AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    The energy change of reactions (HT only): energy must be supplied to break bonds in the reactants

    Chemical bonds hold atoms together, so pulling them apart requires work against the attractive forces.

    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Chemical bonds hold atoms together, so pulling them apart requires work against the attractive forces. In a reaction, the first stage is breaking the bonds in the reactant molecules, and this step is endothermic: energy is taken in from the surroundings. For example, splitting H₂ into two H atoms needs energy input, and breaking O₂ into two O atoms also needs energy input. The total energy needed equals the sum of the bond energies of all bonds broken in the reactants. This energy is not lost; it is stored in the separated atoms. Only after this input can new bonds form in the products. Understanding this clause lets you calculate the overall energy change of a reaction by comparing energy in (breaking) with energy out (forming).

    What earns the marks

    • Breaking a chemical bond requires an input of energy because work must be done against the electrostatic attraction holding the atoms together.
    • The energy supplied to break bonds in the reactants is endothermic and is taken in from the surroundings, so the reactant mixture cools if no other process occurs.
    • The total energy required to break all bonds in the reactants equals the sum of the bond energies for every bond broken, using values in kJ mol⁻¹.
    • Bond breaking produces separated atoms or fragments with higher potential energy than the bonded reactants, so the process is not spontaneous without an energy source.
    • In an overall calculation, energy in from bond breaking is compared with energy out from bond formation to decide whether the reaction is exothermic or endothermic.

    Common mistakes and how to put them right

    • Thinking that breaking bonds releases energy because reactions can feel hot.

      Put it right by stating that bond breaking always takes energy in, while bond formation releases energy.

    • Adding bond energies for bonds in the products when calculating energy supplied to break reactant bonds.

      Put it right by summing only the bonds broken in the reactants.

    • Writing that energy is destroyed or created during bond breaking.

      Put it right by saying energy is transferred from the surroundings and stored in the separated atoms.

    Exam tips

    • Underline the words reactants and supplied in the question so you do not accidentally use product bond energies.
    • When calculating, list every bond broken in the reactants and sum their bond energies before doing any subtraction.
    • Use the phrase energy is taken in from the surroundings when explaining why bond breaking is endothermic.
    • Questions requiring you to use bond energies to calculate energy supplied are only found on Higher Tier papers.

    Try the lesson

    The start of the The energy change of reactions (HT only): energy must be supplied to break bonds in the reactants lesson, made from checked teaching notes for this specification point.

    Have a go first

    In a chemical reaction, what must happen to the bonds in the reactant molecules before new bonds can form?

    Listen

    Chemical bonds hold atoms together, so pulling them apart requires work against the attractive forces. In a reaction, the first stage is breaking the bonds in the reactant molecules, and this step is endothermic: energy is taken in from the surroundings. For example, splitting H₂ into two H atoms needs energy input, and breaking O₂ into two O atoms also needs energy input. The total energy needed equals the sum of the bond energies of all bonds broken in the reactants. This energy is not lost; it is stored in the separated atoms. Only after this input can new bonds form in the products. Understanding this clause lets you calculate the overall energy change of a reaction by comparing energy in (breaking) with energy out (forming). A strong answer earns marks for these points. Breaking a chemical bond requires an input of energy because work must be done against the electrostatic attraction holding the atoms together. The energy supplied to break bonds in the reactants is endothermic and is taken in from the surroundings, so the reactant mixture cools if no other process occurs. The total energy required to break all bonds in the reactants equals the sum of the bond energies for every bond broken, using values in kJ mol⁻¹. Bond breaking produces separated atoms or fragments with higher potential energy than the bonded reactants, so the process is not spontaneous without an energy source. A common slip: Thinking that breaking bonds releases energy because reactions can feel hot. Instead, stating that bond breaking always takes energy in, while bond formation releases energy. Exam tip: Underline the words reactants and supplied in the question so you do not accidentally use product bond energies.

    See it

    A flowchart running from top to bottom. The first box reads 'Bonds in reactants' and points to a second box, 'Energy supplied to break bonds'. From this second box four arrows lead to: 'Energy taken in from the surroundings', 'Endothermic step', 'Separated atoms with higher potential energy', and 'Total energy needed equals sum of bond energies of bonds broken'. The box 'Separated atoms with higher potential energy' then points to a final box, 'New bonds form in the products'.
    It shows that energy must be supplied to break bonds in the reactants, which is endothermic, takes energy in from the surroundings and leaves separated atoms with higher potential energy before new bonds form in the products.

    Key words

    Say what each word means before you turn the card.

    Bond breaking

    Try answering aloud or in your head before turning the card.

    Energy transfer between surroundings and reacting particles

    Try answering aloud or in your head before turning the card.

    Conservation of energy in chemical reactions

    Try answering aloud or in your head before turning the card.

    See every definition
    Bond breaking
    Energy must be supplied to break bonds in the reactants, and this step is endothermic because energy is taken in from the surroundings.
    Energy transfer between surroundings and reacting particles
    Energy is taken in from the surroundings to break bonds, and this energy is not lost but stored in the separated atoms.
    Conservation of energy in chemical reactions
    Energy is not lost when bonds break; it is stored in the separated atoms, and overall change is found by comparing energy in with energy out.

    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 flowchart running from top to bottom. The first box reads 'Bonds in reactants' and points to a second box, 'Energy supplied to break bonds'. From this second box four arrows lead to: 'Energy taken in from the surroundings', 'Endothermic step', 'Separated atoms with higher potential energy', and 'Total energy needed equals sum of bond energies of bonds broken'. The box 'Separated atoms with higher potential energy' then points to a final box, 'New bonds form in the products'.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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