Stopcock closed → open → gases diffuse and mix
14.7 Diffusion
Molecules present in gases are in a constant state of random motion. Due to this molecular motion the gas particles spread out and intermix from an area of high concentration to an area of low concentration. This property of gases is called diffusion. When a bottle of body perfume is opened in one corner of a room its sweet smell slowly spreads throughout the room after sometime due to the process of diffusion. Figure14.6. shows the diffusion of hydrogen gas from one container to another one. Diffusion is a spontaneous process during which gas particles spread out.
The difference in the rates of diffusion of gases can be explained with the help of kinetic particle theory of gases. According to the kinetic theory, molecules of all the gases possess same average kinetic energy at constant temperature. Since hydrogen is lighter than oxygen its molecules will move faster than oxygen at a given temperature. The rate at which hydrogen gas will diffuse is thus much faster than that of oxygen.
Rate of diffusion increases with the increase in temperature as the particles have more kinetic energy and hence they move faster. This eventually leads to rapid mixing and spreading.
Two cotton plugs soaked in hydrogen chloride gas and ammonia solutions are introduced in the open ends of a 100 cm long glass tube simultaneously as shown in Figure 14.7. The two gases produce white dense fumes of ammonium chloride at the point at which they meet in the tube. HCl molecules travel a distance of 40.5 cm while ammonia molecules cover 59.5 cm in the same duration.
Thus ammonia diffuses faster than hydrogen chloride gas because it is lighter than hydrogen chloride gas.
1. Which gas among the following will diffuse at the fastest speed and which will diffuse at the slowest speed under similar conditions of temperature and pressure? NO, SO₂, H₂S
✅ Molar masses: NO = 30 g/mol, SO₂ = 64 g/mol, H₂S = 34 g/mol. Lighter gas diffuses faster.
Fastest: NO (lightest), Slowest: SO₂ (heaviest).
2. Explain with the help of kinetic particle theory why does ammonia gas diffuse faster than hydrogen chloride gas?
✅ At same temperature, average kinetic energy (½mv²) is same for both gases. Since NH₃ (M=17) is lighter than HCl (M=36.5), its molecules have higher rms speed. Faster moving molecules diffuse quicker. Hence ammonia diffuses faster.
“Light speed diffusion”
Lighter gas = faster diffusion. NH₃ (17) beats HCl (36.5).
• Diffusion: high concentration → low concentration.
• Rate ∝ 1/√(M) (Graham).
• Temperature ↑ → rate ↑.
Perfume spray at one end of room – time to smell. Fan increases temp/speed.
00-05 min: open perfume demo – introduce diffusion.
05-12 min: kinetic theory explanation (light gases move faster).
12-18 min: Activity 14.2 (NH₃ & HCl tube) + calculation.
18-25 min: Quick check & Graham’s law (qualitative).
25-35 min: 10 MCQs quiz & discussion.
• Diffusion is from high to low concentration – spontaneous.
• At same T, all gases have same average KE; lighter moves faster.
• Graham’s law: rate₁/rate₂ = √(M₂/M₁).
• In HCl-NH₃ tube, white ring forms closer to heavier gas (HCl).
• Attempt quiz without notes first, then check key.