Diffusion

1. Overview

Diffusion is the process by which particles spread out and mix with each other without being stirred. It provides essential evidence for the kinetic particle theory, demonstrating that particles in liquids and gases are in constant, random motion. Understanding diffusion is vital for explaining how substances move in biological systems and industrial chemical processes.

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Key Definitions

• Diffusion: The net movement of particles from a region of their higher concentration to a region of their lower concentration down a concentration gradient, as a result of their random movement.
• Kinetic Particle Theory: The theory that all matter is made up of very small particles (atoms, molecules, or ions) that are in constant motion.
• Concentration Gradient: The difference in the concentration of a substance between two regions.
• Relative Molecular Mass ($M_r$): The average mass of a molecule relative to one-twelfth of the mass of an atom of carbon-12.

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Core Content

Diffusion and Kinetic Particle Theory

• Random Motion: Particles in liquids and gases move randomly. They collide with each other and the walls of their container, causing them to spread out.
• Concentration: Particles move from where there are many of them (high concentration) to where there are fewer (low concentration) until they are evenly distributed.
• States of Matter:
  • Gases: Diffusion is rapid because particles are far apart and move at high speeds.
  • Liquids: Diffusion is much slower because particles are closer together and have more frequent collisions, which impedes their movement.
  • Solids: Diffusion does not occur in solids because particles are held in fixed positions and can only vibrate.

Factors Affecting the Rate of Diffusion

1. Temperature: As temperature increases, particles gain more kinetic energy and move faster. This leads to an increased rate of diffusion.
2. State of Matter: Gases diffuse faster than liquids due to higher particle speeds and more space between particles.

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Extended Content (Extended Only)

The Effect of Relative Molecular Mass ($M_r$)

The rate at which a gas diffuses is dependent on its Relative Molecular Mass ($M_r$).

• Lighter particles (lower $M_r$) move faster and diffuse more quickly than heavier particles (higher $M_r$) at the same temperature.

Worked Example: The Ammonia and Hydrogen Chloride Experiment

In this classic experiment, cotton wool soaked in aqueous ammonia and cotton wool soaked in concentrated hydrochloric acid are placed at opposite ends of a glass tube.

The Reaction:

• Word Equation: ammonia + hydrogen chloride → ammonium chloride
• Symbol Equation: $NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)$

Calculating $M_r$:

• $M_r$ of $NH_3$: $14 + (1 \times 3) = 17$
• $M_r$ of $HCl$: $1 + 35.5 = 36.5$

Observation and Explanation:

• A white ring of solid ammonium chloride ($NH_4Cl$) forms inside the tube.
• The ring forms closer to the hydrochloric acid end.
• Reasoning: Because $NH_3$ has a lower $M_r$ (17) than $HCl$ (36.5), the ammonia particles move faster and travel a further distance in the same amount of time.

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Key Equations

• Relative Molecular Mass ($M_r$): $M_r = \text{Sum of all Relative Atomic Masses } (A_r) \text{ in a molecule}$
• Relationship between Mass and Rate: $\text{Lower } M_r = \text{Faster Diffusion Rate}$ $\text{Higher } M_r = \text{Slower Diffusion Rate}$

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Common Mistakes to Avoid

• ❌ Wrong: Saying diffusion happens because the particles "want" to fill the space.
• ✓ Right: Diffusion happens due to the random motion of particles.
• ❌ Wrong: Forgetting to include state symbols in the $NH_4Cl$ reaction.
• ✓ Right: Always write $NH_4Cl(s)$, as the white "smoke" is actually tiny particles of solid.
• ❌ Wrong: Claiming that diffusion occurs in solids.
• ✓ Right: Diffusion only occurs in liquids and gases (fluids).
• ❌ Wrong: Thinking heavier molecules diffuse faster because they have more "force."
• ✓ Right: Heavier molecules move more slowly and therefore diffuse more slowly.

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Exam Tips

• Command Word - "Explain": When asked to explain diffusion, you must mention random motion and movement from high to low concentration.
• Command Word - "State": If asked to state the effect of mass, a simple "The higher the mass, the slower the rate" is sufficient.
• Common Contexts: Look out for questions involving smells (perfume/cooking) or the movement of colored gases ($Br_2$ or $NO_2$) and liquids (ink in water or $KMnO_4$ crystals).
• Calculation Practice: Be prepared to calculate $M_r$ values using the Periodic Table provided in the exam to justify why one gas diffuses faster than another. Common $M_r$ values to recognize include $NH_3$ (17) and $HCl$ (36.5).
• Numerical Values: Be familiar with temperatures used in these experiments (e.g., room temperature is approx. 20-25°C or 293-298K).