Introduction to Energy Changes
In every chemical reaction, energy is either absorbed from or released to the surroundings. This is known as an energy change. In the Singapore O-Level Pure Chemistry syllabus (Syllabus 6092), understanding the distinction between exothermic and endothermic reactions is crucial for scoring in both Paper 1 (MCQ) and Paper 2 (Structured Questions).
Exothermic Reactions: Releasing Energy
An exothermic reaction is one that releases heat energy to the surroundings. As a result, the temperature of the surroundings rises.
- Key Examples: Combustion of fuels, neutralisation (acid + base), respiration, and the rusting of iron.
- Energy Level: The total energy of the products is less than the total energy of the reactants.
- Enthalpy Change (ΔH): Always negative (-ΔH).
Endothermic Reactions: Absorbing Energy
An endothermic reaction is one that absorbs heat energy from the surroundings. This causes the temperature of the surroundings to drop.
- Key Examples: Photosynthesis, thermal decomposition (e.g., heating calcium carbonate), and dissolving certain salts like ammonium nitrate in water.
- Energy Level: The total energy of the products is more than the total energy of the reactants.
- Enthalpy Change (ΔH): Always positive (+ΔH).
Bond Breaking and Bond Forming
Students often get confused about why energy is absorbed or released. Remember this simple rule:
- Bond Breaking: Absorbs energy (Endothermic). Think of it like snapping a pencil — you need to put effort in.
- Bond Forming: Releases energy (Exothermic).
The overall ΔH of a reaction is calculated by: Energy taken in to break bonds - Energy given out when bonds are formed.
Energy Profile Diagrams: Must-Know for Exams
In your exams, you will frequently be asked to draw or interpret energy profile diagrams. Look out for these three components:
- Activation Energy (Ea): The minimum energy that reacting particles must possess to react. It is always a positive value, represented by the arrow from reactants to the peak of the curve.
- Enthalpy Change (ΔH): The difference in energy between reactants and products.
- Reactants vs. Products: In exothermic diagrams, products are lower than reactants. In endothermic diagrams, products are higher.
Common Exam Pitfalls
- Confusing Temperature with Energy: In an exothermic reaction, the system loses energy, but the thermometer (part of the surroundings) shows a temperature increase.
- Mislabeling Arrows: ΔH and Ea arrows must be single-headed or double-headed according to specific SEAB marking schemes. Check your latest school notes for the preferred style!
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