The energy change of reactions (HT only) — AQA GCSE Combined Science
Test yourself on The energy change of reactions (HT only) with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
The energy change of reactions (HT only) explained
This statement introduces the higher-tier treatment of energy changes during a chemical reaction.
Read the full explanation
Energy is transferred when bonds in reactants are broken and new bonds in products are formed. Breaking bonds takes in energy from the surroundings, while forming bonds releases energy. The overall energy change is the difference between energy needed to break bonds and energy released when new bonds form. If more energy is released than taken in, the reaction is exothermic and the surroundings warm up. If more energy is taken in, it is endothermic and the surroundings cool. For example, burning methane is exothermic because the bonds formed release more energy than was needed to break the bonds in methane and oxygen.
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. 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).
energy is released when bonds in the products are formed.
After reactant bonds break, atoms rearrange and new bonds form in the products. Bond formation is exothermic: as atoms come together, attractive forces pull them into a lower-energy arrangement, and surplus energy is released to the surroundings, often warming the reaction mixture. For example, when H atoms and O atoms combine to form H₂O, new O–H bonds form and energy is given out. The total energy released equals the sum of the bond energies of all bonds formed in the products. This clause is the second half of the energy balance: overall energy change = energy supplied to break reactant bonds − energy released when product bonds form. If more energy is released than supplied, the reaction is exothermic overall.
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. 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.
The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed is the overall energy change of the reaction.
The overall energy change of a chemical reaction is calculated by finding the difference between the energy required to break reactant bonds and the energy released when product bonds form. The formula used is: Overall energy change = Total energy needed to break bonds - Total energy released forming bonds. For instance, if breaking bonds requires 678 kJ/mol and forming bonds releases 862 kJ/mol, the overall change is 678 - 862 = -184 kJ/mol. A negative overall energy change indicates an exothermic reaction, as more energy is released than absorbed. A positive value indicates an endothermic reaction. These calculations use average bond energies, meaning the results are approximations. This topic is exclusively assessed at Higher Tier.
In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.
Every reaction involves breaking bonds in the reactants and forming bonds in the products. Breaking a bond requires an input of energy, while forming a bond releases energy. In an exothermic reaction, the total energy released when new bonds form is greater than the total energy needed to break the existing bonds. The overall energy change is therefore negative, transferring energy to the surroundings and warming them. For example, in methane combustion (CH₄ + 2O₂ → CO₂ + 2H₂O), the bonds formed in CO₂ and H₂O release more energy than is needed to break the bonds in CH₄ and O₂. Higher Tier students must be able to explain this difference between energy released and energy absorbed as the overall energy change of the reaction.
In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.
In any reaction, energy must be supplied to break bonds in the reactants, and energy is released when new bonds form in the products. In an endothermic reaction, the total energy needed to break the existing bonds is greater than the total energy released when the new bonds form. The overall energy change is therefore positive, taking in energy from the surroundings and cooling them. For example, in the thermal decomposition of calcium carbonate (CaCO₃ → CaO + CO₂), the energy needed to break bonds in CaCO₃ is greater than the energy released when bonds form in CaO and CO₂. Higher Tier students must explain this difference between energy absorbed and energy released as the overall energy change.
Students should be able to calculate the energy transferred in chemical reactions using bond energies supplied.
Bond breaking takes in energy; bond making releases it. To find the overall energy transferred, add the energies of all bonds broken in the reactants, add the energies of all bonds made in the products, then subtract: energy transferred = energy of bonds broken − energy of bonds made. A negative result means more energy is released than absorbed, so the reaction is exothermic. For example, in H₂ + Cl₂ → 2HCl, breaking H–H and Cl–Cl absorbs energy, while forming two H–Cl bonds releases energy; the difference gives the overall change. Higher Tier students must draw displayed formulae first so no bond is missed, and remember that a bond appearing twice must be counted twice.
Your focus
- Describe bond breaking as endothermic and bond forming as exothermic.
- Calculate the overall energy change of a reaction from bond energies.
- Classify a reaction as exothermic or endothermic from its overall energy change and the temperature change of the surroundings.
Show all 24 objectives
- Describe bond breaking in reactants as a process that requires an input of energy from the surroundings.
- Calculate the total energy supplied to break all bonds in the reactants using bond energy values.
- Explain why separated atoms have more stored energy than the bonded reactants.
- Describe bond formation in products as a process that releases energy to the surroundings.
- Calculate the total energy released when all bonds in the products are formed using bond energy values.
