Diffusion (Cambridge O Level Chemistry)

• This is the process by which different gases or different liquids mix and is due to the random motion of their particles
• Diffusing particles move from an area of high concentration to an area of low concentration
• Eventually the concentration of particles is even as they spread out to occupy all of the available space
• Diffusion happens on its own and no energy input is required although it occurs faster at higher temperatures

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

• Diffusion occurs much faster in gases than in liquids as gaseous particles move much quicker than liquid particles
• At the same temperature, different gases do not diffuse at the same rate.
• This is due to the difference in their relative molecular masses
• Lighter gas particles can travel faster and hence further, therefore the lower its relative mass the faster a gas will diffuse
• This can be demonstrated in the reaction between ammonia, NH₃, and hydrogen chloride gas, HCl, inside a long glass tube
• Where the two gases meet a white smoke of ammonium chloride, NH₄Cl, is formed
• This does not occur in the middle of the tube as you might expect, but much closer to the end with the hydrogen chloride (M_r = 36.5) and the ammonia (M_r = 17) molecules are smaller and lighter

NH₃ molecules have less mass than the HCl molecule, so diffuse faster, hence the product (a white smoke of NH₄Cl) forms closer to the end where the HCl is