Chemistry – States of matter - Diffusion | e-Consult

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.