197. The driving force for mass transfer by molecular diffusion is the difference in
1. Potential energy
2. Vapour pressure
3. Gibbs free energy
4. Chemical potential

Introduction:

Mass transfer is a fundamental concept in processes like chemical engineering, biology, and environmental science. It refers to the movement of molecules from regions of higher concentration to lower concentration. The process of molecular diffusion is driven by differences in certain thermodynamic properties. Understanding these driving forces is crucial in optimizing processes such as filtration, reaction engineering, and material science.

Molecular Diffusion and Its Driving Force:

Molecular diffusion is the process by which molecules move due to concentration gradients. For this process to occur, there needs to be a difference between the properties of the two regions involved in the diffusion process.

The driving force for molecular diffusion is related to the difference in chemical potential. The chemical potential is a thermodynamic quantity that represents the potential energy per particle and drives the movement of particles from high to low chemical potential regions.

Key Thermodynamic Concepts:

• Potential energy: Refers to the stored energy in an object due to its position or state. While potential energy can influence mass transfer in some contexts, it is not the primary driving force for molecular diffusion.

• Vapor pressure: This refers to the pressure exerted by a vapor in equilibrium with its liquid or solid phase. While it can influence the rate of evaporation or condensation, it is not directly related to molecular diffusion in general contexts.

• Gibbs free energy: A thermodynamic potential used to predict the spontaneity of processes, but in terms of mass transfer, the direct driving force is more accurately described by chemical potential.

• Chemical potential: This is the most accurate description of the driving force for molecular diffusion. It represents the change in the Gibbs free energy as the number of particles changes in a system. Diffusion occurs when there is a difference in chemical potential between regions.

Conclusion:

The driving force for mass transfer by molecular diffusion is the difference in chemical potential. This difference causes molecules to move from areas of high chemical potential (higher concentration) to areas of low chemical potential (lower concentration) until equilibrium is reached.

Answer:
4. Chemical potential