Each reaction in a reaction network consists of a predetermined set of operations which can be represented by a directed acyclic graph, the operation graph, where the nodes represent the operations of this reaction and edges represent the required precedence relations among the operations. We use the term "required" here to distinguish it from the implied precedence relations in an existing schedule. The required precedence relations of a chemical reaction sometimes lead to a linear sequence of operations (a path graph), such as the additions of reagents with a fixed addition order followed by a heating operation. In other cases, however, an operation can be required to precede, or to be preceded by, multiple other operations.