Diffusion- CIE iGCSE Chemistry Notes - New Syllabus
Diffusion for iGCSE
Core Syllabus
- Describe and explain diffusion in terms of kinetic particle theory
Supplement Syllabus
- Describe and explain the effect of relative molecular mass on the rate of diffusion of gases
Diffusion
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It happens due to the random motion of particles and continues until the particles are evenly spread out.
Explanation using Kinetic Particle Theory:
- All matter is made up of tiny particles (atoms or molecules) that are always moving.
- In gases and liquids, particles have enough energy to move freely in random directions.
- When a substance is introduced into another (e.g., perfume in air), its particles move and collide with surrounding particles.
- Because of these random collisions, the particles spread out from areas of high concentration to areas of low concentration.
- This movement continues until the concentration becomes uniform throughout the space (equilibrium).
Key Features of Diffusion:
- Passive Process – does not require energy input from the system.
- Faster in gases – gas particles move faster and are far apart, so they mix more quickly.
- Slower in liquids – particles move more slowly and are closer together.
- Very slow in solids – particles can only vibrate in fixed positions, so diffusion is negligible.
Everyday Examples of Diffusion:
- The smell of perfume spreading across a room.
- Food colouring gradually spreading through still water.
- Oxygen diffusing from the lungs into the bloodstream.
Important Terms:
- Concentration gradient: The difference in particle concentration between two areas.
- Random motion: Movement of particles in all directions with no fixed pattern.
- Net movement: The overall direction in which particles move (from high to low concentration).
Example
Explain why a drop of perfume can be smelled from across a room, even though no air is blown or stirred.
▶️Answer/Explanation
The perfume particles evaporate into the air and move randomly due to their kinetic energy. They spread from an area of high concentration (near the source) to areas of low concentration (throughout the room) by diffusion, as explained by the kinetic particle theory.
Effect of relative molecular mass on the rate of diffusion of gases
Effect of relative molecular mass on the rate of diffusion of gases
In gases, the rate of diffusion is inversely related to the relative molecular mass (Mr) of the gas particles. This means:
- Lighter gas particles (lower Mr) diffuse faster
- Heavier gas particles (higher Mr) diffuse more slowly
Explanation using Kinetic Particle Theory:
- At a given temperature, all gas particles have the same average kinetic energy.
- But kinetic energy is related to both mass and speed: \( KE = \frac{1}{2}mv^2 \)
- So, for lighter particles to have the same energy, they must move faster.
- This explains why gases with a lower Mr (like hydrogen or ammonia) diffuse faster than gases with a higher Mr (like oxygen or hydrogen chloride).
Simple Rule:
Lower Mr → Higher speed → Faster diffusion
Higher Mr → Lower speed → Slower diffusion
Examples:
- Hydrogen (Mr = 2) diffuses faster than Oxygen (Mr = 32).
- Ammonia (NH3, Mr = 17) diffuses faster than Hydrogen chloride (HCl, Mr = 36.5).
Example
In an experiment, a white ring forms closer to the hydrochloric acid end when ammonia and hydrogen chloride gases meet in a glass tube. Explain why this happens.
▶️Answer/Explanation
Ammonia (Mr = 17) is lighter than hydrogen chloride (Mr = 36.5), so its particles move faster. The two gases diffuse towards each other, but ammonia travels further in the same time. As a result, the white ring of ammonium chloride forms nearer to the HCl end.
