CIE iGCSE Co-ordinated Sciences-C1.2 Diffusion- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C1.2 Diffusion – Study Notes
CIE iGCSE Co-ordinated Sciences-C1.2 Diffusion – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe and explain diffusion in terms of kinetic particle theory
Supplement
- Describe and explain the effect of relative molecular mass on the rate of diffusion of gases
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Diffusion
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration, due to their random motion, until they are evenly spread out.
Explanation using Kinetic Particle Theory 
- All particles (in gases and liquids) are in constant random motion because they have kinetic energy.
- Particles collide with each other and bounce off in all directions.
- Over time, particles spread from areas where they are crowded (high concentration) to areas where they are less crowded (low concentration).
- This process continues until the particles are evenly distributed, reaching dynamic equilibrium.
- Diffusion happens naturally due to particle motion – no external force is required.
Diffusion in Gases
- Gases diffuse much faster than liquids because gas particles are far apart and move more quickly.
- There are large spaces between gas particles, so they spread out rapidly when mixed.
- Example: The smell of perfume spreading across a room within minutes.
Diffusion in Liquids
- Diffusion occurs more slowly in liquids than in gases because the particles are closer together and move less quickly.
- Collisions between particles happen more often, slowing down the mixing process.
- Example: A drop of ink spreading out in a beaker of water.
Factors Affecting Diffusion
- Temperature: Higher temperature → particles gain more kinetic energy → move faster → diffusion rate increases.
- Particle size (molecular mass): Lighter particles diffuse faster because they move more quickly at the same temperature. Heavier particles diffuse more slowly.
- Medium of diffusion: Diffusion is faster in gases than in liquids because gas particles move more rapidly and have larger spaces between them.
Everyday Examples of Diffusion
- The spreading of cooking smells throughout the house.
- Carbon dioxide escaping from a fizzy drink into the air.
- Oxygen diffusing into the bloodstream in the lungs during breathing.
Experimental Evidence of Diffusion
Experiment 1: Diffusion of ammonia and hydrogen chloride
A long glass tube is filled with air. A cotton wool soaked in concentrated ammonia solution is placed at one end, and another soaked in concentrated hydrochloric acid is placed at the other end.
The two gases (ammonia and hydrogen chloride) diffuse through the tube. Where they meet, they react to form a white ring of ammonium chloride.
The white ring forms closer to the hydrochloric acid end because ammonia (NH3) particles are lighter and diffuse faster than hydrogen chloride (HCl) particles.
Experiment 2: Diffusion of bromine in air
A gas jar filled with bromine gas is placed on top of an empty gas jar, with a glass plate between them. When the plate is removed, the reddish-brown bromine gas slowly diffuses upward into the empty jar, showing diffusion of gases even against gravity.
Key Point: Diffusion provides strong evidence for the kinetic particle theory, as it demonstrates that particles are constantly moving and spreading out.
Example
In a glass tube, cotton wool soaked with concentrated ammonia solution is placed at one end, and cotton wool soaked with concentrated hydrochloric acid is placed at the other end. After some time, a white ring of ammonium chloride is seen inside the tube. Explain why the ring forms closer to the hydrochloric acid end.
▶️ Answer/Explanation
Both ammonia (\( \text{NH}_3 \)) and hydrogen chloride (\( \text{HCl} \)) gases diffuse through the air inside the tube.
The particles of ammonia are lighter and therefore move faster than the heavier hydrogen chloride particles.
Because of this, the ammonia travels further in the same amount of time, and the two gases meet closer to the hydrochloric acid end of the tube.
Where they meet, they react to form solid ammonium chloride (\( \text{NH}_4\text{Cl} \)), which appears as a white ring.
Equation: \( \text{NH}_3 (g) + \text{HCl} (g) \rightarrow \text{NH}_4\text{Cl} (s) \)
Example
A gas jar containing bromine gas is placed upside down on top of an empty gas jar, separated by a glass plate. The plate is removed. After a while, the reddish-brown colour of bromine spreads into the empty jar. Explain why this happens.
▶️ Answer/Explanation
The bromine gas particles are in constant random motion due to their kinetic energy.
When the plate is removed, the particles spread from the region of high concentration (the lower jar) to the region of low concentration (the empty jar).
This spreading out of particles is called diffusion. It occurs even against gravity, because particle motion is random and not dependent on direction.
Effect of Relative Molecular Mass on the Rate of Diffusion
The rate at which a gas diffuses depends on its relative molecular mass (Mr). This can be explained by the kinetic particle theory and Graham’s Law of Diffusion.
Kinetic particle theory explanation
All gas particles have kinetic energy that depends on temperature. At the same temperature, lighter gas particles (with lower Mr) move faster than heavier particles (with higher Mr).Since diffusion is caused by the random movement of particles, lighter particles diffuse more quickly.
Graham’s Law of Diffusion
The rate of diffusion of a gas is inversely proportional to the square root of its relative molecular mass:
\( \text{Rate of diffusion} \propto \dfrac{1}{\sqrt{\text{Mr}}} \)
This means:
- Gases with lower Mr diffuse faster.
- Gases with higher Mr diffuse slower.
Examples
Hydrogen (\( \text{H}_2 \), Mr = 2) diffuses much faster than oxygen (\( \text{O}_2 \), Mr = 32).
Ammonia (\( \text{NH}_3 \), Mr = 17) diffuses faster than hydrogen chloride (\( \text{HCl} \), Mr = 36.5).
Experimental evidence
In the classic experiment with ammonia and hydrogen chloride gases in a tube, the white ring of ammonium chloride forms closer to the hydrochloric acid end. This is because ammonia, having a lower Mr, diffuses faster than hydrogen chloride.
Example
Hydrogen gas (\( \text{H}_2 \), Mr = 2) and oxygen gas (\( \text{O}_2 \), Mr = 32) are kept at the same temperature and pressure. Calculate how many times faster hydrogen diffuses compared to oxygen.
▶️ Answer/Explanation
According to Graham’s Law:
\( \dfrac{\text{Rate of diffusion of H}_2}{\text{Rate of diffusion of O}_2} = \sqrt{\dfrac{\text{Mr of O}_2}{\text{Mr of H}_2}} \)
\( = \sqrt{\dfrac{32}{2}} = \sqrt{16} = 4 \)
Therefore, hydrogen diffuses 4 times faster than oxygen under the same conditions.
Example
Ammonia (\( \text{NH}_3 \), Mr = 17) and hydrogen chloride (\( \text{HCl} \), Mr = 36.5) gases are allowed to diffuse from opposite ends of a glass tube. Which gas diffuses faster and by what factor?
▶️ Answer/Explanation
Using Graham’s Law:
\( \dfrac{\text{Rate of NH}_3}{\text{Rate of HCl}} = \sqrt{\dfrac{36.5}{17}} \)
\( = \sqrt{2.15} \approx 1.47 \)
This means ammonia diffuses about 1.5 times faster than hydrogen chloride. Hence, the white ring of ammonium chloride forms closer to the HCl end of the tube.