Fick's Law: Diffusion Flux Explained

Fick's First Law describes the rate of molecular diffusion. It states that the diffusion flux, which is the amount of substance crossing a unit area per unit time, is directly proportional to the concentration gradient.

Understanding Diffusion Flux

The law can be mathematically represented as:

$ J = -D \frac{dC}{dx} $

Where:

• $J$ represents the diffusion flux (e.g., in mol/m²s).
• $D$ is the diffusion coefficient (a constant for a given system).
• $dC/dx$ represents the concentration gradient – the rate at which concentration changes with distance.

The negative sign indicates that diffusion occurs from a region of higher concentration to a region of lower concentration, effectively down the gradient.

Analyzing the Options

• Concentration Gradient (Option 3): This is the primary driving force for molecular diffusion according to Fick's First Law. A steeper gradient leads to a higher flux.
• Difference in Concentration (Option 4): While a difference in concentration is necessary to *have* a gradient, the term 'gradient' specifically refers to how this difference changes over distance, which is what the law quantifies. Option 3 is more precise.
• Difference in Temperature (Option 1): Temperature differences drive heat transfer (Fourier's Law), not molecular diffusion flux directly, although temperature affects the diffusion coefficient $D$.
• Pressure Difference (Option 2): Pressure differences can cause bulk flow (like flow in a pipe) but are not the basis of Fick's First Law for molecular diffusion.

Therefore, the molecular diffusion flux, according to Fick's First Law, primarily depends on the Concentration Gradient.