Diffusion (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

• 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 KMnO₄ is 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 quicker 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 an area of high concentration
• It diffuses from a high to low concentration
• After 5 minutes, the bromine gas will have diffused from the bottom jar until it is evenly spread throughout both jars

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

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