- Use energy in and energy out values to determine whether a reaction is exothermic or endothermic overall.
- Calculate the total energy required to break all bonds in the reactants using average bond energies.
- Calculate the total energy released when all bonds in the products are formed.
- Explain the energy changes in a reaction in terms of bond breaking and bond making.
- Calculate the overall energy change of a reaction using the sum of bond energies broken and formed.
- Interpret the sign of the overall energy change to classify a reaction as exothermic or endothermic.
- Understand that calculations using average bond energies provide approximate values for the overall energy change.
- State that breaking bonds absorbs energy and forming bonds releases energy.
- Compare total energy released from bond formation with total energy needed for bond breaking in an exothermic reaction.
- Relate the greater energy release to a negative overall energy change and a temperature rise in the surroundings.
- State that breaking bonds absorbs energy and forming bonds releases energy.
- Compare total energy needed for bond breaking with total energy released from bond formation in an endothermic reaction.
- Relate the greater energy absorption to a positive overall energy change and a temperature fall in the surroundings.
- Calculate the total energy absorbed when bonds in the reactants are broken.
- Calculate the total energy released when bonds in the products are formed.
- Use the difference between these totals to determine the overall energy transferred and classify the reaction as exothermic or endothermic.
The energy change of reactions (HT only) exam tips
Marking Points
- Bond breaking is endothermic because it takes in energy from the surroundings.
- Bond forming is exothermic because it releases energy to the surroundings.
- The overall energy change is calculated from the energy required to break bonds in reactants minus the energy released when bonds form in products.
- An exothermic reaction releases more energy than it takes in, so the surroundings warm up.
- An endothermic reaction takes in more energy than it releases, so the surroundings cool down.
- Energy is conserved overall; the energy transferred is not created or destroyed but moved between the reaction and its surroundings.
- 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.
- Forming a chemical bond releases energy because the atoms move to a lower-energy, more stable arrangement as attractive forces pull them together.
- Bond formation in the products is exothermic and transfers energy to the surroundings, which may raise the temperature of the reaction mixture.
- The total energy released when bonds form equals the sum of the bond energies for every bond made in the products, using values in kJ mol⁻¹.
- In an overall calculation, energy released from bond formation is subtracted from energy supplied for bond breaking to find the net energy change.
- If the energy released when product bonds form is greater than the energy supplied to break reactant bonds, the reaction is exothermic overall.
- 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.
- Calculate the total energy required to break all bonds in the reactants (energy in).
- Calculate the total energy released when all bonds in the products are formed (energy out).
- Subtract the total energy released from the total energy required to find the overall energy change.
- Deduce that a negative overall energy change corresponds to an exothermic reaction.
- Deduce that a positive overall energy change corresponds to an endothermic reaction.
- Breaking bonds in reactants requires an input of energy from the surroundings.
- Forming bonds in products releases energy to the surroundings.
- In an exothermic reaction, energy released from bond formation is greater than energy needed for bond breaking.
- The overall energy change is negative because more energy is released than absorbed.
- Energy is transferred to the surroundings, so the temperature of the surroundings typically rises.
- Higher Tier candidates must compare total energy released versus total energy needed, not just one bond.
- Breaking bonds in reactants requires an input of energy from the surroundings.
- Forming bonds in products releases energy to the surroundings.
- In an endothermic reaction, energy needed to break existing bonds is greater than energy released from forming new bonds.
- The overall energy change is positive because more energy is absorbed than released.
- Energy is taken in from the surroundings, so the temperature of the surroundings typically falls.
- Higher Tier candidates must compare total energy needed versus total energy released, not just one bond.
- Identify every bond broken in the reactants and every bond formed in the products from supplied displayed formulae or structural formulae.
- Sum the bond energies for all bonds broken, multiplying each bond energy by how many times that bond appears.
- Sum the bond energies for all bonds made, again multiplying by the number of each bond formed.
- Subtract the total energy of bonds made from the total energy of bonds broken to obtain the overall energy transferred.
- Interpret the sign of the answer: a negative value indicates an exothermic reaction and a positive value indicates an endothermic reaction.
- Higher Tier candidates must correctly apply bond energy calculations to determine the overall energy change.
Examiner Tips
- 💡Write the bond energy calculation clearly, showing energy taken in and energy released before subtracting.
- 💡Use the sign and direction of temperature change to justify whether a reaction is exothermic or endothermic.
- 💡Check that every bond broken and every bond formed has been counted, including coefficients in the balanced equation.
- 💡Remember that calculating overall energy changes using bond energies is assessed at Higher Tier only.
