AQA GCSE Chemistry

Revision Notes

5.1.4 The Energy Change of Reactions

Higher Tier Only

Bond Energies

  • Energy is needed to break bonds which is absorbed from the reaction surroundings, so bond breaking is an endothermic process
  • The opposite occurs for forming bonds as it releases energy back to the surroundings in an exothermic process
  • Both processes occur in the same chemical reaction, for example, in the production of ammonia:

N2 + 3H2 ⟶ 2NH3

  • The bonds in the N-N and H-H molecules must be broken which requires energy while the bonds in the NH3 molecule are formed which releases energy
  • Most reactions occur in a number of steps including steps that are exothermic and steps that are endothermic
  • Whether a reaction is overall endothermic or exothermic depends on the difference between the sum of the exothermic steps and the sum of the endothermic steps

Endothermic

  • If more energy is absorbed than is released, this reaction is endothermic
  • More energy is required to break the bonds than that gained from making the new bonds
  • The change in energy is positive since the reactants have more energy than the products
  • Therefore an endothermic reaction has a positive change in energy

Bond breaking endothermic reaction, IGCSE & GCSE Chemistry revision notes

Energy must be absorbed from the surroundings for bonds to be broken 

Exothermic

  • If more energy is released than is absorbed, then the reaction is exothermic
  • More energy is released when new bonds are formed than energy required to break the bonds in the reactants
  • The change in energy is negative since the reactants have less energy than the products
  • Therefore an exothermic reaction has a negative change in energy

 

Bond making exothermic reaction, IGCSE & GCSE Chemistry revision notes

Making new bonds gives off heat from the reaction to the surroundings

Exam Tip

Remember bond breaking is ENDothermic and results in the END of the bond

Bond Energy Calculations

  • Each chemical bond has a specific bond energy associated with it
  • This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed
  • This energy can be used to calculate how much heat would be released or absorbed in a reaction
  • To do this it is necessary to know the bonds present in both the reactants and products
  • We can calculate the total change in energy for a reaction if we know the bond energies of all the species involved
  • Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’
  • Add together the bond energies for all the bonds in the products – this is the ‘energy out’
  • Calculate the energy change using the equation:

Energy change = Energy taken in – Energy given out

Worked Example

Hydrogen and chlorine react to form hydrogen chloride gas:

H+ Cl2 ⟶ 2HCl

The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic

Calculating energy Changes WE Table 1, downloadable IGCSE & GCSE Chemistry revision notes

Answer

Calculating energy Changes WE1 Answer Table, downloadable IGCSE & GCSE Chemistry revision notes

Worked Example

Hydrogen bromide decomposes to form hydrogen and bromine:

2HBr  H2  + Br2

The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic.

Calculating energy Changes WE Table 2, downloadable IGCSE & GCSE Chemistry revision notes

Answer

Calculating energy Changes WE2 Answer Table, downloadable IGCSE & GCSE Chemistry revision notes

Exam Tip

For bond energy questions, it is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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