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Cambridge IGCSE Chemistry · 0620
Chapter 5: Chemical Energetics — Part 1
Topic 5.1 · Exothermic and endothermic reactions
Exothermic and Endothermic Reactions
Chemical reactions involve a transfer of thermal energy (heat) between the reaction system and its surroundings.
- Exothermic Reaction: A reaction that transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings. Common examples include combustion, neutralisation, and many oxidation reactions. Everyday applications include self-heating cans and hand warmers.
- Endothermic Reaction: A reaction that takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings. Examples include thermal decomposition and the reaction of citric acid with sodium hydrogen carbonate. Sports injury cold packs often utilise endothermic reactions.
Enthalpy Change (?H) and Activation Energy (Ea)
- Enthalpy Change (?H): This is the transfer of thermal energy during a reaction.
- For exothermic reactions, ?H is negative because energy is released (exits the system).
- For endothermic reactions, ?H is positive because energy is absorbed (enters the system).
- Activation Energy (Ea): Defined as the minimum amount of energy that colliding particles must have to react. For a reaction to occur, particles must collide 'successfully' with at least this amount of energy.
Exam Traps
- Do not reverse the signs — exothermic is negative ΔH, not positive.
- Do not confuse Ea with ΔH; Ea is the energy barrier to start the reaction, not the overall energy change.
Reaction Pathway Diagrams
These diagrams show the energy levels of reactants and products over the course of a reaction.
- Exothermic Diagram: The reactants are at a higher energy level than the products, showing that energy was released. The enthalpy change (?H) is shown by an arrow pointing downwards from the reactants to the products.
- Endothermic Diagram: The reactants are at a lower energy level than the products, showing that energy was absorbed. The enthalpy change (?H) is shown by an arrow pointing upwards from the reactants to the products.
- Activation Energy (Ea): In both diagrams, this is represented by an arrow from the energy level of the reactants up to the peak of the curve.
Exam Traps
- Do not draw ΔH from products to reactants on an exothermic diagram — energy is released, so products are lower.
- Do not label Ea as the gap between reactants and products; it is the height from reactants to the transition state peak.
Bond Breaking and Bond Making
The overall enthalpy change of a reaction is determined by the balance between breaking old bonds and forming new ones:
- Bond breaking is an endothermic process; energy must be taken in from the surroundings to break chemical bonds.
- Bond making is an exothermic process; energy is released to the surroundings when new bonds form.
- A reaction is exothermic if the energy released making bonds is greater than the energy taken in to break bonds.
- A reaction is endothermic if the energy taken in to break bonds is greater than the energy released making bonds.
Exam Traps
- Do not say bond breaking releases energy — energy must be supplied to break bonds.
- Do not say bond making absorbs energy — forming bonds releases energy to the surroundings.
Calculations using Bond Energies
The enthalpy change (?H) can be calculated using specific bond energy values (expressed in kJ/mol):
- Calculate 'Energy In': Sum the bond energies for all bonds broken in the reactants.
- Calculate 'Energy Out': Sum the bond energies for all bonds formed in the products.
- Use the Formula: ?H = Energy In - Energy Out.
(Note: Drawing out the displayed formulae of all reactants and products is essential to ensure every bond is counted correctly during calculations.)
Exam Traps
- Do not reverse the formula to energy out − energy in — this gives the wrong sign for ΔH.
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