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Limiting and Excess Reagents

Limiting and Excess Reagents. In a chemical reaction an insufficient quantity of any of the reactants will limit the amount of product that forms. A limiting reagent determines the amount of product that can be formed by a reaction.

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Limiting and Excess Reagents

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  1. Limiting and Excess Reagents • In a chemical reaction an insufficient quantity of any of the reactants will limit the amount of product that forms. • A limiting reagent determines the amount of product that can be formed by a reaction. • The reactant that is not completely used up is called the excess reagent.

  2. Grilled Cheese Sandwiches! • A typical grilled cheese sandwich needs 2 pieces of bread and 1 piece of cheese. • If you have 10 slices of bread and 3 pieces of cheese, you would only be able to make 3 sandwiches even though you have enough bread to make 5. • In this example, the cheese is the limiting reagent and the bread is the excess reagent.

  3. Determining the Limiting Reagent in a Reaction • Copper reacts with sulfur to form copper (I) sulfide according to the following balanced equation. 2Cu (s) + S (s)→ Cu2S (s) • What is the limiting reagent when 80.0 g of Cu reacts with 25.0 g S?

  4. Using a Limiting Reagent to Find the Quantity of a Product • What is the maximum number of grams of Cu2S that can be formed when 80.0 g Cu reacts with 25.0 g of S? 2Cu (s) + S (s)→ Cu2S (s)

  5. Theoretical and Percent Yields • The theoretical yield is the maximum amount of product that could be formed from given amounts of reactants. • In contrast, the amount of product that actually forms is the when the reaction is carried out is called the actual yield. • The percent yield is the ratio of actual yield to the theoretical yield expressed as a percent. • Percent yield = Actual Yield x 100% Theoretical Yield • The percent yield is the measure of the efficiency of a reaction carried out in the laboratory.

  6. Calculating the Theoretical Yield of a Reaction • Calcium Carbonate, which is found in seashells, is decomposed by heating. The balanced equation for this reaction is CaCO3(s)→ CaO(s) + CO2(g) • What is the Theoretical yield of CaO if 24.8 g of CaCO3 is heated?

  7. Calculating the Percent Yield of a Reaction • What is the percent yield if 13.1 g of CaO is actually produced when 24.8 g of CaCO3 is heated? CaCO3(s)→ CaO(s) + CO2(g)

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