Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. This movement occurs randomly and is driven by the kinetic energy of the particles. Kinetic particle theory states that all matter is in constant random motion. The kinetic energy of these particles is directly proportional to the temperature of the substance.

In diffusion, particles are constantly moving and colliding with each other. The higher the temperature, the greater the average kinetic energy of the particles. This means they move faster and collide more frequently. This increased frequency of collisions leads to a faster rate of diffusion.

Particle size also affects the rate of diffusion. Smaller particles diffuse faster than larger particles because they have a greater surface area to volume ratio. This means that a smaller proportion of the particle's kinetic energy is 'wasted' on internal collisions, and more of it contributes to movement across the concentration gradient. Therefore, smaller particles diffuse more rapidly.

Temperature's effect on diffusion rate:

• Higher temperature: Increased kinetic energy, faster particle movement, faster diffusion.
• Lower temperature: Decreased kinetic energy, slower particle movement, slower diffusion.

The observation that diffusion rates increase with temperature is directly explained by kinetic particle theory. As temperature increases, the average kinetic energy of the gas molecules increases. This means the molecules move faster and have more frequent and forceful collisions.

These more energetic collisions result in a greater number of molecules overcoming the resistance to movement from the region of high concentration to the region of low concentration. The increased kinetic energy allows the molecules to diffuse more rapidly. The diagram below illustrates this:

[Image missing: Diffusion with temperature diagram]

Explanation of the diagram:

• Region of high concentration (left): Many molecules are packed closely together.
• Region of low concentration (right): Fewer molecules are present.
• Arrows: Represent the movement of gas molecules from left to right.
• Higher temperature (indicated by the wavy arrows): The molecules have more kinetic energy and move faster, leading to a faster rate of diffusion.

Gases with lower relative molecular masses diffuse faster than gases with higher relative molecular masses because the rate of diffusion is directly related to the average speed of the gas molecules. This relationship is fundamentally linked to the kinetic molecular theory.

Molecular Mass and Average Speed: The kinetic molecular theory states that the average kinetic energy of gas molecules is directly proportional to the absolute temperature and inversely proportional to the molar mass (Mr). This means that lighter molecules (lower Mr) have higher average speeds at a given temperature compared to heavier molecules (higher Mr). The average speed is proportional to the square root of the molar mass.

Diffusion Rate and Average Speed: The rate of diffusion is directly proportional to the average speed of the gas molecules. This is because faster-moving molecules spread out more quickly and efficiently. Therefore, gases with higher average speeds (lower Mr) diffuse faster.

In summary: Lighter molecules have higher average speeds, and faster-moving molecules diffuse faster. The relationship between molecular mass and average speed is key to understanding why lower molecular mass gases diffuse at a faster rate. The faster the molecules move, the more rapidly they spread out and mix, leading to a quicker diffusion rate.