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Stoichiometry

Stoichiometry. The Mathematics of Chemical Equations. 9.1: Balanced Chemical Equations. 1. Provides qualitative and quantitative information. 2. Supports the Law of Conservation of Mass 3. 2H 2 + O 2 → 2H 2 O The above equation is interpreted in terms of particles as follows:

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Stoichiometry

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  1. Stoichiometry The Mathematics of Chemical Equations

  2. 9.1: Balanced Chemical Equations 1. Provides qualitative and quantitative information. 2. Supports the Law of Conservation of Mass 3. 2H2 + O2 → 2H2O The above equation is interpreted in terms of particles as follows: A.2 molecules of H2 react with 1 molecule of O2 to produce 2 molecules of water. The ratio of H2 to O2 to H2O is 2:1:2. OR

  3. 2H2 + O2 → 2H2O B. 20 molecules of H2 react with 10molecules of O2 to produce 20 molecules of water. Again, the ratio of H2 to O2 to H2O is 2:1:2.

  4. 2H2 + O2 → 2H2O C. The original equation can then be interpreted as follows: 2 moles of H2 react with 1 mole of O2 to produce 2 moles of water. The ratio of H2 to O2 to H2O is 2:1:2.

  5. D. It is more convenient to interpret the coefficients as the number of moles, because we measure amounts of substances by grams in the laboratory. We can convert between grams and moles in solving problems involving chemical reactions. 4. Stoichiometry: The study of the mathematical (quantitative) relationships that exist in a formula and in a chemical reaction. 5. The word Stoichiometry comes from the Greek word “stoicheion” meaning element, and “metron”, meaning measure.

  6. 6. Importance: The safe, economical and reproducible manufacture of chemicals (or food) and the safe administration of pharmaceuticals. 7. Proof: The Law of conservation of mass is shown by the balanced equation, or by adding up all the masses of reactants and products to determine if they are equal.

  7. 8. Mole-Mole Problems require 3 steps to determine the molar ratio: Stoichiometryville Map A. Write Equation B. Balance the equation to determine the molar ratio C. Moles Given (Known) to Moles Unknown

  8. Welcome to Stoichiometryville Mass Mass Given Substance Substance to be determined Molar Mass Molar Mass Particles Particles Moles Moles Mole Ratio 6.02 x 1023 6.02 x 1023 22.4 L 22.4 L Volume Volume

  9. Ex #1) Ammonium nitrate decomposes into dinitrogen monoxide and water. How many moles of products are produced from 2.25 moles of reactants? Equation: NH4NO3 N2O + 2 H2O Ratio: 1 : 1 : 2 2.25 mol NH4NO3

  10. Ex #2) Hydrochloric acid reacts with zinc in a single replacement reaction, how many moles of HCl are needed to react with 2.3 moles of Zn? Equation: Ratio:

  11. Stoichiometry Problems: 1. A balanced equation represents a chemical reaction that conforms to the Law of Conservation of Mass 2. Subscriptscan NEVER be changed to balance a chemical equation. 3. The total atoms/mass of the reactants is the same as the total atoms/mass of the products. 4. Coefficients in a balanced equation relate moles of substances in the reaction. 5. Coefficients are used in constructing the molar ratio for a stoichiometric problem. 6. Grams of the known must first be converted to moles, then use the molarratio to relate the 2 substances, finally convert the moles of the unknown to grams.

  12. B. Mass to Mass Problems • What mass of aluminum oxide can be prepared by the reaction of 67.5 g of aluminum and oxygen? Al + O2 Al2O3

  13. 4. The limiting reactant is the startingsubstance that becomes used up first when a chemical reaction occurs. It controls how much or how little product can be formed. Ex) If a recipe for 1 cake is: and your kitchen contains 2 cups flour 7 cups flour 2 eggs 2 dozen eggs 1 cup sugar 9 cups sugar 1½Tbls. baking powder 10 Tbls. baking pwdr 1 cup water unlimited water ⅓ cup oil 3⅓ cups oil • How many cakes can you make given the ingredients present in your kitchen? 3 What ingredient is the limiting reactant?flourHow many eggs are left over after making the cakes? 18

  14. 5. Limiting Reactant: the starting substance that becomes used up first when a chemical reaction occurs. It controls how much or how little product can be formed. 6. 4 NH3 + 5 O2 → 4 NO + 6 H2O ? g 3.50g 3.25g What is the most NO you can make? ______ Which reactant is the limiting reactant? ________ 2.63 g oxygen

  15. Reactants → Products

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