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Section 3: Limiting Reactants

Section 3: Limiting Reactants. Stoichiometry. Learning Goals:. Identify the limiting reactant in a chemical equation . Identify the excess reactant, and calculate the amount remaining after the reaction is complete.

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Section 3: Limiting Reactants

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  1. Section 3: Limiting Reactants Stoichiometry

  2. Learning Goals: • Identify the limiting reactant in a chemical equation. • Identify the excess reactant, and calculate the amount remaining after the reaction is complete. • Calculate the mass of a product when the amounts of more than one reactant are given.

  3. Why Do Reactions Stop? • Reactions proceed until one of the reactants is used up and one is left in excess. • The limiting reactant limits the extent of the reaction and, thereby, determines the amount of product formed. • The excess reactants are all the leftover unused reactants.

  4. Why Do Reactions Stop? • Determining the limiting reactant is important because the amount of the product formed depends on this reactant.

  5. Calculating Products • Ex.) S8(l) + 4Cl2(g) → 4S2Cl2(l) • If 200.0g of sulfur reacts with 100.0g of chlorine, what mass of disulfur dichloride is produced?

  6. Calculating Products • Determine moles of reactants

  7. Calculating Products • Determine whether the two reactants are in the correct mole ratio, as given in the balanced chemical equation. Only 1.808 mol of chlorine is available for every 1 mol sulfur, instead of the 4mol of chlorine required by the balanced chemical equation, thus chlorine is the limiting reactant.

  8. Calculating Products • Calculate the amount of product formed.

  9. Calculating Products • Now that you have determined the limiting reactant and the amount of product formed, what about the excess reactant, sulfur? How much of it reacted? • You need to make a mole-to-mass calculation to determine the mass of sulfur needed to react completely with 1.410 mol of chlorine.

  10. Calculating Products • Next, obtain the mass of sulfur needed:

  11. Calculating Products • Knowing that 200.0g of sulfur is available and only 90.42g is needed, you can calculate the amount of sulfur left unreacted when the reaction ends.

  12. Calculating Products • Using an excess reactant can speed up the reaction. • Using an excess reactant can drive a reaction to completion.

  13. Practice • The reaction between solid white phosphorus (P4) and oxygen produces solid tetraphosphorusdecoxide. • Determine the mass of P4O10 formed if 25.0 g of P4 and 50.0 g of oxygen are combined. • How much of the excess reactant remains after the reaction stops?

  14. Section 4: Percent Yield Stoichiometry

  15. Learning Goals • Calculate the theoretical yield of a chemical reaction from data. • Determine the percent yield for a chemical reaction.

  16. How Much Product? • Laboratory reactions do not always produce the calculated amount of products. • Reactants stick to containers. • Competing reactions form other products.

  17. How Much Product?

  18. How Much Product? • The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant. • The actual yield is the amount of product actually produced when the chemical reaction is carried out in an experiment.

  19. How Much Product? • The percent yield of a product is the ratio of the actual yield expressed as a percent.

  20. Practice • Solid silver chromate (Ag2CrO4) forms when potassium chromate is added to a solution containing 0.500g of silver nitrate. Determine the theoretical yield of silver chromate. Calculate the percent yield if the reaction produces 0.455 g of silver chromate.

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