Calculating Energy Changes
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 reactions
- 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 products have more energy than the reactants
- Therefore an endothermic reaction has a positive change in energy
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 reactants have more energy than the products
- Therefore an exothermic reaction has a negative change in energy
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 that bond!
Bond Energy Calculations
- Each chemical bond has a specific bond energy associated with it
- This is the mean amount of energy required to break the bond or the mean 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:
H2 + Cl2 ⟶ 2HCl
The table below shows the bond energies. Calculate the energy change for the reaction and deduce whether it is exothermic or endothermic:
Answer
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:
Answer
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.
