Diffusion (Cambridge (CIE) O Level Chemistry): Revision Note

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 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 hydrogen chloride end

• Ammonia molecules (M_r = 17) are lighter than hydrogen chloride molecules (M_r = 36.5)

• So the ammonia diffuses faster and travels further along the tube before the two gases meet