CHEMISTRY Chapter 9

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CHEMISTRY Chapter 9

• Stoichiometry

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Chapter 9 Section 1

• Objectives
• Define stoichiometry.
• Describe the importance of the mole ratio in
  stoichiometric calculations.
• Write a mole ratio relating two substances in a
  chemical equation.

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Stoichiometry

• Study of quantitative relationships that can be
  derived from chemical formulas and equations
• 2 types
• 1. composition stoichiometry mass relationships
  of elements in compounds
• 2. reaction stoichiometry mass relationships
  between reactants and products in a chemical
  reaction

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Reaction-stoichiometry

• Use ratios from the balanced equation to convert
  given quantity
• 4 types of problems
• 1. given and unknown quantities are in moles.
• 2. given is in moles and unknown is a mass in
  grams.
• 3. given is a mass in grams and unknown is in
  moles.
• 4. given is a mass in grams and unknown is a
  masss in grams.

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Mole

• The number of atoms in the atomic mass of an
  element
• 6.02 x 1023
• Avogadros number
• Moles are all different sizes b/c all atoms are
  different sizes
• The SI unit for amount
• Mole jars

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Mole Ratio

• A conversion factor that relates the amounts in
  moles of any two substances involved in a
  chemical reaction
• Obtained from a balanced equation
• Must be a conservation of mass
• From Lavoisiers theory
• 1st to measure masses of reactants and products
• 2 Al2O3 ? 4 Al 3 O2
• 2 mol Al2O3 or 4 mol Al
• 4 mol Al 2 mol Al2O3

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Changing a Recipe

• Similar to using mole ratios
• Three bean salad
• 72 red beans
• 72 french-cut green beans
• 72 yellow wax beans
• 25 mL salad oil
• 1/8 cup onion
• 1/8 cup sugar
• 25 mL vinegar
• Develop an ingredients list for 200 people.
• 144 1 gross
• 1000 mL 1 L

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Three Bean Salad for 200

• 100 gross of red beans (14,400)
• 100 gross of french-cut green beans
• 100 gross of yellow wax beans
• 5 L of salad oil
• 25 cups of onion
• 25 cups of sugar
• 5 L of vingegar

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Molar Mass

• Mass in grams of one mole of a substance
• ex. Al2O3 102 g/mole

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Example

• What is the mass of 1 mole of water?
• 2 moles H
• 1 mole O
• Mass of 1 mole atomic mass
• H 1 g/mol(2) 2 g
• O 16 g/mol 16 g
• Total mass 18 g/mol

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Example

• What is the mass of 1 mole of Ca(OH)2?
• 1 mole of Ca
• 2 moles of O
• 2 moles of H
• Mass of 1 mole atomic mass
• Ca 40 g/mol
• O 16g/mol(2) 32 g
• H 1g/mol(2) 2 g
• Total mass 74 g/mol

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Review and Assignment

• 1. List three observations that suggest that a
  chemical reaction has taken place.
• 2. List three requirements for a correctly
  written chemical equation.
• 3. Write a word equation and a formula equation
  for a given chemical reaction.
• Assignment HW 9-1 and WS

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Chapter 9 - Section 2

• Objectives
• 1. Calculate the amount in moles of a reactant or
  product from the amount in moles of a different
  reactant or product.

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Stoichiometric Calculations

• Need balanced equation to start
• Can be one, two, or three step calculations
• Most are theoretical, b/c we do not work in ideal
  situations
• Show the maximum amount of product that can be
  obtained

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One Step Stoichiometric Problems

• Mole to mole calculations
• Need only one conversion factor
• Mole ratio of unknown to the given substance
• Get this from balanced equation
• Given x mole ratio

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Example

• CO2 2 LiOH ? Li2CO3 H2O
• How many moles of LiOH are required to react with
  20 moles of CO2?
• 20 moles CO2 2 mole LiOH 40 moles LiOH
• 1 mole CO2

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Example

• CaCO3 ? Ca CO2
• How many moles of CaCO3 are needed to react with
  5 moles of Ca?
• 5 moles Ca 1 mole CaCO3 5 moles CaCO3
• 1 mole Ca

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Review and Assignment

• 1. Calculate the amount in moles of a reactant or
  product from the amount in moles of a different
  reactant or product.
• Assignment HW 9-2 and WS

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Chapter 9 - Section 2 day 2

• Objectives
• Calculate the mass of a reactant or product from
  the amount in moles of a different reactant or
  product.
• Calculate the amount in moles of a reactant or
  product from the mass of a different reactant or
  product.

