Reactions in series—two reactants

The series-parallel type of reaction outlined in Section 1.10.1 is quite common among industrial processes. For example, ethylene oxide reacts with water to give monoethylene glycol, which may then react with more ethylene oxide to give diethylene glycol. 

In such cases the order with respect to B is usually the same for both the first and the second reaction. Under these circumstances, the level of concentration of B at which the reactions are carried out has no effect on the relative rates of the two reactions, as may be seen by writing these as parallel reactions with respect to B  

These reactions will therefore behave very similarly to the reactions in series above where only one reactant was involved. 

The same general conclusions apply since backmixing of products with reactants should be avoided, a tubular plug-flow reactor or a batch reactor is preferred. However, there is one respect in which a series reaction involving a second reactant B does differ from simple series reaction with one reactant, even when the orders are the same. This is in the stoichiometry of the reaction the reaction cannot proceed completely to the product Q, even in infinite time, if less than two moles 

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