The scaling analysis for the mass-transfer boundary-layer thickness is parallel to that for heat transfer analysis. Use the same method to show that \(\Delta\) is proportional to \(S c^{-1 / 3}\) for a non-reacting system.

Extend the analysis for a reacting system and suggest a suitable expression for \(\Delta\).

Then show that, in the limit of fast reaction, \(D a_{x} \gg 1\), the expression simplifies to Eq. (20.8).

1 A = (Da,Sc) 1/2 (20.8)