Part 1of this series covered the reaction of a non-porous solid.([1]) In this Part 2 of the series, we present a review on the reactions involving a porous solid, which is the most widely encountered type of fluid-solid reactions. The major difference between the reactions of a non-porous and a porous solid is the fact that in the latter, the fluid reactant can reach the interior of the porous solid through the pores even through a portion that contains unreacted solid reactant. The rate expression for the solid at any point depends on, among others, how the area of reaction interface changes with the conversion of the solid. The governing equations, in general, require numerical solution. In this article we describe an approximate closed-form solution to this problem that yielded Sohn's law of additive reaction times. The result is simple to apply and has many useful features, especially for the design and modelling of a multi-particle system.