Ch 2 · Diffusion · O Level Chemistry 5070
Topic 2 of 2 in States of Matter — O Level Chemistry 5070 — 38 lessons.
Introduction
The smell of perfume spreading across a room, a drop of ink colouring a whole beaker of water, and oxygen passing into your blood all happen by diffusion. Diffusion is direct evidence that particles are in constant random motion, so questions on it test whether you can apply kinetic particle theory rather than just describe what you see. The ammonia and hydrogen chloride experiment is the classic example and appears regularly in all three papers.
Key ideas — What diffusion is
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of the random motion of the particles.
🧑🏫 Teacher's explanation
Imagine a crowded, busy shopping mall. If a group of people suddenly starts running randomly through the corridors, they will naturally spread out from the crowded entrance into the empty hallways. Even though individuals might run back and forth, the overall 'crowd' moves from the busy area to the empty area. This is exactly how particles behave.
📖 Definitions
🔍 Worked example — Easy
A gas sample has a concentration of 0.5 mol/dm^3 in a 2 dm^3 container. If the gas diffuses until it occupies a volume of 10 dm^3 at the same concentration, what is the new total number of moles?
🔍 Worked example — Standard
Compare the rate of diffusion of Ammonia (NH_3, M_r = 17) and Hydrogen Chloride (HCl, M_r = 36.5). Which gas diffuses faster and why?
🔍 Worked example — Exam-style
In an experiment with a glass tube containing a cotton wool soaked in NH_3(g) at one end and HCl(g) at the other, a white ring of NH_4Cl(s) forms. If the total length of the tube is 100 cm, and the ring forms 75 cm from the NH_3 end, calculate the ratio of the rates of diffusion of NH_3 to HCl.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — Relative molecular mass and the rate of diffusion
At the same temperature all gas particles have the same average kinetic energy, so lighter molecules move faster than heavier ones. The lower the relative molecular mass (M_r) of a gas, the faster it diffuses.
🧑🏫 Teacher's explanation
Imagine a crowded hallway at school. If a group of heavy, slow-moving students (like seniors) and a group of light, energetic toddlers both start running from opposite ends, the toddlers will cover much more distance in the same amount of time. Diffusion works exactly like this, but with molecules instead of people.
📖 Definitions
🔍 Worked example — Easy
Calculate the relative molecular mass (M_r) of ammonia, NH_3, given A_r of H = 1 and N = 14.
🔍 Worked example — Standard
A gas molecule has an M_r of 44. Another gas molecule has an M_r of 28. Which gas will diffuse faster at the same temperature?
🔍 Worked example — Exam-style
In a diffusion experiment using NH_3 (M_r = 17) and HCl (M_r = 36.5), a white ring of NH_4 extCl forms. If the total length of the tube is 100 cm, and the NH_3 travels 60 cm from its end, how far from the HCl end does the ring form?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Predict which gas will diffuse faster. [1] (b) Explain your answer in terms of kinetic particle theory. [2] [3 marks]
Key ideas — Interpreting experiments with coloured substances
Coloured substances make diffusion visible, and you may be asked to describe observations and explain them.
🧑🏫 Teacher's explanation
Imagine you are standing in a crowded hallway and someone starts spraying a strong perfume. Even if you aren't standing next to them, you will eventually smell it. This happens because the perfume particles are moving randomly and bumping into air molecules, spreading out until they are everywhere. This 'spreading out' is what we call diffusion.
📖 Definitions
🔍 Worked example — Easy
A crystal of potassium manganate(VII) is placed in a beaker of water. Describe the observation after 24 hours.
🔍 Worked example — Standard
In a gas experiment, a brown gas (bromine) is placed at the bottom of a tube and air is at the top. Explain why the brown color eventually reaches the top.
🔍 Worked example — Exam-style
A student observes a blue color spreading through a beaker of water from a copper(II) sulfate crystal. If the temperature of the water is increased, what happens to the rate of diffusion and why?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why a white ring forms. (b) If the tube was much longer, would the white ring form closer to the ammonia end or the hydrochloric acid end? Explain your answer in terms of relative molecular mass. [4 marks]
Worked example
Question. In the experiment shown in the figure, cotton wool soaked in concentrated ammonia solution and concentrated hydrochloric acid was placed at opposite ends of a glass tube at the same time. After a few minutes a white ring formed 15 cm from the hydrochloric acid end of a 40 cm tube.
🔍 Worked example — Easy
Calculate the relative molecular mass (M_r) of ammonia (NH_3) and hydrogen chloride (HCl). (Use H=1, N=14, Cl=35.5)
🔍 Worked example — Standard
A glass tube is 50 cm long. Ammonia gas (NH_3) and hydrogen chloride gas (HCl) are placed at opposite ends. If the ammonia travels 30 cm before reacting, how far has the hydrogen chloride traveled?
🔍 Worked example — Exam-style
In an experiment, a white ring of ammonium chloride forms in a tube. The tube is 40 cm long. The ring forms 10 cm from the HCl end. Calculate the ratio of the distance traveled by NH_3 to the distance traveled by HCl.
⚠️ Common errors
Common mistakes
Practice
Answers The net movement of particles from a region of higher concentration to a region of lower concentration [B1] as a result of their random motion [B1]. The crystal dissolves and a purple colour appears around it [B1]; the colour slowly spreads through the water until the whole solution is uniformly purple [B1]; dissolved particles move randomly [B1]; there is a net movement from the region of high concentration near the crystal to regions of low concentration [B1]. Gas particles are far apart and move quickly with few collisions, so they spread rapidly [B1]; particles in a liquid are close together and collide constantly, so they move only short distances between collisions and diffusion is slow [B1]. Hydrogen travels further [B1]; it has a much lower relative molecular mass, so its molecules move faster and diffuse more quickly [B1].