Diffusion
Section: 1. States of Matter | Syllabus: Cambridge IGCSE Chemistry (0620)
What is Diffusion?
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of their own random motion.
- Particles are always moving randomly in all directions
- Where there are more particles, more of them happen to move outward than move in from the (less crowded) surroundings
- The overall effect is a net movement from high to low concentration, even though each individual particle is just moving randomly
- Diffusion continues until the particles are evenly spread out; even then, the particles keep moving, but there is no further net movement
- Diffusion needs no energy input - it happens automatically because particles are always in motion
Examples: a drop of food colouring spreading through water; the smell of perfume spreading across a room.
Diffusion is the net spreading of particles from high to low concentration until they are evenly distributed
Rate of Diffusion
Effect of State of Matter
| State | Rate of diffusion | Reason |
|---|---|---|
| Gas | Fastest | Particles are far apart and move quickly, with few collisions to slow them down |
| Liquid | Slower | Particles are close together, so they collide often, which slows their progress |
| Solid | Extremely slow / negligible | Particles can only vibrate about fixed positions |
Effect of Temperature
Increasing temperature increases the rate of diffusion, because particles have more kinetic energy and move faster.
Effect of Relative Molecular Mass
- At the same temperature, particles of different substances have approximately the same average kinetic energy
- Since kinetic energy depends on both mass and speed, a particle with a smaller relative molecular mass (Mr) must move faster to have the same kinetic energy as a heavier particle
- Faster-moving particles diffuse more quickly, so gases with a lower Mr diffuse faster than gases with a higher Mr
Classic Diffusion Experiments
Ammonia and Hydrogen Chloride
Cotton wool soaked in concentrated ammonia is placed at one end of a long glass tube, and cotton wool soaked in concentrated hydrochloric acid at the other end. Both substances give off gas (ammonia, NH3, and hydrogen chloride, HCl) which diffuse along the tube from opposite ends. Where the two gases meet, they react to form a white ring of solid ammonium chloride.
- Mr(NH3) = 17; Mr(HCl) = 36.5
- Ammonia has the smaller Mr, so it diffuses faster and travels further along the tube before meeting the hydrogen chloride
- The white ring forms closer to the hydrochloric acid end of the tube
The white ring forms closer to the hydrochloric acid end because the lighter ammonia molecules diffuse faster and travel further
Bromine Diffusing Into Air
Orange-brown bromine liquid placed in a sealed gas jar evaporates at room temperature (well below its boiling point of 59°C). The bromine vapour then diffuses until it fills the jar completely and evenly.
Worked Example: Ammonia and Hydrogen Chloride
- Question: in the ammonia/hydrogen chloride tube experiment, a strip of damp universal indicator paper is placed in the middle of the tube. Which gas reaches the indicator paper first, and what colour does the paper turn? Explain your answer. [3]
- Answer: ammonia reaches the indicator paper first [1], because it has a smaller relative molecular mass (17, compared with 36.5 for hydrogen chloride) and so its particles move faster and diffuse more quickly [1]; ammonia dissolves in the water in the paper to form an alkali, so the indicator paper turns blue [1]
Common Mistakes
- ❌ Thinking heavier particles diffuse faster
- ✓ at the same temperature, lighter particles move faster (to have the same kinetic energy as heavier particles), so lighter particles diffuse faster
- ❌ Thinking particles stop moving once diffusion is complete
- ✓ particles keep moving randomly even once they are evenly spread out - there is just no further net movement in any direction
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