Chapter 5

Rate of Diffusion

  • Definition: The rate at which a substance moves through a medium, e.g., air.

  • Example: A bottle of perfume releases scent, and over time, everyone in the room can smell it due to diffusion.

  • Molecular movement: Molecules randomly collide with one another, facilitating diffusion.

Brownian Motion

  • Definition: The random movement of particles suspended in a fluid (liquid or gas) resulting from their collision with fast-moving molecules in the fluid.

  • Rate of diffusion correlation: The rate of diffusion is dependent on the rate of Brownian motion.

  • Relationship: Increasing Brownian motion leads to an increase in the rate of diffusion.

Factors Affecting Diffusion

Temperature

  • Effect: Increasing the temperature enhances Brownian motion by increasing molecular activity.

  • Examples of temperature effects:

    • At 25 degrees Celsius, molecules move at a certain pace.

    • At lower temperatures (e.g., 4 degrees), molecular action slows down.

    • At higher temperatures (e.g., 50 degrees), molecules collide more energetically, increasing diffusion.

Molecular Weight

  • Inverse relationship: As molecular weight increases, the rate of diffusion decreases.

  • Analogy: Heavier objects (like a refrigerator) are harder to move, similar to heavier molecules diffusing slower.

Type of Medium

  • Medium specifications: Diffusion occurs in gas, liquid, or solid mediums.

  • Behavior in different media:

    • Gases: Molecules diffuse more quickly due to less mass.

    • Liquids: Diffusion occurs but is slower than in gases.

    • Solids: Very slow diffusion due to tightly packed molecules.

Experimental Setup

Experiment Objective

  • To illustrate and test the effects of molecular weight and temperature on the rate of diffusion using methyl orange and potassium permanganate.

Procedure Overview

  • Materials: Use plates with holes, methyl orange, potassium permanganate.

  • Temperature settings: 4 degrees, 25 degrees, and 37 degrees Celsius environments.

  • Molecular weights:

    • Potassium permanganate (58 g/mol) is lighter than methyl orange (327 g/mol).

Anticipated Results

  • Diffusion: Expect potassium permanganate to diffuse faster than methyl orange due to lower molecular weight.

  • Impact of temperature: The reaction at 37 degrees will show faster diffusion rates compared to 4 and 25 degrees due to increased molecular motion.

Data Recording

  • Measurement: After ten minutes of diffusion, measure diameter and convert to radius in millimeters.

  • Calculating diffusion rates:

    • Convert radius to micrometers for standardization.

    • Note time taken for diffusion to measure rate in micrometers per second.

Final Calculations and Observations

  • Diameter to radius: Diameter (e.g., 50 mm) divided by 2 gives radius (e.g., 25 mm).

  • Conversion: Radius in millimeters to micrometers (e.g., 25 mm = 25,000 micrometers).

  • Time factor: Adjust for seconds when calculating diffusion over a 10-minute period (600 seconds).

  • Rate formula: Final diffusion rate will be calculated as micrometers divided by elapsed time.