Energetics – Exothermic and Endothermic Reactions
Understand how energy is transferred during chemical reactions
Energy in Reactions
Release and Absorption
Exothermic reactions release heat energy to the surroundings. When these reactions occur, the temperature of the surroundings increases. The enthalpy change (ΔH) is negative because the products contain less energy than the reactants.
Common examples include combustion (burning fuels like methane or wood), neutralisation (acid + alkali reactions), and oxidation reactions like rusting of iron. Hand warmers use exothermic reactions—when calcium oxide reacts with water, it releases heat.
Endothermic reactions absorb heat energy from the surroundings. The temperature of the surroundings decreases during these reactions. The enthalpy change (ΔH) is positive because products contain more energy than reactants.
Examples include thermal decomposition (breaking down compounds by heating), photosynthesis (plants absorbing light energy), and dissolving ammonium nitrate in water. Sports cooling packs use this principle—when ammonium nitrate dissolves in water, it absorbs heat, cooling the injured area.
Energy diagrams show the energy levels of reactants and products. In exothermic reactions, the products line is lower than reactants. In endothermic reactions, products are higher than reactants.
Activation energy (Ea) is the minimum energy that colliding particles need for a reaction to occur. It appears as a peak or "hump" on energy diagrams. Even exothermic reactions need activation energy to get started—like striking a match.
Key Exam Point
A catalyst lowers the activation energy by providing an alternative reaction pathway. It speeds up reactions without being consumed and does not change the overall energy released or absorbed (ΔH stays the same).
Exothermic Reaction
Energy is released to surroundings. Products have less energy than reactants. ΔH is negative.
Question:
When ammonium chloride is dissolved in water, the beaker feels cold. Is this reaction exothermic or endothermic? Explain your answer.
Answer:
This is an endothermic reaction.
The beaker feels cold because the dissolving process absorbs heat energy from the surroundings (including the beaker and your hand).
In endothermic reactions, energy is transferred from the surroundings into the reaction, causing the temperature of the surroundings to decrease. The ΔH value for this reaction is positive.
In an exothermic reaction, energy is: