MOLECULAR DIFFUSION

Molecular diffusion

• Mass transfer process can take place in a gas or vapour or in a liquid, and it can result from the random velocities of the molecules (molecular diffusion) or from the circulating or eddy currents present in a turbulent fluid (eddy diffusion).
• Molecular diffusion occurs due to movement of individual molecule through a substance by virtue of their thermal energy
• The phenomenon of molecular diffusion ultimately leads to completely uniform concentration of substances throughout solution which may initially have been nonuniform.
• For example:-

1. If a drop of blue copper sulphate solution is placed in a beaker of water, the copper sulphate eventually permeate the entire liquid. the blue color in times everywhere uniform, and no subsequent changes occurs.
2. In a two phase system not at equilibrium, such as layer of ammonia and air as a gas solution in contact with a layer of liquid water, spontaneous alteration through molecular diffusion also occurs, ultimately bringing the entire system to a state of equilibrium, after that diffusion stops.

• Hence in molecular diffusion ,if solution is not uniform everywhere in concentration of its constituents, the solution is spontaneously brought to uniformity by diffusion.
• It means that molecular diffusion is the mechanism of mass transfer in stagnant fluids or in fluids which are moving in laminar flow and the mass transfer takes place from a place of high concentration to low concentration.
• The rate at which a solute moves at any point in any direction must therefore depend upon concentration gradient at that point and in that direction.
• rate = molar flux [mol/ (area)(time)]
• The area is measured in the direction normal to the diffusion