Revision notes · Energy changes
Exothermic and endothermic reactions
Energy transfer during exothermic and endothermic reactions4.5.1.1
Definition: An exothermic reaction transfers energy to the surroundings, raising the surrounding temperature, whereas an endothermic reaction takes in energy from the surroundings, lowering the surrounding temperature.
Chemical reactions involve an energy transfer between the reacting chemicals and their surroundings.
| Exothermic | Endothermic | |
|---|---|---|
| Energy | Transferred to the surroundings — temperature increases | Taken in from the surroundings — temperature decreases |
| Examples | Combustion, neutralisation, oxidation | Thermal decomposition; citric acid + sodium hydrogencarbonate |
| Everyday use | Self-heating cans, hand warmers | Instant cold packs |
⚠️ Common mistake: Thinking that an exothermic reaction causes a temperature drop because it 'loses' energy, when in fact the measured temperature increases.
🧠 Remember: EXothermic = energy EXits (temperature goes up); ENdothermic = energy ENters (temperature goes down).
Reaction profiles4.5.1.2
Definition: A reaction profile is a diagram showing the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.
Reaction profiles show how the energy of chemicals changes as reactants turn into products.
- •Activation energy is the minimum amount of energy that reacting particles must have to collide successfully and react.
- •In an exothermic reaction profile, the products are at a lower energy level than the reactants because energy has been released.
- •In an endothermic reaction profile, the products are at a higher energy level than the reactants because energy has been absorbed.
- •The peak of the curve from the reactant level to the highest point represents the activation energy.
- •The vertical difference between the reactants and products represents the overall energy change.
⚠️ Common mistake: Drawing the activation energy arrow from the product level instead of starting from the reactant level.
🧠 Remember: EXO profiles drop down to products; ENDO profiles step up to products.
The energy change of reactions4.5.1.3
Definition: The overall energy change of a reaction is the difference between the total energy needed to break bonds in reactants and the total energy released forming bonds in products.
During a reaction, energy must be supplied to break existing bonds, and energy is released when new bonds form.
- •Breaking bonds is an endothermic process because energy must be taken in from the surroundings.
- •Making bonds is an exothermic process because energy is released to the surroundings.
- •In an exothermic reaction, the energy released forming new bonds is greater than the energy needed to break existing bonds.
- •In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released forming new bonds.
- •Energy change = total energy needed to break bonds minus total energy released making bonds.
⚠️ Common mistake: Thinking bond breaking releases energy, when in fact bond breaking requires energy and bond making releases energy.
🧠 Remember: BEND MEX: Breaking is ENDothermic, Making is EXothermic.
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