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Beng 130 Lecture 7 - Molecular Motion Transport Processes...

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Molecular Motion Transport Processes and Properties Transport Processes Transport Processes Diffusion Diffusion - Thermodynamic driving force - Frick Frick s first law. s first law. - Frick s second law. - Stokes - Einstein Equation Viscosity Viscosity Molecular Physical Chemistry – Lecture 7 Reading: Chapter 6 and Lecture Notes • Till recently we focused on systems at equilibrium where there is no net flow of heat, work, or matter. At the microscopic level, what we have been studying is time averaged behavior . • There are cases where time average of forces acting on a system results in a flow of material –This leads to transport phenomena Transport Processes 4 such phenomena we encounter are: – Diffusion – Electrical conduction – Fluid flow (convection) – Heat flow (conduction) Each of these represents net movement in the direction of the gradient from a higher to a lower potential Transport Processes The gradients arise from differences in – Chemical potential – Electrical potential – Temperature – Pressure The general equation for all forms of transport: The flow (J) of material in the x-direction is proportional to the gradient of force of type A in the x-direction . B is a constant. A BF dx dA B J 0 0 Transport Processes
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For diffusion - Fick’s Law B=D, the diffusion constant For electrical conduction - Ohm’s Law B= ț , the conductivity For fluid movement - Poiseuille’s Law B=C, the hydraulic conductivity For heat transfer - Fourier’s Law B=K T , the thermal conductivity coefficient A BF dx dA B J 0 0 Transport Processes The flux of material along a direction through a unit area in a unit time (second). dx dC D J 0 J – Flux; mol cm -2 s -1 D – Diffusion coefficient; cm 2 s -1 Diffusion – The Definition Let us assume that the concentration of species x in solution is allowed to vary only in the x-direction. There is a gradient in
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