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STOICHIOMETRY

STOICHIOMETRY. Stoichiometry is the science of using balanced chemical equations to determine exact amounts of chemicals needed or produced in a chemical reaction. Determining Ratios in Balanced Chemical Reactions.

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STOICHIOMETRY

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  1. STOICHIOMETRY

  2. Stoichiometry is the science of using balanced chemical equations to determine exact amounts of chemicals needed or produced in a chemical reaction.

  3. Determining Ratios in Balanced Chemical Reactions • What is the balanced chemical reaction for the reaction between nitrogen gas (N2) and hydrogen gas (H2) to produce ammonia (NH3)? • This means that for every one mole of N2 you have (28.02 g) you need to have 3 mole of H2 (3 × 2.02 g = 6.06 g)… (34.08 g of reactants) • You make 2 mole of NH3 ( 14.01 + 3.03 = 17.04 * 2 = 34.08 g)

  4. Mole Ratios • These are what we call mole ratios • Example 1: If you had 5 mol of N2, how many mol of H2 would you need? How many mol of NH3 would you make?

  5. Example Problem 2: Calculations Involving Mass of Reactants • Propane, C3H8(g), is a gas that is commonly used in barbeques. Calculate the mass of oxygen gas, O2, that is needed to burn 15 g of propane. (Produces CO2 and H2O)

  6. Example Problem 2: Calculations involving Numbers of Entities and Mass • How many molecules of oxygen are produced from the decomposition of 12 g of water into its elements? • Recall: O2 and H2 are produced

  7. Practice Problems: • Bauxite ore contains aluminum oxide, Al2O3, which is decomposed using electricity to produce aluminum metal and oxygen gas (O2). What masse of aluminum metal can be produced from 125 g of aluminum oxide? How many grams of O2 are produced? • Potassium metal, K(s), reacts with hydrochloric acid, HCl(aq), to produce aqueous potassium chloride and hydrogen gas, H2. How many grams of potassium are required to produce 5.00 g of hydrogen gas? • Potassium chlorate, KClO3, decomposes when heated to form solid potassium chloride and oxygen gas. How many grams of KClO3 must decompose to produce 0.96 g of O2?

  8. Limiting Reactant, Excess Reactant and Percent Yield • In the real world, you never have perfect amounts of every reactant you need…

  9. Example • In the synthesis of water, we require exactly 2 mol of H2 and exactly 1 mol O2 to create exactly 2 mol of H2O… • What would happen if we have a 2 mol H2 and 8 mol of O2? • Would this change the amount f H2Othat would be created?

  10. No… • There would be 7mol of O2 left over. • O2 is an excess reagent; H2 is limiting

  11. A chemical reaction will stop once any one of the reactants runs out. • This reactant is known as the limiting reagent (limiting reactant) • Any other reactant is an excess reagent

  12. Example 1: Table Salt, BaCL, can be formed by the reaction of sodium metal and chlorine gas. A reaction mixture contains 45.9 g of sodium and 142.0 g of chlorine. Calculate the mass of sodium chloride that is produced.

  13. Example 2: • Determine the mass of carbon monoxide that is produced when 32.1 g of methane, CH4 undergoes incomplete combustion with 160.0 g of oxygen (products: CO and H2O)

  14. Example 3: • Phosphorus, P4, reacts with chlorine gas to produce solid phosphorus pentachloride as the only product. Determine the mas of phosphorus pentachloride that is produced from a reaction between 123.88 g of phosphorus and 950.00 g of chlorine.

  15. Percentage Yield • Yield: the quantity of product produced in a chemical reaction • Actual Yield: the quantity of product that is actually produced in a chemical reaction • Theoretical Yield: the quantity of product calculated from a balanced chemical equation (using stoichiometry)

  16. There are many reasons why the amount of a product predicted for a reaction may not actually form • Reactions may not go to completion • Some of the reactant may be impure • There may be competing side reactions • It may be difficult to collect the product

  17. Example 1: • Iron is produced from its ore, hematite, Fe2O3, by heating hematite with carbon monoxide in a blast furnace. If 635 g of iron is obtained from 1150 g of hematite, what is the percentage yield of iron?

  18. Practice 1: • The most common ore of arsenic, FeSAs, can be heated to produce arsenic and iron(II) sulfide. When 250 g of the ore was processed industrially, 95.3 kg of arsenic was obtained. Calculate the yield of arsenic.

  19. Homework… • You have a test on Monday, November 24 at 6:30 pm, sharp. • I strongly recommend you study for it.

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