Bond EnergiesAQA GCSE Chemistry: Revision notes
Section 1
What Is an Exothermic Reaction?
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases.
- Examples: combustion, many oxidation reactions, neutralisation
- Everyday uses: self-heating cans, hand warmers
Section 2
What Is an Endothermic Reaction?
An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.
- Examples: thermal decomposition reactions, the reaction of citric acid with sodium hydrogencarbonate
- Everyday use: some sports injury cold packs are based on endothermic reactions
Exothermic and endothermic reactions can be distinguished by measuring the temperature change of the surroundings during the reaction.
If the surroundings get warmer, the reaction is exothermic; if they get colder, it's endothermic — think of it from the surroundings' point of view.
Section 3
How Do Reaction Profiles Show Energy Changes?
A reaction profile (energy level diagram) shows the relative energies of reactants and products, and the activation energy needed to start the reaction:
- For an exothermic reaction, the products have less energy than the reactants; the diagram slopes downward overall
- For an endothermic reaction, the products have more energy than the reactants; the diagram slopes upward overall
- In both cases, there is a 'hump' up to a peak before the reactant/product line — this represents the activation energy
The activation energy is the minimum energy that particles must have when they collide in order to react. Reactions only happen when particles collide with at least this much energy.
Section 4
How Do We Calculate Energy Changes Using Bond Energies? (HT)
Every chemical bond stores a certain amount of energy. During a reaction:
- Breaking bonds in the reactants requires an input of energy (endothermic step)
- Forming new bonds in the products releases energy (exothermic step)
The overall energy change of the reaction depends on the balance between these two steps:
- If more energy is released forming new bonds than was used breaking old bonds, the overall reaction is exothermic
- If more energy is needed to break bonds than is released forming new ones, the overall reaction is endothermic
Overall energy change = (energy needed to break bonds in reactants) − (energy released forming bonds in products)
If breaking all the bonds in the reactants needs 500 kJ, but forming the new bonds in the products releases 700 kJ, the reaction releases 200 kJ overall — it is exothermic.
Do not mix up the signs — breaking bonds always takes in energy, forming bonds always releases energy, whatever the overall reaction type.
Must Know
- Exothermic reactions transfer energy to the surroundings (temperature rises); e.g. combustion, neutralisation
- Endothermic reactions take in energy from the surroundings (temperature falls); e.g. thermal decomposition
- Reaction profiles show reactant/product energy and the activation energy hump between them
- Activation energy is the minimum energy particles need when they collide to react
- (HT) Breaking bonds requires energy in; forming bonds releases energy out
- (HT) Overall energy change = energy to break bonds − energy released forming bonds; positive result overall release = exothermic, positive net input = endothermic
That's the notes covered.
Carry on to the next subtopic.