Endothermic processes are processes that take in heat energy (thermal energy) from the surroundings.

Key characteristics:

  • Take in thermal energy from the surroundings
  • Lead to a decrease in the temperature of the surroundings
  • The system gains energy
  • The surroundings become cooler

Examples of endothermic processes:

  • Photosynthesis in green plants
  • Evaporation of a volatile liquid
  • Reaction between nitrogen and oxygen during lightning strikes
  • Electrolysis
  • Thermal decomposition
  • Cold packs for sports injuries

System vs Surroundings 📌

  • System: the reacting chemicals themselves
  • Surroundings: anything other than the chemicals reacting (the container, air, environment)

The energy within the system comes from the chemical bonds themselves, which can be considered as tiny stores of chemical energy.

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