Conversion and its related terms yield and selectivity are important terms in chemical reaction engineering. They are described as ratios of how much of a reactant has reacted (X — conversion, normally between zero and one), how much of a desired product was formed (Y — yield, normally also between zero and one) and how much desired product was formed in ratio to the undesired product(s) (S — selectivity). There are conflicting definitions in the literature for selectivity and yield, so each author's intended definition should be verified. Conversion can be defined for (semi-)batch and continuous reactors and as instantaneous and overall conversion. Assumptions The following assumptions are made: The following chemical reaction takes place: , where and are the stoichiometric coefficients. For multiple parallel reactions, the definitions can also be applied, either per reaction or using the limiting reaction. Batch reaction assumes all reactants are added at the beginning. Semi-Batch reaction assumes some reactants are added at the beginning, the rest is fed during the batch. Continuous reaction assumes reactants are fed and products leave the reactor continuously and in steady state. Conversion Conversion can be separated into instantaneous conversion and overall conversion. For continuous processes the two are the same, for batch and semi-batch there are important differences. Furthermore, for multiple reactants, conversion can be defined overall or per reactant. Instantaneous Conversion Semi-Batch In this setting there are different definitions. One definition regards the instantaneous conversion as the ratio of the instantaneously converted amount to the amount fed at any point in time: . with as the change of moles with time of species i. This ratio can become larger than 1. It can be used to indicate whether reservoirs are built up and it is ideally close to 1. When the feed stops, its value is not defined. In semi-batch polymerisation, the instantaneous conversion is defined as the total mass of polymer divided by the total mass of monomer fed: . Overall Conversion Batch (This is the generally stated form) Semi-Batch Total conversion of the formulation: Total conversion of the fed reactants: Continuous (This is the generally stated form) Yield Yield in general refers to the amount of a specific product (p in 1..m) formed per mole of reactant consumed (Definition 1). However, it is also defined as the amount of product produced per amount of product that could be produced (Definition 2). If not all of