Chemical Reactions

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Chapter 5
Chemical Reactions
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Chemical Reactions Chemical Equations
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Reactants
Products
2H2 O2 ? 2H2O
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Chemical Reactions Equations

• Balance the following equations.

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Equations with Polyatomic Ions
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Equations with Polyatomic Ions
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Atomic Weights
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Molecular Weight and Formula Weight
NaCl
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Formula weight is the sum of the atomic masses
(in amu) in a formula unit of an ionic compound.
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• Alternate Example 3.1 Calculating the Formula
  Weight from a Formula
• Calculate the formula weight of the following
  compounds from their formulas
• calcium hydroxide, Ca(OH)2
• methylamine, CH3NH2.

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Molecular Weight and Formula Weight
H2O
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Molecular mass (or molecular weight) is the sum
of the atomic masses (in amu) in a molecule.
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http//www.uksafari.com/postcard/mole.htm
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http//www.dumville.org/moles/starnosed.html
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1 dozen 12 items
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6.02 x 1023 somethings 1
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Mass and Moles of a Substance

• The Mole Concept

1-octanol (C8H17OH)
Mercury(II) Iodide (HgI2)
Sulfur (S8)
Methanol (CH3OH)
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One seat
One tricycle
Three tires
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Amu grams/mole
6.02 x 1023 somethings 1mole
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Molar Mass
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Molar Mass

• Molar mass (formula weight) can be used to
  convert from grams to moles, and from moles to
  grams

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Mass and Moles of a Substance

• Mole calculations
• Suppose we have 100.0 grams of iron (Fe). The
  atomic weight of iron is 55.8 g/mol. How many
  moles of iron does this represent?

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Mass and Moles of a Substance

• Mole calculations
• Conversely, suppose we have 5.75 moles of
  magnesium (atomic wt. 24.3 g/mol). What is its
  mass?

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Mass and Moles of a Substance

• Mole calculations
• This same method applies to compounds. Suppose we
  have 100.0 grams of H2O. How many moles does this
  represent?

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Alternate Example 3.4 Converting Moles of
Substance to Grams A sample of nitric acid
contains 0.253 mol HNO3. How many grams is this?
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First Alternate Example 3.5 Converting Grams of
Substance to Moles Calcite is a mineral composed
of calcium carbonate, CaCO3. A sample of calcite
composed of pure calcium carbonate weighs 23.6 g.
How many moles of calcium carbonate is this?
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Mass and Moles of a Substance

• Mole calculations
• Suppose we have 3.25 moles of glucose, C6H12O6.
  What is its mass?

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Second Alternate Example 3.5 Converting Grams of
Substance to Moles The average daily requirement
of the essential amino acid leucine, C6H14O2N, is
2.2 g for an adult. How many moles of leucine are
required daily? The formula weight for leucine
is 132 g/mol.
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Grams to Moles

• Calculate the number of moles of sodium ions,
  Na, in 5.63 g of sodium sulfate, Na2SO4

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How many moles of H are in 72.5 g of C3H8O ?
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Molar Interpretation of a Chemical Equation

• The balanced chemical equation can be interpreted
  in numbers of molecules, but generally chemists
  interpret equations as mole-to-mole
  relationships.

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• The balanced chemical equation can be interpreted
  in numbers of molecules, but generally chemists
  interpret equations as mole-to-mole
  relationships.

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Molar Interpretation of a Chemical Equation

• Suppose we wished to determine the number of
  moles of NH3 we could obtain from 4.8 mol H2.

Reaction coefficients
FW of A
FW of B
grams A
moles A
moles B
grams B
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Alternate Example 3.13 Relating the Quantity of
Reactant to Quantity of Product Propane, C3H8, is
normally a gas, but it is sold as a fuel
compressed as a liquid in steel cylinders. The
gas burns according to the equation C3H8(g)
5O2(g) 3CO2(g) 4H2O(g) How many grams of CO2
are produced when 20.0 g of propane is burned?
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Mass Relationships in Chemical Equations

• Amounts of substances in a chemical reaction by
  mass.
• How many grams of HCl are required to react with
  5.00 grams manganese dioxide according to this
  equation?

Reaction coefficients
MW of A
MW of B
grams A
moles A
moles B
grams B
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Limiting Reagent

• The limiting reactant (or limiting reagent) is
  the reactant that is entirely consumed when the
  reaction goes to completion.

• The limiting reagent ultimately determines how
  much product can be obtained.

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Limiting Reagent

• Zinc metal reacts with hydrochloric acid by the
  following reaction.

• If 0.30 mol Zn is added to hydrochloric acid
  containing 0.52 mol HCl, how many moles of H2 are
  produced?

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Urea, (NH2)2CO, is prepared by reacting ammonia
with carbon dioxide 2NH3 CO2 ? (NH2)2CO
H2O In one process, 637.2 g of NH3 are
treated with 1142 g of CO2. Which of the two
reactants is the limiting reagent? Calculate the
mass of (NH2)2CO formed. The formula weight of
Urea is 60.06 g/mol.
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• The theoretical yield of product is the maximum
  amount of product that can be obtained from given
  amounts of reactants.

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Chemical ReactionsAn Introduction
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• What happens when we mix aqueous solutions of two
  different ionic compounds?

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• A precipitate is an insoluble solid compound
  formed during a chemical reaction in solution.

potassium iodide
potassium iodide
mercury(II) nitrate
lead(II) nitrate
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Pb(NO3)2(aq) 2KI(aq) ? PbI2(s) 2KNO3(aq)
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Types of Chemical Reactions

• Most of the reactions we will study fall into one
  of the following categories
• Precipitation Reactions
• Acid base reactions (later)
• Oxidation-Reduction Reactions

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Figure 4.11 Molecular Blow-out of Reaction of
Fe with Cu2(aq) to Yield Fe2(aq) and Cu(s)
Iron nail
Copper sulfate
Copper sulfate
Iron nail
Iron nail with copper plating
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Leo the lion says gerrrrr
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Oxidation-Reduction

• An alternative definition of oxidation-reduction

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Heat of Reaction

• In almost all chemical reactions, heat is either
  given off or absorbed.