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Diffusion x Concentration

🧪 INSTRUCTIONS:
1st Watch the video & take detailed notes.
2nd Review & compare your notes to the slides below
3rd Add any additional important information to your notes
4th Then complete the Quick Check Questions
5th Screenshot your name + score to the Google Classroom when finish

Diffusion Video

In this video you will learn about Diffusion of Gases. Take notes! Credit: Fuse School. (2023).
Diffusion — CAST Chemistry Notes

DIFFUSION & CONCENTRATION GRADIENTS

High School Chemistry — Particle Motion & Mixing in Matter
Definition of Diffusion
Diffusion is the random spreading of particles from HIGH concentration to LOW concentration.
HIGH → LOW
It happens because particles are already moving randomly and colliding.
Goal outcome: particles become more evenly distributed in the available space.
What “Random Motion” Means
Gas particles move constantly in all directions.
  • They travel in straight lines until they collide.
  • Collisions change direction → motion looks random.
  • Over time, random motion causes mixing.
This is explained by the Kinetic Molecular Theory (KMT).
Kinetic Molecular Theory (KMT)
Particles have kinetic energy because they are moving.
  • Temperature measures average kinetic energy.
  • Higher temperature → particles move faster → diffusion happens faster.
  • Lower temperature → particles move slower → diffusion happens slower.
Key idea: diffusion doesn’t need “pushing” — motion already exists.
How Diffusion Works in Gases
Gases are made of tiny particles that move fast and are far apart.
  • When two gases meet, their particles intermix due to random motion.
  • Mixing continues until the gases are evenly distributed throughout the container.
  • Diffusion is fast in gases because there is lots of empty space between particles.
In a closed container, diffusion leads to a uniform mixture over time.
Everyday Example: Smells Spreading
If perfume is sprayed in one corner, the smell spreads across the room.
  • At first, perfume molecules are most concentrated near the spray.
  • Random motion + collisions cause perfume molecules to spread into the air.
  • Eventually, you can smell it far away because particles have mixed.
This is diffusion: molecules move from more crowded to less crowded regions.
What Is a Concentration Gradient?
A concentration gradient is the difference in concentration between two regions.
Diffusion moves DOWN the gradient.
  • Large gradient (big difference) → diffusion is usually faster.
  • Small gradient (small difference) → diffusion is usually slower.
“Down the gradient” means from high to low.
What Is Equilibrium?
Diffusion continues until particles are evenly distributed.
At equilibrium, particles still move randomly — but there is NO NET CHANGE in concentration from one area to another.
  • Movement continues.
  • Concentrations stay the same overall.
Gases vs. Liquids vs. Solids
Diffusion rate depends on particle spacing and particle speed.
  • Gases: fastest diffusion (far apart + fast motion).
  • Liquids: slower diffusion (closer together + more resistance).
  • Solids: very slow diffusion (particles vibrate in fixed positions).
In solids, particles can’t freely move past each other, so mixing is extremely slow.
Factors That Affect Diffusion Rate
Diffusion can speed up or slow down depending on conditions.
  • Temperature: higher T → faster particles → faster diffusion.
  • Concentration gradient: bigger difference → faster diffusion.
  • Distance: longer distance → slower to spread through the space.
  • Particle mass: lighter particles generally diffuse faster than heavier particles.
These factors are consistent with KMT: faster motion and easier movement → faster mixing.
Diffusion vs. Effusion (Gas Behavior)
Diffusion = gas particles mix through space.
Effusion = gas particles pass through a tiny opening into another container.
  • Both involve random motion.
  • Lighter gases tend to diffuse/effuse faster than heavier gases.
(Optional extension: this idea connects to Graham’s Law in later chemistry.)
Does Diffusion Require Energy?
Diffusion is passive: it happens without adding energy from outside.
NO ATP • NO EXTERNAL ENERGY INPUT
Particles already have kinetic energy, so they move and spread naturally.
Closing Summary
Diffusion explains how mixtures form when particles spread out due to random motion.
  • Particles move from high to low concentration.
  • A concentration gradient drives the net movement.
  • At equilibrium, motion continues but concentrations don’t change overall.
  • Diffusion is fastest in gases, slower in liquids, and very slow in solids.
Big picture: diffusion is a particle-level reason why gases mix and why smells spread.
Check for Understanding
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