Zumdahl

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Zumdahl Zumdahl DeCoste
World of

• CHEMISTRY

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Chapter 9
Chemical Quantities
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Goals of Chapter 9

• Understand molecular and molar mass given in
  balanced equation
• Use balanced equation to determine the
  relationships between moles of reactants and
  moles of products
• Relate masses of reactants and products in a
  chemical reaction
• Perform mass calculations that involve scientific
  notation
• Understand the concept of limiting reactants
• Recognize the limiting reactant in a reaction
• Use the limiting reactant to do stoichiometric
  calculations

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Information Given by Chemical Equations

• Reactions are described by equations
• Give the identities of the reactants products
• Show how much of each reactant and product
  participates in the reaction.
• Numbers or coefficients enable us to determine
  how much product we get from a given quantity of
  reactants

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Information Given by Chemical Equations

• Atoms are rearranged in a chemical reaction (not
  created or destroyed)
• Must have same number of each type of atom on
  both sides of equation.
• Coefficients give relative number of molecules,
  meaning we can multiply by any number and still
  have a balanced equation.

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Table 9.1
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Combustion of Propane

• Propane reacts with oxygen to produce heat and
  the products carbon dioxide and water
• C3H8(g) 5O2(g) ? 3CO2(g) 4H2O(g)

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Interpretation of Equation

• 1 molecule of C3H8 reacts with 5 molecules of O2
  to give 3 molecules of CO2 plus 4 molecules of
  H2O
• 1 mole of C3H8 reacts with 5 moles of O2 to give
  3 moles of CO2 plus 4 moles of H2O

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Nuts Bolts of Chemistry Activity

• 2 Nuts (N) react with 1 bolt (B) to form a
  nut-bolt molecule
• 2N B ? N2B
• Note difference between coefficient and subscript
• Construct nut-bolt molecules
• Which is limiting reactant? Why?

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Average mass of bolt 10.64 g average mass of
nut 4.35 g If you are given about 1500 g of
each, answer the following questions 1. How
many bolts are in 1500 g? How many nuts are in
1500 g? 2. Which is limiting reactant? Why? 3.
What is largest possible mass of product?
How many products can you make? 4. What is
mass of leftover reactant?
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Mole-Mole Relationships

• 2H2O(l) ? 2H2(g) O2(g)
• Equation tells us that 2 mol of H2O yields 2 mol
  of H2 and 1 mol of O2
• If we decompose 4 mol of water, how many moles of
  products do we get?
• 4H2O(l) ? 4H2(g) 2O2(g)
• If we decompose 5.8 mol of water, how many moles
  of products do we get?
• 5.8H2O(l) ? 5.8H2(g) ?O2(g)

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Mole Ratios conversion factors based on balanced
chemical equations

• From initial equation
• 2 mol H2O 2 mol H2 1 mol O2
• Can use equivalent statement perform
  dimensional analysis
• 5.8 mol H2O x 1 mol O2_ 2.9 mol O2
• 2 mol H2O

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Mass Calculations

• What mass of oxygen (O2) is required to react
  with exactly 44.1 g of propane (C3H8)?

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Step 1 Write balanced equation

• C3H8(g) 5O2(g) ? 3CO2(g) 4H2O(g)

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Step 2 Convert grams of propane to moles of
propane

• 44.1 g C3H8 x 1 mol C3H8 1.00 mol C3H8
• 44.09 g C3H8

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Step 3 Use coefficients in equation to determine
moles of oxygen required

• 1.00 mol C3H8 x 5 mol O2 5.00 mol O2
• 1 mol C3H8

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Step 4 Use molar mass of O2 to calculate the
grams of oxygen

• 5.00 mol O2 x 32.0 g O2 160 g O2
• 1.00 mol O2

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Can perform conversion in on long step

• 44.1 g C3H8 x 1 mol C3H8 x 5 mol O2 x
  32.0 g O2 160 g O2
• 44.09 g C3H8 1 mol C3H8
  1.00 mol O2

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Mass Calculations Using Scientific Notation

• Step 1 Balance the equation for the reaction
• Step 2 Convert the masses of reactants or
  products to moles
• Step 3 Use the balanced equation to set up the
  appropriate mole ratio(s)
• Step 4 Use the mole ratio(s) to calculate the
  number of moles of the desired product or
  reactant.
• Step 5 Convert from moles back to mass

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Stoichiometry

• The process of using a chemical equation to
  calculate the relative masses of reactants and
  products involved in a reaction

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Mass Calculations Comparing Two Reactions

• Antacids are used to neutralize excess
  hydrochloric acid secreted by the stomach. Which
  antacid is more effective baking soda, NaHCO3,
  or milk of magnesia, Mg(OH)2?
• Determine how many moles of stomach acid (HCl)
  will react with 1.00 g of each acid.

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The Concept of Limiting Reactants
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Consider the reaction that forms ammonia

• N2(g) 3H2(g) ? 2NH3(g)
• These gases are mixed in a closed vessel and
  begin to react

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Container (1) of N2(g) and H2(g).
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Before and after the reaction.
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This reaction contained the exact number of
molecules to make ammonia molecules with no
unreacted molecules left over.Before the
reaction, there were 15 H2 molecules and 5 N2
molecules which gives the exact ratio to make
ammonia, 31.This type of mixture is called a
stoichiometric mixture contains the relative
amounts of reactants that matches the numbers in
balanced equation
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What happens when the ratio is not the same as in
the chemical equation?
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Container (2) of N2(g) and H2(g). (58 ratio)
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Before and after the reaction. (Some N2
molecules are left over)
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Limiting Reactant

• The reactant that runs out first and thus limits
  the amounts of products that can form
• H2 is limiting reactant in previous slide

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Ammonia, which is used as a fertilizer, is made
by combining nitrogen from the air with hydrogen.
The hydrogen is produced by reacting methane
(CH4) with water. If you have 249 grams of
methane, how much hydrogen will be produced and
how much water will you need to convert all of
the methane to hydrogen?
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Step 1 Write balanced equation

• CH4(g) H2O(g) ? 3H2(g) CO(g)

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Figure 9.1 A mixture of 5CH4 and 3H2O molecules.
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Step 2 Convert mass of methane to moles

• 249 g CH4 x 1 mol CH4 15.5 mol CH4
• 16.04 g CH4

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Step 3 Determine moles of H2O needed

• 15.5 mol CH4 x 1 mol H2O 15.5 mol H2O
• 1 mol CH4

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Step 4 Determine mass of water

• 15.5 mol H2O x 18.02 g H2O 279 g H2O
• 1 mol H2O

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Reacting 279 grams of water with 249 grams of
methane will cause both reactants to run out at
the same time.If 300 grams of water is reacted
with 249 grams of methane, the methane will run
out first limiting reactant (it limits the
reaction)
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Figure 9.2 A map of the procedure used in
Example 9.7. (see for determining limiting
reactant)
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Steps for solving stoichiometric problems
involving limiting reactants

• Step 1 Write and balance chemical equation
• Step 2 Convert known masses to moles
• Step 3 Using number of moles of reactants and
  mole ratios, determine limiting reactant
• Step 4 Use amount of limiting reactant and mole
  ratios to calculate number of moles of product
• Step 5 Convert from moles to mass

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Theoretical Yield

• Calculated yield from chemical reaction (amount
  of product form mole ratio calculations)
• Maximum amount that can be produced
• Amount predicted is seldom obtained
• Side reactions occur
• Actual yield amount of product actually obtained

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Percent Yield

• Actual Yield x 100 percent
• Theoretical Yield yield