Skip to content
Skip to content
Teachers
Go to pupils
New
New

    Breaking and making bonds: using bond energies

    I can describe how bond making and breaking use or release energy, and use bond energies to calculate whether a reaction is exothermic or endothermic.

    New
    New

      Breaking and making bonds: using bond energies

      I can describe how bond making and breaking use or release energy, and use bond energies to calculate whether a reaction is exothermic or endothermic.

      Lesson details

      Key learning points

      1. The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.
      2. The difference between the energy needed to break bonds and the energy released is the overall energy change.

      Keywords

      • Endothermic - An endothermic chemical reaction is a type of reaction in which energy is transferred from the surroundings to the products, e.g. photosynthesis.

      • Exothermic - An exothermic chemical reaction is a type of reaction in which energy is transferred from the reactants to the surroundings, e.g. combustion.

      • Activation energy - The minimum energy that the particles must have in order to react is known as the activation energy.

      • Bond energy - The energy required to break a covalent bond, or released when making a covalent bond. is known as the bond energy.

      Common misconception

      Pupils often make mistakes when identifying the different types of bonds and the number of each in a molecule. They may also reverse the calculation of the overall energy change, as make - break.

      Molecules can be complex and should be drawn to show all the bonds. Emphasis that double and triple covalent bonds are different to single bonds. Pupils should be instructed to consider whether a reaction is exothermic or endothermic.

      Remind pupils how many bonds each particular atom can make e.g. carbon makes 4. Give pupils example molecules and ask them to identify the different types of bond present and count the number of each one.
      Teacher tip

      Licence

      This content is © Oak National Academy Limited (2026), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

      Lesson video

      Skip lesson video

      Loading...

      Prior knowledge starter quiz

      Download quiz pdf

      6 Questions

      Q1.
      What does an energy profile diagram show?

      Correct answer: the energy changes during a chemical reaction
      the mass changes during a reaction
      the temperature changes during a reaction

      Q2.
      What occurs during a chemical reaction?

      The state of matter is changed.
      Substances are mixed without a reaction.
      Correct answer: Atoms are rearranged and chemical bonds are broken/formed.

      Q3.
      What is an exothermic reaction?

      Correct answer: a reaction where energy is transferred from the reactants to the surrounding
      a reaction that absorbs energy from the surroundings
      a reaction that does not involve energy change

      Q4.
      What is an endothermic reaction?

      Correct answer: a reaction in which energy is transferred from the surroundings to the products
      a reaction that does not involve energy change
      a reaction that releases energy to the surroundings

      Q5.
      What is activation energy?

      the energy released during a reaction
      the energy stored in reactants
      Correct answer: the minimum energy that the particles must have in order to react

      Q6.
      In which energy profile does the blue arrow show the energy change of an exothermic reaction?

      energy profile A
      energy profile B
      energy profile C

      Assessment exit quiz

      Download quiz pdf

      6 Questions

      Q1.
      The overall energy change for an exothermic reaction is...

      Correct answer: negative, because the energy released is greater than the energy absorbed.
      positive, because the energy absorbed is greater than the energy released.
      zero, because no energy change occurs.

      Q2.
      Given the following data: reactants energy = 200 kJ, products energy = 300 kJ, what is the overall energy change? Is the reaction endothermic or exothermic?

      Correct answer: +100 kJ
      -100 kJ
      Correct answer: endothermic reaction
      exothermic reaction

      Q3.
      Given the following data: reactants energy = 400 kJ, products energy = 250 kJ, what is the overall energy change? Is the reaction endothermic or exothermic?

      +150 kJ
      Correct answer: -150 kJ
      endothermic reaction
      Correct answer: exothermic reaction

      Q4.
      During the combustion of methane, the reactants have an energy of 500 kJ and the products have an energy of 300 kJ. What is the overall energy change? Is the reaction endothermic or exothermic?

      +200 kJ, indicating an endothermic reaction
      -200 kJ, indicating an endothermic reaction
      +200 kJ, indicating an exothermic reaction
      Correct answer: -200 kJ, indicating an exothermic reaction

      Q5.
      How many C-H bonds are there in the reactants and in the products of this reaction?

      An image in a quiz
      6 C-H bonds in the reactants
      Correct answer: 5 C-H bonds in the reactants
      6 C-H bonds in the products
      Correct answer: 4 C-H bonds in the products

      Q6.
      What is the overall energy change when ethanol (C₂H₅OH) reacts to form ethene (C₂H₄) and water (H₂O)? Give your answer in kJ/mol.

      An image in a quiz
      Correct Answer: +45 kJ/mol, 45 kJ/mol, +45, 45

      Lesson appears in

      UnitChemistry / Energy changes in reactions

      AQAChemistry

      Show Higher unit

      EdexcelChemistry

      Show Higher unit

      OCRChemistry

      Show Higher unit

      UnitCombined science / Energy changes in reactions

      AQACombined science

      Show Higher unit

      EdexcelCombined science

      Show Higher unit

      OCRCombined science

      Show Higher unit