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Stoichiometry

Stoichiometry. Quantitative Analysis. Composition Stoichiometry. Find the molar mass of: Al 2 O 3 = Al = O 2 =. Deals with mass relationships of elements in compounds Formula (molar) mass Converting grams to moles to atoms/molecules. Reaction Stoichiometry.

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Stoichiometry

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  1. Stoichiometry Quantitative Analysis

  2. Composition Stoichiometry Find the molar mass of: Al2O3 = Al = O2 = • Deals with mass relationships of elements in compounds • Formula (molar) mass • Converting grams to moles to atoms/molecules

  3. Reaction Stoichiometry Involves the mass relationships between reactants and products in a chemical reaction Based on balanced chemical equations and the law of conservation of mass Mole ratio = a conversion factor that relates the amounts in moles of any two substances involved in a chemical rxn

  4. RXN Stoichiometry Problems For every 2 moles of Al2O3 , you produce 4 moles of Al and 3 moles of O2 The coefficients represent the mole ratios of all elements and compounds in a balanced equation. How many moles of oxygen will be produced if you decompose 5.8 moles of aluminum oxide? Amt (moles) of given → amt (moles) of unknown Ex. 2Al2O3 → 4Al + 3O2

  5. RXN Stoichiometry Problems For every 2 moles of Al2O3 , you produce 4 moles of Al and 3 moles of O2 What was the mass of aluminum oxide decomposed if 5 moles of oxygen is produced? Amt of given→amt of unknown→mass of unknown 2Al2O3 → 4Al + 3O2

  6. RXN Stoichiometry Problems Mass of given Moles of given Moles of unknown 2Al2O3 → 4Al + 3O2 If 150 grams of aluminum oxide is decomposed, how many moles of aluminum are produced?

  7. RXN Stoichiometry Problems Mass of given Moles of given Moles of unknown Mass of unknown 2Al2O3 → 4Al + 3O2 If 300 grams of aluminum oxide is decomposed, what is the mass of aluminum produced?

  8. Ideal Conditions? Exact required amount of reactants based on equation? Not realistic We will run out of supplies of one of the reactants and have excess of the others

  9. Limiting Reagent (reactant) = the reactant that limits the amount of the products that can be produced (because we have a limited amt) Excess reactant = the substance that is NOT used up completely

  10. Determining the Limiting Reagent Whichever reactant produces the least of the product is the limiting reactant. Given: equation and masses of reactants Unknown: limiting reactant Write and balance chemical equation Convert masses (g) of reactants into moles of reactants using their molar masses Multiply mole amount of each reactant by mole ratio to determine mole amount of products

  11. Example 1 N2H4 (l) + H2O (l) → N2 (g) + H2O (g) Which is the limiting reactant when 0.750 mol of N2H4 is mixed with 0.500 moles of H2O?

  12. Example 2 Fe (s) + H2O (g) → Fe3O4 (s) + H2 (g) When 36.0 g of water is mixed with 67.0 g of iron, which is the limiting reactant?

  13. Calculating Yield (=how much of product is made) After limiting reactant is identified, you can use it to determine theoretical yield (= the amt of product predicted by the balanced equation) • Use mole amount of limiting reactant and mole ratio of equation to determine how much product is produced (just like you did last week)

  14. Example 1 Zn (s) + S8 (s) → ZnS (s) 2.00 mol Zn reacts with 1.00 mol of S8 • What is the limiting reactant? • How many moles of the product are produced (what is the theoretical yield)?

  15. C (s) + H2O (g) → H2 (g) + CO (g) 2.40 mol of C react with 3.10 mol of steam • What is the limiting reactant? • Calculate theoretical yield of each product

  16. REMINDERS Grams → Moles → Moles → Grams When you can calculate the moles used or produced, you can always convert it into grams USING MOLAR MASS!!! When you are given the mass of something, you MUST FIRST convert it into moles (USING MOLAR MASS!!) before you can begin a problem

  17. % Yield • You may be given a value for the ACUTAL yield of product (amount measured during the ACTUAL performance of the reaction in a lab) • Using the theoretical yield (which you can now find), you can calculate the % yield % yield = actual yield_ X 100 theoretical yield

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