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Limiting and Excess Reactants

Percentage Yield. Limiting and Excess Reactants. Some Definitions. Limiting Reactant – The substance that controls the quantity of product that can form in a chemical reaction Excess Reactant – The substance that is NOT used up completely in the reaction Yield – The mass of product formed

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Limiting and Excess Reactants

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  1. Percentage Yield Limiting and Excess Reactants

  2. Some Definitions • Limiting Reactant – The substance that controls the quantity of product that can form in a chemical reaction • Excess Reactant – The substance that is NOT used up completely in the reaction • Yield – The mass of product formed • Theoretic – assuming no loss or error, the mathematical calculation of the mass of product formed • Actual – the measured amount of the product from a reaction

  3. Stoichiometry • We use stoichiometry to determine: • Limiting Reactant • Excess Reactant • Theoretic Yield

  4. Limiting vs. Excess Reactant • The Limiting reactant will form the LEAST amount of product. • The Excess reactant will form the MOST amount of the product. • We can determine these using stoichiometry if we know how much mass (or how many moles) we have of each reactant.

  5. Example Zn + 2HCl  ZnCl2 + H2 • If you combine 0.23 mol Zn and 0.60 mol HCl, would they react completely? • In other words, will they produce the same amount of product, or will one produce more ZnCl2 than the other?

  6. Example #2 PCl3 + 3H2O  H3PO3 + 3HCl • Determine the limiting and excess reactants in this equation if you have 75.0 g PCl3 and 75.0 g H2O. • How about 1.o mol of PCl3 and 50.0 g of H2O?

  7. Determining Theoretic Yield • The theoretic yield is what you determined when you determined the limiting reactant. This is the most product that can be produced in ideal circumstances. • There will be instances when you will have more product than you should. Why might that be? • There will be instances when you will have less product. Why??

  8. Determining Actual Yield • Actual yield is determined by measurement after the reaction is complete.

  9. Determining Percentage Yield • This is a ratio of the actual yield of a reaction to its theoretic yield. • This ratio determines the efficiency of the reaction. Percent Yield = Actual Yield Theoretic Yield

  10. Examples • If the theoretic yield for a reaction is 738 g, and the actual yield is 591 g, what is the percentage yield? • How many grams of CH3COOC5H11 should form if 4808 g are theoretically possible, and the percentage yield is 80.5%?

  11. One more Example N2 + 3H2 2NH3 • 14.0 g N2 react with 3.15 g H2 to give an actual yield of 14.5 g of NH3. Determine the limiting reactant and the percentage yield for this reaction.

  12. #6, p. 319 • A chemist reacts 8.85 g of iron with an excess of hydrogen chloride to form hydrogen gas and iron (II) chloride. Calculate the theoretical yield and the percentage yield of hydrogen if 0.27 g of H2are collected.

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