AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The energy change of reactions (HT only): energy is released when bonds in the products are formed.
After reactant bonds break, atoms rearrange and new bonds form in the products.
Study notes
From the checked teaching notes for this specification point.
The idea
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.
What earns the marks
- 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.
Common mistakes and how to put them right
Saying energy is taken in when bonds form because atoms need to collide.
Put it right by stating that bond formation releases energy as atoms become more stable.
Using reactant bond energies when calculating energy released.
Put it right by summing only the bonds formed in the products.
Forgetting that bond formation releases energy to the surroundings, not into the bonds themselves.
Put it right by describing energy transfer to the surroundings.
Exam tips
- 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.
Try the lesson
The start of the The energy change of reactions (HT only): energy is released when bonds in the products are formed. lesson, made from checked teaching notes for this specification point.
Have a go first
In a chemical reaction, new bonds are made between atoms to form the products. What happens to energy when these bonds in the products are formed?
Listen
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. A strong answer earns marks for these points. 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. A common slip: Saying energy is taken in when bonds form because atoms need to collide. Instead, stating that bond formation releases energy as atoms become more stable. Exam tip: Highlight bonds formed in the products and sum their bond energies separately from bonds broken.
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