Diffusion
This phenomenon occurs in both gases and liquids. It does not occur in solids because solid particles are fixed in position and can only vibrate, preventing them from moving freely to mix with other substances.
Diffusion is explained by the kinetic particle theory. This theory states that all particles are in constant, random motion.
- Particles possess kinetic energy, which allows them to move.
- Because this motion is random, particles collide with each other and bounce off in different directions.
- Over time, these random collisions cause particles to spread out from areas of high concentration to areas of low concentration.
Building on the concept of kinetic energy, the speed at which a particle moves depends on its mass and temperature. At a constant temperature, all gas particles have the same average kinetic energy (E_k), regardless of their mass.
Kinetic Particle Theory is the model used to explain the properties of matter. Key postulates relevant to diffusion are:
- All matter is made up of tiny particles (atoms, molecules, or ions).
- These particles are in constant random motion.
- The particles have kinetic energy.
- As temperature increases, the average kinetic energy of the particles increases, causing them to move faster.
The Rule: Lighter gases (lower M_r) diffuse faster than heavier gases (higher M_r).
Why? The Physics Explanation:
At a constant temperature, all gas particles have the same average kinetic energy (E_k). The formula for kinetic energy is:
E_k = \frac{1}{2}mv^2
Where:
- m is the mass of the particle (proportional to its relative molecular mass, M_r).
- v is the velocity (speed) of the particle.
Since E_k is constant for all gases at the same temperature, if the mass (m) decreases, the velocity (v) must increase to keep the equation balanced. Therefore, particles with a lower mass move faster and diffuse more quickly.
This relationship is quantitatively described by Graham's Law, which states that the rate of diffusion (r) is inversely proportional to the square root of the relative molecular mass (M_r):
r \propto \frac{1}{\sqrt{M_r}}
For exam purposes, you must understand the qualitative trend: Lower M_r = Faster Diffusion. This explains why hydrogen (H_2, M_r=2) diffuses much faster than oxygen (O_2, M_r=32).
Step 1: Calculate Relative Molecular Masses (M_r)
- CH_4: 12 + (4 \times 1) = 16
- CO_2: 12 + (2 \times 16) = 44
- SO_2: 32 + (2 \times 16) = 64
Step 2: Apply the Rule
- Methane has the lowest M_r (16).
- Therefore, methane particles are lighter and move faster.
Answer: Methane (CH_4) diffuses most quickly.
Experiment Example: Ammonia and Hydrogen Chloride
In a classic experiment, cotton wool soaked in concentrated ammonia (NH_3) is placed at one end of a dry glass tube, and cotton wool soaked in concentrated hydrochloric acid (HCl) is placed at the other end.
- NH_3 gas and HCl gas diffuse towards each other.
- They react to form a white solid ring of ammonium chloride: NH_3(g) + HCl(g) \rightarrow NH_4Cl(s).
- Observation: The white ring forms closer to the HCl end.
Explanation:
- M_r of NH_3 = 17.
- M_r of HCl = 36.5.
- Since NH_3 has a lower M_r, it diffuses faster and travels further down the tube before meeting the slower HCl particles.
Why it loses marks: This describes the macroscopic result, not the microscopic mechanism. Examiners require an explanation based on kinetic particle theory.
The Correction: You must explicitly mention:
- Particles/Molecules: Refer to "ammonia molecules" or "gas particles," not just "the gas."
- Random Motion: State that particles are in "constant random motion" or "move randomly."
- Collisions: Mention that particles "collide" or "bounce off" each other, which causes the spreading.
Correct Phrasing: "Ammonia molecules move randomly and collide with air particles, causing them to spread from the region of high concentration to low concentration."
Why it loses marks: While 'lighter' is colloquially understood, it is imprecise. 'Density' depends on both mass and volume/pressure, which can vary. 'Atomic mass' applies to atoms, not molecules.
The Correction: Always use the term Relative Molecular Mass (M_r) or mass of the particle.
Correct Phrasing: "But-1-ene has a lower relative molecular mass than butane, so its particles move faster at the same temperature."
Why examiners accept this: Examiners look for specific keywords that prove you understand the microscopic mechanism, not just the macroscopic observation. You must link the movement of individual particles to the overall spread.
Key Markscheme Phrases to Include:
- "Particles are in constant random motion." (Essential first point)
- "Particles collide / bounce off each other." (Explains the mechanism of spreading)
- "Movement is from high concentration to low concentration." (Defines the direction)
Example Answer Structure:
"The purple crystals dissolve, releasing particles into the solution. These particles are in constant random motion. They collide with water particles and each other, causing them to spread out from the area of high concentration (near the crystal) to the area of low concentration until uniformly distributed."
Why examiners accept this: You must explicitly link the mass to the speed. Simply stating "it has a lower mass" is often insufficient without linking it to movement speed.
Key Markscheme Phrases to Include:
- "Lower relative molecular mass (M_r)." (Identify the property)
- "Particles move faster / have higher velocity." (Link property to motion)
- "At the same temperature, all particles have the same kinetic energy." (Optional but strong justification for why lighter = faster)
Example Answer Structure:
"Hydrogen chloride (HCl) diffuses slower than ammonia (NH_3). This is because HCl has a higher relative molecular mass (M_r) than NH_3. Therefore, at the same temperature, HCl particles have a lower average speed and diffuse more slowly."
- Particles are in constant random motion. [1]
- Particles move from an area of high concentration to an area of low concentration. [1]
- CH_4 (Methane). [1]
- It has the lowest relative molecular mass (M_r) / is the lightest gas. [1]