Learning focus
Explain electrochemical and energetic processes using ions, electrons, balanced equations, observations, energy pathways and quantitative evidence.
Bond breaking
Energy must be supplied to separate bonded atoms. Bond breaking is therefore endothermic and contributes a positive amount to the energy calculation.
Bond making
When atoms form new bonds, energy is released. Bond making is exothermic and lowers the energy of the product system.
Net result
Delta H = total energy required to break reactant bonds – total energy released when product bonds form. A negative result means more energy was released than absorbed.
Average bond energies
Bond-energy values are averages across different compounds, so calculated enthalpies are estimates. The method is most suitable for reactions involving gaseous molecules when values are supplied.
Worked example
If breaking bonds requires 900 kJ and making bonds releases 1100 kJ, Delta H = 900 – 1100 = -200 kJ/mol.
Practical or data skill
Sort listed processes into bond breaking or bond making and assign positive or energy-releasing contributions.
Examination tip
Do not reverse the subtraction: broken minus formed.
Review questions and suggested answers
Question 1
Is bond breaking exothermic or endothermic?
Suggested answer
Endothermic.
Question 2
Write the bond-energy relationship.
Suggested answer
Delta H = sum of bond energies broken – sum of bond energies formed.
Question 3
What does a negative result mean?
Suggested answer
The reaction is exothermic.
