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Unit 9- Stoichiometry

Unit 9- Stoichiometry. What is Stoichiometry?. Using chemical equations to make predictions about the quantities of reactants or products. Predicting a precipitate. Practice with precipitates (SOLID). H 2 SO 4 + BaCl 2  Ba SO 4(s) + HCl

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Unit 9- Stoichiometry

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  1. Unit 9- Stoichiometry

  2. What is Stoichiometry? • Using chemical equations to make predictions about the quantities of reactants or products.

  3. Predicting a precipitate

  4. Practice with precipitates (SOLID) • H2SO4 + BaCl2 BaSO4(s)+ HCl • Al2(SO4)3 + NH4OH  Al(OH) 3 + (NH4)2SO4 • Which is the precipitate or Solid formed??? • AgNO3 + H2S  Ag2S + HNO3

  5. A sandwich analogy… • Suppose you wanted to make turkey sandwiches. You use the following: 2 bread slices + 2 turkey slices + 1 cheese slice  1 sandwich If you have 10 slices of bread, how many sandwiches could you make? If you had unlimited bread, 8 slices of cheese and 14 slices of turkey, how many could you make?

  6. We do the same with chemical reactions!!! • N2 (g)+ 3H2 (g)  2NH3 (g) What does this mean?

  7. Calculations • Mole – Mole Calculations: • Coefficients from the balanced chemical equation are used to write conversion factors called mole to mole ratios. • With these ratios you can relate moles of to one compound to moles of another compound.

  8. Mole to mole ratio • Mole Ratios for Ammonia: • N2 + 3H2 2NH3 • 1 mol N2 – 3 mol H2 • 2 mol NH3 – 1 mol N2 • 3 mol H2 – 2 mol NH3

  9. Practice • Using the following equation – give the mole ratio of oxygen gas to water. • 2H2S + 3O2 2SO2 + 2 H2O • Answer: 3 mole O2 / 2 mol H2O

  10. Moles of Product • How many moles of ammonia are produced when 0.60 mol of nitrogen reacts with hydrogen? • N2 + 3H2 2NH3 • Known: moles of nitrogen = 0.60 mol of N2 • Unknown: moles of ammonia?

  11. Answer: • 0.60 mol N2 x 2 mol NH3 = 1.2 mol NH3 1 mol N2

  12. Practice Problem • 4 Al + 3O2 2Al2O3 • How many moles of aluminum are needed to form 3.7 mol Al2O3? • Known: Mole of Al2O3 • Unknown: mole of Al

  13. Answer • 3.7 mol Al2O3 X 4 mole Al = 7.4 mol Al 2 mol Al2O3

  14. Gram to mole • How many moles of ammonia can be produced from 15g of N2? What we know… Molar mass of N2 is 28.02 g/mol We are given: 15 g of N2 We need to find: moles of NH3

  15. How to solve gram to mole… Start with the balanced chemical equation: 3 H2 + N2 2 NH3 15g N2 1mol N2 2 mol NH3 28.02 g N2 1 mol N2 = 1.1 mol

  16. Now try this one… H2 + Br2 2 HBr How many moles of HBr will be produced from 12.45 grams of H2?

  17. Mass to Mass Calculations • Calculate the number of grams of NH3 produced by the reaction of 5.40 g of H2 and excess nitrogen. The balanced equation is: N2 + 3H2 2NH3

  18. Grams to Grams!!!

  19. Practice Known: 5.00 g of CaC2 Unknown: g of C2H2

  20. Answer • 5.00 g CaC2 X 1 mol CaC2 X 1 mol C2H2 X 26.0 g C2H2 = 2.03 g C2H2 64.0 g CaC2 1 mol CaC2 1 mol C2H2

  21. What is a limiting Reagent? • Reactant that limits the amount of product that is produced. • Why? – It runs out first.

  22. Identify the limiting reagent… • When magnesium metal is dropped into hydrochloric acid, hydrogen gas and magnesium chloride are produced. If the magnesium is consumed, and HCl remains, what is the limiting reagent?

  23. Reaction Yields • Theoretical Yield – maximum amount of product that could be formed from the amounts of reactants • Actual Yield – amount of product actually produced in the reaction • Often less than the theoretical yield

  24. Percent Yield • Ratio of the actual yield to the theoretical yield • Measure of the efficiency of the reaction • Actual / theoretical yield x 100= percent yield!

  25. Why is percent yield usually less than 100%? • Reaction doesn’t go to completion • Impurities in reactants • Product might be lost during experimental process

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