- 💡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.
- 💡Highlight bonds formed in the products and sum their bond energies separately from bonds broken.
- 💡Use the equation energy change = energy in (breaking) − energy out (forming) and keep the signs consistent.
- 💡Check whether the question asks for energy released or overall energy change; they are not the same quantity.
- 💡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.
- 💡Write down the formula 'Energy change = Energy in - Energy out' before substituting your calculated values to avoid reversing the subtraction.
- 💡Check if the question asks you to state whether the reaction is exothermic or endothermic based on your calculated sign.
- 💡As this is Higher Tier content, be prepared to rearrange the formula to find an unknown bond energy if the overall energy change is given.
- 💡Use the phrase 'energy released from forming bonds is greater than energy needed to break bonds' to make the comparison explicit.
- 💡Link the bond-energy comparison to the sign of the overall energy change and to the observed temperature rise of the surroundings.
- 💡As a Higher Tier student, when given bond energies, calculate total energy absorbed and total energy released separately before comparing them.
- 💡Use the phrase 'energy needed to break bonds is greater than energy released from forming bonds' to make the comparison explicit.
- 💡Link the bond-energy comparison to the sign of the overall energy change and to the observed temperature fall of the surroundings.
- 💡As a Higher Tier student, when given bond energies, calculate total energy absorbed and total energy released separately before comparing them.
- 💡Write out a clear table of bonds broken and bonds made before doing any arithmetic, so nothing is missed.
- 💡Show each multiplication and the two totals, because method marks are often available even if the final value is wrong.
- 💡As a Higher Tier student, check the sign and unit of your final answer, and state whether the reaction is exothermic or endothermic in the context of the question.
Common Mistakes
- Saying that breaking bonds releases energy: breaking bonds always takes in energy, while forming bonds releases energy.
- Reversing the subtraction: overall energy change is energy in for bond breaking minus energy out for bond forming, not the other way round.
- Assuming that an exothermic reaction needs no activation energy: it still needs an initial energy input to break bonds before new bonds form.
- Thinking that breaking bonds releases energy because reactions can feel hot: correct this 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: correct this by summing only the bonds broken in the reactants.
- Writing that energy is destroyed or created during bond breaking: correct this by saying energy is transferred from the surroundings and stored in the separated atoms.
- Saying energy is taken in when bonds form because atoms need to collide: correct this by stating that bond formation releases energy as atoms become more stable.
- Using reactant bond energies when calculating energy released: correct this by summing only the bonds formed in the products.
- Forgetting that bond formation releases energy to the surroundings, not into the bonds themselves: correct this by describing energy transfer to the surroundings.
- Failing to account for the balancing numbers (stoichiometry) in the chemical equation. Correction: Always 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. Correction: Remember that 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. Correction: Draw the full displayed formula to accurately count all bonds (CH₄ has four C-H bonds).
- Subtracting the energy required from the energy released (Energy out - Energy in). Correction: Always use the formula: Overall energy change = Energy required to break bonds - Energy released forming bonds.
- Ignoring the negative sign in the final answer for an exothermic reaction. Correction: The sign is crucial; always include the negative sign to denote an exothermic overall energy change.
- Assuming that average bond energies give an exact value for the enthalpy change of a specific reaction. Correction: Recognise that average bond energies are averaged across different molecules, so the calculated energy change is an approximation.
- Thinking that exothermic reactions release energy because bonds break; correction: breaking bonds absorbs energy, and it is bond formation that releases energy.
- Comparing only one bond broken with one bond formed; correction: compare the total energy needed to break all reactant bonds with the total energy released forming all product bonds.
- Stating that exothermic reactions do not need any energy input; correction: an initial energy input is needed to break bonds, but overall more energy is released than absorbed.
- Thinking that endothermic reactions release energy because new bonds form; correction: bond formation releases energy, but in endothermic reactions the energy needed to break bonds is greater overall.
- Comparing only one bond broken with one bond formed; correction: compare the total energy needed to break all reactant bonds with the total energy released forming all product bonds.
- Stating that endothermic reactions absorb no energy at all; correction: they absorb more energy than they release, giving a positive overall energy change.
- Adding bonds made to bonds broken instead of subtracting; correct by always using energy transferred = bonds broken − bonds made.
- Counting a bond only once when it appears multiple times, such as two H–Cl bonds in 2HCl; correct by multiplying each bond energy by its number of occurrences.
- Reversing the subtraction, for example bonds made − bonds broken; correct by checking that an exothermic reaction gives a negative value.