Diffusion and gas behaviourIB MYP Chemistry: Revision notes
Section 1
What is diffusion?
Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated, until they are spread evenly. It happens because particles are always moving randomly.
Diffusion happens in gases and in liquids, for example a smell spreading across a room or a drop of ink spreading in water. It is much faster in gases than in liquids, because gas particles are further apart and move faster.
Diffusion does not happen in solids because the particles are held in fixed positions.
Particles do not move because they want to spread out. They move randomly all the time, and the overall result is that they spread out.
Section 2
What affects how fast diffusion happens?
- Temperature: at a higher temperature the particles have more kinetic energy and move faster, so diffusion is faster.
- Particle mass: at the same temperature, lighter particles move faster than heavier ones, so they diffuse faster.
Worked example: ammonia and hydrogen chloride gases are released from opposite ends of a sealed tube. Ammonia particles are lighter (relative mass 17) than hydrogen chloride particles (36.5), so they move faster and travel further. The gases meet and make a ring of white ammonium chloride closer to the hydrochloric acid end.
Section 3
Brownian motion
In 1827 Robert Brown saw tiny grains jiggling in water. We now call this Brownian motion: the random, jerky movement of small visible particles in a liquid or gas.
It happens because the tiny, fast-moving particles of the liquid or gas collide with the visible particle from all sides. At any moment the collisions are uneven, so the particle is pushed in a changing direction.
Brownian motion is evidence that matter is made of particles which are in constant random motion.
Say that the small, invisible particles hit the visible one from different directions and unevenly.
Section 4
Gas pressure
Gas particles move randomly at high speed and collide with the walls of their container. Each collision gives the wall a tiny push. Millions of collisions every second add up to a steady gas pressure.
Pressure is not caused by the particles pushing on each other. It is caused by particles hitting the walls. More collisions per second, or harder collisions, means a higher pressure.
Section 5
Temperature, volume and pressure
Two qualitative rules for a fixed amount of gas:
- Heat it in a rigid container (fixed volume): the particles move faster, so they hit the walls more often and harder. The pressure increases. This is why aerosol cans must not be heated.
- Squeeze it into a smaller volume (same temperature): the particles have less space, so they hit the walls more often. The pressure increases.
Cooling a gas or letting it expand has the opposite effect, so the pressure decreases.
Must Know
- Diffusion is the net movement from a higher to a lower concentration, in gases and liquids
- It is faster at higher temperature and for lighter particles
- Brownian motion is evidence that particles are in constant random motion
- Gas pressure is caused by particles colliding with the walls
- Heating a gas at fixed volume, or reducing its volume, increases the pressure
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Diffusion and gas behaviour
- A teacher in Johannesburg places cotton wool soaked in concentrated ammonia solution at one end of a long, horizontal glass tube, and cotton wool soaked in concentrated hydrochloric acid at the other end. Both ends are sealed with bungs. After a few minutes a ring of white solid (ammonium chloride) forms inside the tube, closer to the hydrochloric acid end than to the ammonia end.A particle of ammonia has a relative mass of 17 and a particle of hydrogen chloride has a relative mass of 36.5. Explain why the white ring forms closer to the hydrochloric acid end.2 marks
- A student in Oslo uses a microscope to look at smoke in a small glass cell lit from the side. The student sees tiny bright specks of smoke ash that jiggle about in random, jerky paths, constantly changing direction. The specks never stop moving, even though the air in the cell is perfectly still.Explain why the smoke specks move in random directions rather than in a straight line.2 marks
- A student in Wellington investigates how temperature affects diffusion in a liquid. She drops one small crystal of potassium manganate(VII), which is purple, into each of three beakers containing 200 cm³ of water at different temperatures, without stirring. She uses a stopwatch to time how long it takes for the purple colour to spread evenly through the water, judged by eye. She wears safety glasses throughout.State a testable hypothesis for this investigation, with a scientific reason.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).