Stoichiometry The Mathematics of Chemical Equations

9.1: Balanced Chemical Equations 1. Provides qualitative and quantitative information. Conservation of Mass 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: 2 molecules1 molecule 2 molecules A. 2 molecules of H 2 react with 1 molecule of O 2 to produce 2 molecules of water. 2:1:2 The ratio of H 2 to O 2 to H 2 O is 2:1:2. OR

2H 2 +O 2 →2H 2 O B. 20 molecules of H 2 react with 10 molecules of O 2 to produce 20 molecules of water. 2:1:2 Again, the ratio of H 2 to O 2 to H 2 O is 2:1:2.

C. The original equation can then be interpreted as follows: 12 2 moles of H 2 react with 1 mole of O 2 to produce 2 moles of water. 2:1:2 The ratio of H 2 to O 2 to H 2 O is 2:1:2. 2H 2 +O 2 →2H 2 O

grams 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. mathematical (quantitative) relationships 4. Stoichiometry: The study of the mathematical (quantitative) relationships that exist in a formula and in a chemical reaction. element measure 5. The word Stoichiometry comes from the Greek word “stoicheion” meaning element, and “metron”, meaning measure.

reproducible 6. Importance: The safe, economical and reproducible manufacture of chemicals (or food) and the safe administration of pharmaceuticals. balanced 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.

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

Volume 22.4 L Mass Particles Molar Mass 6.02 x Given Substance Substance to be determined

Ex #1) Ammonium nitrate decomposes into dinitrogen monoxide and water. How many moles of products are produced from 2.25 moles of reactants? Equation: NH 4 NO 3  N 2 O + 2 H 2 O Ratio: 1 : 1 : mol NH 4 NO 3

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:

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

B. Mass to Mass Problems 1)What mass of aluminum oxide can be prepared by the reaction of 67.5 g of aluminum and oxygen? Al + O 2 Al 2 O 3

starting first muchlittle 5. The limiting reactant is the starting substance 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 eggs2 dozen eggs 1 cup sugar9 cups sugar 1½Tbls. baking powder10 Tbls. baking pwdr 1 cup waterunlimited water ⅓ cup oil3 ⅓ cups oil How many cakes can you make given the ingredients present in your kitchen? 3 What ingredient is the limiting reactant? flour How many eggs are left over after making the cakes? 18

NH O 2 → 4 NO + 6 H 2 O What is the most NO you can make? ______ Which reactant is the limiting reactant? ________ 2.63 g oxygen 3.25g 3.50g ? g

Reactants → Products