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Two Step Stoichiometric Problems

• Mole to mass calculations
• Need two conversion factors
• Mole ratio from the balanced equation
• Molar mass from the periodic table
• Given amount x mole ratio x mass of unknown

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Example

• CO2 2 LiOH ? Li2CO3 H2O
• How many grams of LiOH are required to react with
  20 moles of CO2?
• 20 moles CO22 mole LiOH 40 moles LiOH
• 1 mole CO2
• 40 moles LiOH 24 g LiOH 960 g of LiOH
• 1 mole LiOH

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Two Step Stoichiometric Problems

• Mass to mole calculations
• Need two conversion factors
• Molar mass from periodic table
• Mole ratio from balanced equation
• Given mass x molar mass x mole ratio

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Example

• 4 NH3 5 O2 ? 4 NO 6 H2O
• How many moles of NO are formed when you begin
  with 824 g of NH3?
• 824 g NH3 1 mole NH3 48 mole NH3
• 17 g NH3
• 48 moles NH3 4 mole NO 48 mole NO
• 4 mole NH3

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Example

• N2 3 H2 ? 2 NH3
• How many moles of NH3 are formed with 184.6 g of
  N2?
• 184.6 g N2 1 mole N2 6.6 mole N2
• 28 g N2
• 6.6 mole N2 2 mole NH3 13.2 mole NH3
• 1 mole N2

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Review and Assignment

• 1. Calculate the amount in moles of a reactant or
  product from the amount in moles of a different
  reactant or product.
• Assignment HW 9-3 and WS

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Chapter 9 - Section 2 day 3

• Objectives
• Calculate the mass of a reactant or product from
  the mass of a different reactant or product.

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Three Step Stoichiometric Problems

• mass to mass calculations
• Need two conversion factors
• Molar mass from periodic table
• Mole ratio from balanced equation
• More practical b/c we can measure grams and not
  moles
• Given mass x molar mass of given x mole ratio x
  molar mass of unknown

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Example

• Sn 2 HF ? SnF2 H2
• How many grams of SnF2 are produced from 30.00g
  of HF?
• 30.00 g HF 1 mole HF 1.5 moles HF
• 20 g HF
• 1.5 moles HF 1 mole SnF2 0.75 mole SnF2
• 2 mole HF
• 0.75 moles SnF2 157 g SnF2 118 g SnF2
• 1 mole SnF2

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Review and Assignment

• 1. Calculate the mass of a reactant or product
  from the mass of a different reactant or product.
• Assignment HW 9-4 and WS

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Chapter 9 - Section 3

• Objectives
• Describe a method for determining which of two
  reactants is a limiting reactant.
• Calculate the amount in moles or mass in grams of
  a product, given the amounts in boles or masses
  in grams of two reactants, one of which is in
  excess.

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Limiting Reactant

• The reactant that limits the amount of the other
  reactants that can combine and the amount of
  product that can form in a chemical reaction
• Limits the reaction

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Excess Reactant

• The substance that is not used up completely in a
  reaction

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Making Cookies

• Figure out which ingredient will run out first
• Use that to figure out how many batches of
  cookies you can make

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Example

• If 2.0 mole of HF are exposed to 4.5 moles of
  SiO2, which is the limiting reactant?
• SiO2 4 HF ? SiF4 2 H2O
• 2.0 mole HF 1 mole SiO2 0.5 mole SiO2
• 4 mole HF
• Have enough SiO2, so HF is limiting reactant

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Example

• Which is the limiting reactant in this reaction,
  when 0.750 moles of N2H4 is mixed with 0.500
  moles of H2O2?
• N2H4 2 H2O2 ? N2 4 H2O
• 0.750 mole N2H4 2 mole H2O2 1.5 mole H2O2
• 1 mole N2H4
• Not enough H2O2, so it is the limiting reactant

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Example (continued)

• How much of the excess reactant (in moles) is
  unchanged?
• MUST use limiting reactant to find this.
• 0.50 mole H2O2 1 mole N2H4 0.25 mole N2H4
• 2 mole H2O2
• 0.750 mole 0.25 mole 0.500 mole N2H4

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Review and Assignment

• Describe a method for determining which of two
  reactants is a limiting reactant.
• Calculate the amount in moles or mass in grams of
  a product, given the amounts in boles or masses
  in grams of two reactants, one of which is in
  excess.
• Assignment HW 9-5 and WS

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Chapter 9 - Section 3

• Objectives
• Distinguish between theoretical yield, actual
  yield, and percent yield.
• Calculate percent yield, given the actual yield
  and quantity of a reactant.

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Theoretical Yield

• The maximum amount of product that can be
  produced from a given amount of reactant

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Actual Yield

• Amount of a product obtained from a reaction
• Usually less than the theoretical yield
• Used in side reactions
• Lost during purification

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Percent Yield

• The ratio of the actual yield to the theoretical
  yield multiplied by 100
• Percent yield actual yield x 100
• theoretical yield

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Example

• C6H6 Cl2 ? C6H5Cl HCl
• When 36.8 g of C6H6 react with excess Cl2, the
  actual yield of C6H5Cl is 38.8 g, What is the
  percent yield of C6H5Cl?
• Theoretical yield 36.8 g C6H6 1 mol C6H6
  0.47 mol C6H6
• 78 g C6H6
• 0.47 mol C6H6 1 mol C6H5Cl 0.47 mol C6H5Cl
• 1 mole C6H6
• 0.47 mole C6H5Cl 113 g C6H5Cl 53.0 g C6H5Cl
• 1 mole C6H5Cl
• Percent yield (38.8 g/53.0 g) x 100 73.2

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Review and Assignment

• 1. Distinguish between theoretical yield, actual
  yield, and percent yield.
• 2. Calculate percent yield, given the actual
  yield and quantity of a reactant.
• Assignment HW 9-6