Explain why ammonia gas diffuses faster than bromine vapour at the same temperature.

Cambridge International IGCSE Chemistry (0620) — 1.2 Diffusion · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Ammonia (NH₃) has a relative molecular mass of 17. Bromine vapour (Br₂) has a relative molecular mass of 160.

Model answer (4 marks)

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
Ammonia has a lower relative molecular mass than bromine vapour.
At the same temperature, particles of both gases have the same average kinetic energy.
Because ammonia molecules are lighter but have the same kinetic energy, they move at a greater speed, so diffuse faster.

Examiner tips

  • Use the definition of diffusion first, then compare molecular masses, mention equal kinetic energy, finish with speed argument.
  • Show the logical sequence: definition → mass difference → kinetic energy equality → speed difference → faster diffusion.
  • Use the exact wording from the mark scheme where possible.

Common mistakes

  • Failing to state that diffusion is movement from high to low concentration.
  • Confusing diffusion with effusion or not recognising equal kinetic energy at same temperature.
  • Using vague terms like ‘lighter’ without linking to speed or mass difference.

Mark scheme (4 marks)

  1. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration
  2. Ammonia has a lower relative molecular mass than bromine vapour
  3. At the same temperature, particles of both gases have the same average kinetic energy
  4. Because ammonia molecules are lighter but have the same kinetic energy, they move at a greater speed / faster, so diffuse faster

Related

More Diffusion questions

▶ Try answering this question with AI marking (free) →