AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
During a chemical reaction: energy must be supplied to break bonds in the…
Full specification wording
The energy change of reactions (HT only): During a chemical reaction: energy must be supplied to break bonds in the reactants energy is released when bonds in the products are formed.
Chemical reactions involve breaking the bonds in the reactants and making new bonds in the products.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Chemical reactions involve breaking the bonds in the reactants and making new bonds in the products. Breaking a bond requires an input of energy because the atoms are pulled apart against the attraction holding them together, so bond breaking is endothermic. Forming a bond releases energy as the atoms come together and the attraction pulls them into a lower-energy arrangement, so bond making is exothermic. The overall energy change is the balance of these two processes: total energy needed to break reactant bonds minus total energy released when product bonds form. If more energy is released than supplied, the reaction is exothermic overall; if more is supplied than released, it is endothermic. Bond energies, usually quoted in kJ/mol, allow this balance to be calculated.
What earns the marks
- Bond breaking in reactants requires an input of energy and is endothermic.
- Bond formation in products releases energy to the surroundings and is exothermic.
- The overall energy change equals the energy needed to break bonds in the reactants minus the energy released when bonds form in the products.
- A negative overall energy change indicates an exothermic reaction; a positive value indicates an endothermic reaction.
- Bond energies are average values in kJ/mol and can be used to calculate the overall energy change.
- The same bonds may appear in reactants and products, so only the net difference contributes to the overall change.
Common mistakes and how to put them right
Stating that bond breaking releases energy.
Put it right: Bond breaking always requires energy input, while bond formation releases energy.
Subtracting in the wrong order, for example products minus reactants when the convention used is reactants minus products.
Put it right: Keep one consistent order and interpret the sign.
Assuming a reaction is exothermic simply because bonds form in the products, ignoring the energy needed to break the reactant bonds first.
Exam tips
- Write the calculation as a clear subtraction of total bond energies, showing the sum for bonds broken and the sum for bonds made.
- Check the sign of your final answer against the reaction type: negative for exothermic, positive for endothermic.
- Quote bond energy values with units of kJ/mol and keep significant figures consistent through the calculation.
Try the lesson
The start of the The energy change of reactions (HT only): During a chemical reaction: energy must be supplied to break bonds in the reactants energy is released when bonds in the products are formed. lesson, made from checked teaching notes for this specification point.
Have a go first
During a chemical reaction, what happens to energy when bonds in the reactants are broken?
Listen
Chemical reactions involve breaking the bonds in the reactants and making new bonds in the products. Breaking a bond requires an input of energy because the atoms are pulled apart against the attraction holding them together, so bond breaking is endothermic. Forming a bond releases energy as the atoms come together and the attraction pulls them into a lower-energy arrangement, so bond making is exothermic. The overall energy change is the balance of these two processes: total energy needed to break reactant bonds minus total energy released when product bonds form. If more energy is released than supplied, the reaction is exothermic overall; if more is supplied than released, it is endothermic. Bond energies, usually quoted in kJ/mol, allow this balance to be calculated. A strong answer earns marks for these points. Bond breaking in reactants requires an input of energy and is endothermic. Bond formation in products releases energy to the surroundings and is exothermic. The overall energy change equals the energy needed to break bonds in the reactants minus the energy released when bonds form in the products. A negative overall energy change indicates an exothermic reaction; a positive value indicates an endothermic reaction. Exam tip: Write the calculation as a clear subtraction of total bond energies, showing the sum for bonds broken and the sum for bonds made.
Key words
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Bond breaking
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Bond making
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Overall energy change
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Bond energies
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See every definition
- Bond breaking
- Energy must be supplied to break bonds in the reactants, because the atoms are pulled apart against the attraction holding them together.
- Bond making
- Energy is released when bonds in the products are formed, as the atoms come together into a lower-energy arrangement.
- Overall energy change
- The balance of the two processes: total energy needed to break reactant bonds minus total energy released when product bonds form.
- Bond energies
- Average values, usually quoted in kJ/mol, that allow the balance of bond breaking and bond making to be calculated.
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