Diffusion (Cambridge International IGCSE Chemistry)

Spec point — 1.1.4 · Describe & explain diffusion in terms of kinetic particle theory

Diffusion

• Diffusion occurs in gases and liquids, due to the random motion of their particles

• It is where particles move from an area of high concentration to an area of low concentration

• Eventually the concentration of particles is even as the particles are evenly spread throughout the available space

• Diffusion happens on its own and no energy input is required

  • Although, it occurs faster at higher temperatures because the particles have more kinetic energy

Diffusion in liquids

• Potassium manganate(VII), KMnO₄ , in water is a typical demonstration of diffusion in liquids:

Diffusion of potassium manganate(VII), KMnO₄ , in water. After a few hours, the concentration of KMnO₄is the same throughout the solution

Diffusion in gases

• Diffusion is faster in gases than in liquids

  • This is because gaseous particles have more energy and move faster than liquid particles

• For example, the diffusion of bromine gas and air:

Bromine diffuses until it is evenly spread throughout the container

• At the start, the orange-brown bromine gas is in an area of high concentration

• It diffuses from an area of high concentration to an area of low concentration

• After 5 minutes, the bromine gas will have diffused from the bottom jar until it is evenly spread throughout both containers

  • The same can be said for the air, although it is less obvious as it is colourless

Exam Hint When you are talking about diffusion, you should (where appropriate) include:

• Ideas about areas of high and low concentration
• The energy / movement of the particles

Diffusion & Molecular Mass

Spec point — 1.1.4 · Describe & explain the effect of relative molecular mass on the rate of diffusion of gases

Extended tier only

How molecular mass affects diffusion

• At the same temperature, different gases do not diffuse at the same rate.

• This is due to the difference in their relative molecular masses

• Gases with a lower relative molecular mass are "lighter" which means that they:

  • Travel faster
  • Travel further in the same amount of time

• The reverse argument is true for gases with a high relative molecular mass, they:

  • Travel slower
  • Do not travel as far in the same amount of time

• For example, the reaction between ammonia and hydrogen chloride

Diffusion of ammonia and hydrogen chloride

• Ammonia gas and hydrogen chloride gas react together to form solid ammonium chloride

NH₃(g) + HCl (g) → NH₄Cl (s)

• The M_r of ammonia is 17, while the M_r* *of hydrogen chloride is 36.5

  • This means that ammonia will travel faster and further than hydrogen chloride

  • Therefore, the ammonium chloride will form nearer to the hydrogen chloride

    • This is seen as a white "smoke" ring inside the gas tube

Ammonia molecules have less mass than HCl molecules so they diffuse faster and the product forms closer to the HCl end