Energy changes — GCSE Chemistry Revision
Everything you need to revise energy changes for GCSE Chemistry: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Exothermic and Endothermic Reactions
Chemical reactions involve energy changes. Exothermic reactions release energy into the surroundings, usually as heat, causing the temperature of the surroundings to rise. Examples include combustion and neutralisation. Endothermic reactions absorb energy from the surroundings, typically as heat, causing the temperature of the surroundings to fall. Photosynthesis is an important endothermic process. These energy changes are crucial for understanding how reactions proceed and their applications in everyday life and industry. You can often feel the temperature change directly.
Reaction Profiles and Activation Energy
Reaction profiles are diagrams showing how energy changes during a chemical reaction. They plot the energy of reactants and products against the reaction pathway. The activation energy is the minimum amount of energy particles must have to react when they collide. In exothermic reactions, the products have lower energy than the reactants, releasing energy. In endothermic reactions, products have higher energy than reactants, absorbing energy. The 'hump' on the profile represents the activation energy barrier that must be overcome for the reaction to occur, regardless of whether it's exothermic or endothermic.
Bond Breaking and Bond Making
Energy changes in chemical reactions are directly linked to the breaking and making of chemical bonds. Breaking bonds always requires an input of energy – it's an endothermic process. Making bonds always releases energy – it's an exothermic process. The overall energy change of a reaction is the difference between the energy required to break bonds in the reactants and the energy released when new bonds are formed in the products. If more energy is released than absorbed, the reaction is exothermic. If more energy is absorbed than released, it's endothermic.
Calculating Energy Changes using Bond Energies
Bond energy is the amount of energy required to break one mole of a specific type of bond, or the energy released when one mole of that bond is formed. These values are usually given in kJ/mol. To calculate the overall energy change (enthalpy change, ΔH) for a reaction, sum the energy needed to break all bonds in the reactants (positive value) and subtract the sum of energy released when forming all bonds in the products (negative value). A negative ΔH indicates an exothermic reaction; a positive ΔH indicates an endothermic reaction.
Key facts
- Exothermic reactions release energy (ΔH < 0).|Endothermic reactions absorb energy (ΔH > 0).|Activation energy is needed to start any reaction.|Breaking bonds requires energy (endothermic).|Making bonds releases energy (exothermic).|Overall energy change = energy in (bonds broken) - energy out (bonds made).
Key terms
- Exothermic reaction::A reaction that releases energy to the surroundings, usually as heat, causing the temperature to rise.|Endothermic reaction::A reaction that absorbs energy from the surroundings, usually as heat, causing the temperature to fall.|Activation energy::The minimum amount of energy that reacting particles must possess to collide successfully and react.|Reaction profile::A diagram showing the energy changes of reactants and products during the course of a chemical reaction.|Bond energy::The energy required to break one mole of a specific bond, or released when one mole of that bond forms.|Enthalpy change (ΔH)::The overall energy change of a reaction, calculated as energy absorbed minus energy released.
Common mistakes
- Confusing 'releases energy' with 'absorbs energy' when describing exothermic and endothermic reactions.|Forgetting that activation energy is required for both exothermic and endothermic reactions.|Mixing up whether bond breaking 'releases' or 'requires' energy – breaking always requires energy.|Not balancing the chemical equation correctly before calculating bond energy changes.
Exam tips
- Always draw a reaction profile diagram for a 4-mark question on exothermic/endothermic reactions.|Remember to include units (kJ/mol) for bond energies and energy changes in your answers.|Clearly state whether a reaction is exothermic (ΔH is negative) or endothermic (ΔH is positive).|Practice calculating bond energy changes for common molecules like H2, O2, H2O, CH4, CO2.
Quick quiz
1. Which statement best describes an exothermic reaction?
- It absorbs energy from the surroundings.
- It releases energy into the surroundings.
- It has products with higher energy than reactants.
- It always requires a catalyst.
Show answer
It releases energy into the surroundings. — Exothermic reactions are defined by the release of energy to the surroundings, typically as heat, causing a temperature increase.
2. What is represented by the 'hump' on a reaction profile diagram?
- The overall energy change of the reaction.
- The energy of the products.
- The activation energy.
- The energy of the reactants.
Show answer
The activation energy. — The 'hump' on a reaction profile diagram illustrates the activation energy, which is the minimum energy required for the reaction to start.
3. What happens to energy when chemical bonds are broken?
- Energy is released.
- Energy is absorbed.
- Energy remains unchanged.
- Energy is converted to matter.
Show answer
Energy is absorbed. — Breaking chemical bonds always requires an input of energy; it is an endothermic process.
4. If the products of a reaction have higher energy than the reactants, what type of reaction is it?
- Exothermic
- Endothermic
- Combustion
- Neutralisation
Show answer
Endothermic — In an endothermic reaction, energy is absorbed, meaning the products end up with more energy than the initial reactants.
5. A reaction has a ΔH value of -150 kJ/mol. What does this indicate?
- It is an endothermic reaction.
- It is an exothermic reaction.
- It requires a catalyst.
- It involves bond breaking only.
Show answer
It is an exothermic reaction. — A negative value for the enthalpy change (ΔH) signifies that energy has been released, characteristic of an exothermic reaction.
Exam-style questions
Draw a reaction profile diagram for an endothermic reaction. Label the reactants, products, activation energy, and overall energy change (ΔH). [4 marks]
Show mark scheme
- Correct shape for endothermic (products higher than reactants).
- Reactants and products labelled correctly on energy axis.
- Activation energy labelled correctly (from reactants to peak).
- Overall energy change (ΔH) labelled correctly (from reactants to products).
Explain, in terms of bond breaking and bond making, why a reaction might be exothermic. [3 marks]
Show mark scheme
- Bond breaking requires energy (is endothermic).
- Bond making releases energy (is exothermic).
- A reaction is exothermic if more energy is released by making new bonds than is absorbed by breaking old bonds.
Consider the reaction: H₂(g) + Cl₂(g) → 2HCl(g). Use the bond energies provided to calculate the overall energy change (ΔH) for this reaction. Bond energies: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol. [4 marks]
Show mark scheme
- Energy absorbed (bonds broken) = (436 + 242) = 678 kJ/mol.
- Energy released (bonds made) = (2 x 431) = 862 kJ/mol.
- Overall energy change ΔH = Energy absorbed - Energy released.
- ΔH = 678 - 862 = -184 kJ/mol.
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