Limiting Reactants

 Section 9.2 Limiting Reactants and Percentage Yield

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In the previous section, we assumed that 100% of the reactants changed into products.

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That is what should happen theoretically.

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In the real world, other factors can limit the yield of a reaction.

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the amounts of all reactants

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the completeness of the reaction

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product lost in the process

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Whatever reactant is in short supply will limit the quantity of product made.

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Reactants of a reaction are seldom present in ratios equal to the mole ratio in the balanced equation, so one of the reactants is used up first.

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For example, If you combine 0.23 mol Zn and 0.60 mol HCl, would they react completely?

Zn + 2HCl ® ZnCl₂ + H₂

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The coefficients indicate that 0.23 mol Zn forms 0.23 mol H₂, and 0.60 mol HCl forms 0.30 mol H₂.

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Zinc is called the limiting reactant because it limits the amount of product that can form. It runs out first.

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The HCl is the excess reactant because there is more than enough HCl present to react with all of the Zn. It is left over at the end of the reaction.

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The maximum quantity of product that a reaction could theoretically make if everything about the reaction works perfectly is called the

theoretical yield.

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The theoretical yield of a reaction should always be calculated based on the limiting reactant.

Sample Problem E

Identify the limiting reactant and the theoretical yield of phosphorous acid, H₃PO₃, if 225 g of PCl₃ is mixed with 125 g of H₂O.

PCl₃ + 3H₂O ® H₃PO₃ + 3HCl