Question

The driving force for diffusion is

• A. temperature gradient
• B. pressure gradient
• C. chemical potential gradient
• D. magnetic field

Hint:

Remember that "uphill diffusion" shows that concentration gradient is not the ultimate driving force; atoms can move up a concentration gradient to go down a chemical potential gradient.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the fundamental thermodynamic driving force that causes atomic or molecular diffusion.

Step 2: Key Formula or Approach:
Thermodynamically, the diffusion flux (\( J_i \)) of a species \( i \) is proportional to the gradient of its chemical potential (\( \mu_i \)):
\[ J_i = - L_i \frac{d\mu_i}{dx} \]
where:
\( L_i \) is a phenomenological mobility coefficient, and
\( \frac{d\mu_i}{dx} \) is the chemical potential gradient.

Step 3: Detailed Explanation:

• Chemical Potential vs. Concentration Gradient: While Fick's first law defines diffusion in terms of a concentration gradient (\( \frac{dC}{dx} \)), this is an approximation valid only for ideal solutions.
In non-ideal solutions, species can sometimes diffuse from low-concentration regions to high-concentration regions (known as uphill diffusion), which happens during spinodal decomposition or phase separation.

• Thermodynamic Driving Force: The true driving force is always the minimization of the Gibbs free energy, which is represented by the gradient of chemical potential. Atoms migrate to equalize chemical potentials.

• Analysis of Other Options:
-

Temperature gradients (Option A) can cause thermal diffusion (Soret effect), but they are not the primary driving force for ordinary mass diffusion.
-

Pressure gradients (Option B) can drive fluid bulk flow (advection) or pressure-induced diffusion, but not standard atomic diffusion in materials.
-

Magnetic fields (Option D) affect magnetic domains but do not drive mass diffusion.


Step 4: Final Answer:
Therefore, the ultimate driving force for diffusion is the chemical potential gradient, matching Option (C).