Diffusion in gases
5 marks · HardA student investigates the rate of diffusion of hydrogen and air using the apparatus shown. In a sealed glass tube, a small amount of hydrogen is released at one end and air at the other. After some time, the gases mix completely.
Describe, using the particle model, how the hydrogen and air particles move inside the tube during diffusion.
Suggest one reason, in terms of particles, why hydrogen diffuses faster than air.
The tube is cooled to a much lower temperature. Predict and explain the effect this will have on the rate of diffusion, using ideas about particle movement.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show Particles move randomly in all directions, Particles collide with each other/with air
(b) Hydrogen particles are lighter/have less mass than air particles so they move faster · must show hydrogen is lighter/less massive
(c) Diffusion will slow down because at lower temperature, particles move more slowly, so they spread less quickly · must show rate decreases, particles have less kinetic energy/move slower
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Describes particles moving randomly or in all directions
M2 (a) · 1 mark · Mentions particles colliding with each other/air
M3 (b) · 1 mark · Relates faster diffusion of hydrogen to lighter/smaller particles or greater speed
M4 (c) · 1 mark · Correctly predicts lower temperature decreases diffusion rate
M5 (c) · 1 mark · Links reduced diffusion rate to slower movement/less kinetic energy of particles
Worked solution
(a) The hydrogen and air particles both move randomly in all directions inside the tube. As they move, the particles keep colliding and bouncing off each other, changing direction many times until they eventually mix evenly.
(b) Hydrogen diffuses faster because its particles are lighter than the particles that make up air. Lighter particles move faster at a given temperature.
(c) Lowering the temperature will slow down the rate of diffusion. When the tube is cooled, all the particles have less kinetic energy, so they move more slowly, causing diffusion to take longer.