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The Energy Change of Reactions (AQA GCSE Chemistry: Combined Science)
Revision Note
Bond energies
Higher Tier Only
Energy changes
- Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break existing bonds and the energy released when the new bonds are formed
- Bond breaking is always an endothermic process as energy needs to be taken in from the surroundings to break the chemical bonds
- Bond making is always an exothermic process as energy is transferred to the surroundings as the new bond is formed
Endothermic reactions
- If more energy is absorbed than is released, this reaction is endothermic
- More energy is required to break the bonds than is released from making the new bonds
- The change in energy is positive since the products have more energy than the reactants
- Therefore an endothermic reaction has a positive change in energy
Bond breaking
Energy must be absorbed from the surroundings for bonds to be broken
Exothermic reactions
- 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 products have less energy than the reactants
- Therefore an exothermic reaction has a negative change in energy
Bond making
Making new bonds gives off heat from the reaction to the surroundings
Examiner 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–H + Cl–Cl ⟶ H–Cl H–Cl
The bond energies are given in the table below.
| Bond | Energy (kJ) |
| H–H | 436 |
| Cl–Cl | 242 |
| H–Cl | 431 |
Calculate the overall energy change for this reaction and use this value to explain whether the reaction is exothermic or endothermic.
Answer:
- Calculate the energy in
- 436 + 242 = 678 (kJ)
- Calculate the energy out
- 2 x 431 = 862 (kJ)
- Calculate the energy change
- 678 - 862 = –184 (kJ)
- Since the energy change is a negative number, energy is being released (to the surroundings)
- Therefore, the reaction is exothermic
Worked example
Hydrogen reacts with iodine to form hydrogen iodide.
H2 + I2 ⟶ 2HI
The relevant bond energies are shown in the table below.
| Bond | Energy (kJ) |
| H–I | 366 > 295 |
| H–H | 436 |
| I–I | 151 |
Calculate the overall energy change for this reaction and use this value to explain why the reaction is endothermic.
Answer:
- Calculate the energy in
- 436 + 151 = 587 (kJ)
- Calculate the energy out
- 2 x 295 = 590 (kJ)
- Calculate the energy change
- 587 - 590 = -3 (kJ)
- The reaction is exothermic because:
- More energy is released than taken in
Worked example
Hydrogen bromide decomposes to form hydrogen and bromine:
2HBr ⟶ H2 + Br2
The overall energy change for this reaction is +103 kJ.
The relevant bond energies are shown in the table below.
| Bond | Energy (kJ) |
| H–Br | 366 |
| Br–Br | |
| H–H | 436 |
Calculate the bond energy of the Br–Br bond.
Answer:
- Calculate the energy in
- 2 x 366 = 732 (kJ)
- State the energy out
- 436 + Br–Br
- Overall energy change = energy in - energy out
- +103 = 732 - (436 + Br–Br)
- +103 = 732 - 436 - Br–Br
- Calculate the bond energy of the Br–Br bond
- Br–Br = 732 - 436 - 103
- Br–Br = +193 (kJ)
Examiner 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. Don't forget to take into account the balancing numbers when working out how many of each type of bond is being broken/formed.
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