How energy is transferred during chemical reactions.
Exothermic: Energy is transferred from the system to the surroundings. The surroundings get hotter.
Endothermic: Energy is transferred from the surroundings to the system. The surroundings get colder.
| Reaction Type | Energy Flow | Temp. Change | Common Examples |
|---|---|---|---|
| Exothermic | System → Surroundings | Increases | Combustion, Neutralisation |
| Endothermic | Surroundings → System | Decreases | Photosynthesis, Thermal decomposition |
Q1: Define an exothermic reaction in terms of energy transfer.
A reaction that transfers energy from the system to the surroundings as heat.
The minimum energy that reacting particles must have for a successful collision to occur.
A catalyst lowers the activation energy by providing an alternative reaction pathway. It does not change the overall energy change ($\Delta H$).
Never say a catalyst makes a reaction "more exothermic". It only affects the $E_a$ (peak), not the difference between reactants and products.
In exam answers, always use this formal phrasing:
Caveat: Bond energies are average values, so calculated energy changes are estimates, not exact values. Energy change is measured in kilojoules per mole (kJ/mol).
$$ \text{Energy Change} = \text{Bonds Broken} - \text{Bonds Formed} $$
Positive result = Endothermic | Negative result = Exothermic
Q1: Why is an overall reaction exothermic?
Because the energy released when forming new bonds is greater than the energy required to break existing bonds.
Hydrogen fuel cells are redox reactions where hydrogen is oxidised and oxygen is reduced. Unlike batteries, fuel cells do not run out; they continue to produce electricity as long as fuel (H₂) and oxygen are supplied. At the anode, hydrogen is oxidised; at the cathode, oxygen is reduced.
[Image of hydrogen fuel cell]| Hydrogen Fuel Cell Pros | Hydrogen Fuel Cell Cons |
|---|---|
| No toxic waste (only water) | Hydrogen is highly flammable/explosive |
| Continuous supply (doesn't "run out") | Hydrogen is hard to store (requires high pressure) |
Q1: Which substance is reduced in a hydrogen fuel cell?
Oxygen is reduced.
1. Don't forget that bond energies are averages—this is a common "evaluate" question.
2. Don't say endothermic reactions "don't happen naturally"; they just need continuous energy input.
3. Don't confuse the catalyst's effect; it never changes the overall energy change ($\Delta H$).
4. Don't use "Bendo Mexo" in your final exam answer; write out the full "breaking requires/making releases" sentences.