Diffusion & Molecular Mass
The invisible race that happens in every breath you take
The Definition (Learn This)
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient, as a result of their random movement.
The Story: A Race Between Molecules
Imagine you're at a marathon. But this isn't a normal race—the runners are invisible gas molecules, and they're all moving randomly, bouncing off each other like bumper cars at a carnival. Who wins? Not the one who tries hardest—but the one who's lightest.
Picture a heavyweight boxer (Bromine, Mr = 160) versus a lightweight gymnast (Hydrogen, Mr = 2). Both are running through a crowded room. The gymnast zips between people effortlessly. The boxer keeps bumping into everyone, slowing down. That's diffusion.
⚡ The Physics Behind It:
At the same temperature, all gas molecules have the same average kinetic energy. But kinetic energy = ½mv². Since lighter molecules have smaller mass (m), they must have higher velocity (v) to keep the energy constant. Result? Light molecules zoom. Heavy molecules crawl.
Interactive Experiments
🏁 The Molecular Drag Race
Hydrogen (H₂, Mr = 2) vs Bromine (Br₂, Mr = 160). Both released at the same time. Who reaches the finish line first?
⚠️ Common Exam Traps
Wrong:
"Particles move from high to low concentration."
✓ Correct:
"NET movement of particles from higher to lower concentration down a concentration gradient, due to random movement."
Wrong:
"Light gases move faster."
✓ Correct:
"Gases with lower relative molecular mass (Mr) diffuse faster at the same temperature."
🎯 Exam Success Checklist
Always mention "random movement"
Diffusion happens because particles move randomly in all directions
Use "NET movement" not just "movement"
Particles move both ways, but MORE move from high to low
Explain the Mr effect quantitatively
Lower Mr = lighter = higher velocity = faster diffusion
Link temperature to kinetic energy
Higher temp → higher KE → faster particle movement → faster diffusion
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