Chapter 14

How is the concentration of a commodity defined? How is the concentration gradient defined? How is the diffusion rate of a commodity related to the concentration gradient?

Give examples for (a) liquid-to-gas, $(b)$ solid-to-liquid, (c) solid-to-gas, and ( $d$ ) gas-to-liquid mass transfer.

What is the driving force for $(a)$ heat transfer, (b) electric current flow, $(c)$ fluid flow, and $(d)$ mass transfer?

Both Fourier's law of heat conduction and Fick's law of mass diffusion can be expressed as $\dot{Q}=-kA(dT/dx)$. What do the quantities $\dot{Q},k,A$, and $T$ represent in $(a)$ heat conduction and $(b)$ mass diffusion?

How does mass transfer differ from bulk fluid flow? Can mass transfer occur in a homogeneous medium?

What do $(a)$ homogeneous reactions and $(b)$ heterogeneous reactions represent in mass transfer? To what do they correspond in heat transfer?

Someone claims that the mass and the mole fractions for a mixture of ${\mathrm{CO}}_2$ and ${\mathrm{N}}_2\mathrm{O}$ gases are identical. Do you agree? Explain.

Mark these statements as being True or False for a binary mixture of substances $A$ and $B$. (a) The density of a mixture is always equal to the sum of the densities of its constituents. (b) The ratio of the density of component $A$ to the density of component $B$ is equal to the mass fraction of component $A$. (c) If the mass fraction of component $A$ is greater than $0.5$, then at least half of the moles of the mixture are component $A$. (d) If the molar masses of $A$ and $B$ are equal to each other, then the mass fraction of $A$ will be equal to the mole fraction of $A$. (e) If the mass fractions of $A$ and $B$ are both $0.5$, then the molar mass of the mixture is simply the arithmetic average of the molar masses of $A$ and $B$.

Fick's law of diffusion is expressed on the mass and mole basis as ${\dot{m}}_{\mathrm{diff},A}=-\rho A{D}_{AB}\left(d{w}_A/dx\right)$ and ${\dot{N}}_{\mathrm{diff},A}=$ $-CA{D}_{AB}\left(d{y}_A/dx\right)$, respectively. Are the diffusion coefficients $D_{AB}$ in the two relations the same or different?

How does the mass diffusivity of a gas mixture change with $(a)$ temperature and $(b)$ pressure?