AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    The energy change of reactions (HT only): The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies.

    Bond energies represent the average energy required to break one mole of a specific covalent bond in the gaseous state.

    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Bond energies represent the average energy required to break one mole of a specific covalent bond in the gaseous state. Energy must be supplied to break existing bonds in the reactants (an endothermic process). Conversely, energy is released when new bonds form to create products (an exothermic process). To calculate the total energy needed to break bonds, identify every reactant bond and multiply its average bond energy by its frequency. For example, in H₂ + Cl₂ → 2HCl, breaking one H–H (436 kJ/mol) and one Cl–Cl (242 kJ/mol) requires 678 kJ/mol. Forming two H–Cl bonds releases 2 × 431 = 862 kJ/mol. This quantitative analysis is assessed at Higher Tier only.

    What earns the marks

    • Identify and count all the covalent bonds present in the reactant molecules using the balanced equation.
    • Multiply the number of each type of reactant bond by its specific average bond energy and sum them to find the total energy required.
    • Identify and count all the covalent bonds present in the product molecules, accounting for the stoichiometry of the reaction.
    • Multiply the number of each type of product bond by its average bond energy and sum them to find the total energy released.
    • State that bond breaking is an endothermic process, whereas bond making is an exothermic process.

    Common mistakes and how to put them right

    • Failing to account for the balancing numbers (stoichiometry) in the chemical equation.

      Put it right: Multiply the bond energy of a molecule by its coefficient in the balanced equation (e.g., 2H₂O means 4 O-H bonds).

    • Confusing the processes of bond breaking and bond making.

      Put it right: Breaking bonds always requires energy (endothermic), while making bonds always releases energy (exothermic).

    • Miscounting the number of bonds within a single molecule, such as assuming CH₄ has only one C-H bond.

      Put it right: Draw the full displayed formula to accurately count all bonds (CH₄ has four C-H bonds).

    Exam tips

    • Always sketch the displayed structures of all reactants and products to ensure you do not miss any hidden bonds, especially double bonds like O=O.
    • Organise your calculation into two distinct sections: 'Energy required to break bonds' and 'Energy released making bonds' to prevent calculation errors.
    • Remember that bond energy calculations are Higher Tier only, so expect them to be integrated with multi-step problem-solving questions.

    Try the lesson

    The start of the The energy change of reactions (HT only): The energy needed to break bonds and the energy released when bonds are formed can be calculated from bond energies. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Hydrogen reacts with chlorine to form hydrogen chloride: H₂ + Cl₂ → 2HCl. The bond energies are: H-H = 436 kJ/mol, Cl-Cl = 242 kJ/mol, H-Cl = 431 kJ/mol. What is the overall energy change for this reaction?

    Listen

    Bond energies represent the average energy required to break one mole of a specific covalent bond in the gaseous state. Energy must be supplied to break existing bonds in the reactants (an endothermic process). Conversely, energy is released when new bonds form to create products (an exothermic process). To calculate the total energy needed to break bonds, identify every reactant bond and multiply its average bond energy by its frequency. For example, in H₂ + Cl₂ → 2HCl, breaking one H–H (436 kJ/mol) and one Cl–Cl (242 kJ/mol) requires 678 kJ/mol. Forming two H–Cl bonds releases 2 × 431 = 862 kJ/mol. This quantitative analysis is assessed at Higher Tier only. A strong answer earns marks for these points. Identify and count all the covalent bonds present in the reactant molecules using the balanced equation. Multiply the number of each type of reactant bond by its specific average bond energy and sum them to find the total energy required. Identify and count all the covalent bonds present in the product molecules, accounting for the stoichiometry of the reaction. Multiply the number of each type of product bond by its average bond energy and sum them to find the total energy released. A common slip: Failing to account for the balancing numbers (stoichiometry) in the chemical equation. Instead, multiply the bond energy of a molecule by its coefficient in the balanced equation (e.g., 2H₂O means 4 O-H bonds). Exam tip: Always sketch the displayed structures of all reactants and products to ensure you do not miss any hidden bonds, especially double bonds like O=O.

    See it

    A top-down flowchart. On the left, a node labelled Reactant bonds leads down to Energy required to break bonds, which leads to two nodes: Bond breaking is endothermic, and Multiply each reactant bond by its average bond energy and sum. On the right, a node labelled Product bonds leads down to Energy released when bonds form, which leads to two nodes: Bond making is exothermic, and Multiply each product bond by its average bond energy and sum.
    It shows that breaking reactant bonds requires energy (endothermic) while forming product bonds releases energy (exothermic), and how each total is calculated from bond energies.

    Key words

    Say what each word means before you turn the card.

    bond energy

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    endothermic process

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    exothermic process

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    stoichiometry

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    See every definition
    bond energy
    The average energy required to break one mole of a specific covalent bond in the gaseous state.
    endothermic process
    A process in which energy must be supplied, such as breaking existing bonds in the reactants.
    exothermic process
    A process in which energy is released, such as when new bonds form to create products.
    stoichiometry
    The balancing numbers in the chemical equation, by which the bond energy of a molecule must be multiplied.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. On the left, a node labelled Reactant bonds leads down to Energy required to break bonds, which leads to two nodes: Bond breaking is endothermic, and Multiply each reactant bond by its average bond energy and sum. On the right, a node labelled Product bonds leads down to Energy released when bonds form, which leads to two nodes: Bond making is exothermic, and Multiply each product bond by its average bond energy and sum.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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