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One reactant (A) is chosen, and the balanced chemical equation is used to determine the amount of the other reactant (B) necessary to react with A. If the amount of B actually present exceeds the amount required, then B is in excess and A is the limiting reagent. If the amount of B present is less than required, then B is the limiting reagent.
Stoichiometric equations are used to determine the limiting reagent or reactant—the reactant that is completely consumed in a reaction. The limiting reagent determines the theoretical yield—the relative quantity of moles of reactants and the product formed in a chemical reaction. Other reactants are said to be present in excess.
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 ...
From the general mole balance on some species , where for a CSTR steady state and perfect mixing are assumed, + = + = = Assuming a constant volumetric flow rate , which is the case for a liquid reactor or a gas phase reaction with no net generation of moles,
To find the limiting reagent and the mass of HCl produced by the reaction, we change the above amounts by a factor of 90/324.41 and obtain the following amounts: 90.00 g FeCl 3, 28.37 g H 2 S, 57.67 g Fe 2 S 3, 60.69 g HCl. The limiting reactant (or reagent) is FeCl 3, since all 90.00 g of it is used up while only 28.37 g H 2 S are consumed.
As an example, consider the gas-phase reaction NO 2 + CO → NO + CO 2.If this reaction occurred in a single step, its reaction rate (r) would be proportional to the rate of collisions between NO 2 and CO molecules: r = k[NO 2][CO], where k is the reaction rate constant, and square brackets indicate a molar concentration.
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If we assume a local steady state, then the rate at which B reaches is the limiting factor and balances the reaction. Therefore, the steady state condition becomes 1. [] = where is the flux of B, as given by Fick's law of diffusion, 2.