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General Chemistry: Chapter 4
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Philip Dutton
University of Windsor, Canada

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Chapter 4: Chemical Reactions
General Chemistry
Principles and Modern Applications
Petrucci • Harwood • Herring
8th Edition
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General Chemistry: Chapter 4
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Contents
4-1 Chemical Reactions and Chemical Equations
4-2 Chemical Equations and Stoichiometry
4-3 Chemical Reactions in Solution
4-4 Determining the Limiting reagent
4-5 Other Practical Matters in Reaction Stoichiometry
Focus on Industrial Chemistry
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4-1 Chemical Reactions and
Chemical Equations
As reactants are converted to products we observe:
Color change
Precipitate formation
Gas evolution
Heat absorption or evolution
Chemical evidence may be necessary.
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Chemical Reaction
Nitrogen monoxide + oxygen → nitrogen dioxide
Step 1: Write the reaction using chemical symbols.
NO + O2 → NO2
Step 2: Balance the chemical equation.
2
1
2
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Molecular Representation
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Balancing Equations
Never introduce extraneous atoms to balance.
NO + O2 → NO2 + O
Never change a formula for the purpose of balancing an equation.
NO + O2 → NO3
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Balancing Equation Strategy
Balance elements that occur in only one compound on each side first.
Balance free elements last.
Balance unchanged polyatomics as groups.
Fractional coefficients are acceptable and can be cleared at the end by multiplication.
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Example 4-2
Writing and Balancing an Equation: The Combustion of a Carbon-Hydrogen-Oxygen Compound.
Liquid triethylene glycol, C6H14O4, is used a a solvent and plasticizer for vinyl and polyurethane plastics. Write a balanced chemical equation for its complete combustion.
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C6H14O4 + O2 → CO2 + H2O
6
2. Balance H.
2 C6H14O4 + 15 O2 → 12 CO2 + 14 H2O
4. Multiply by two
Example 4-2
3. Balance O.
and check all elements.
Chemical Equation:
1. Balance C.
6 7
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4-2 Chemical Equations and Stoichiometry
Stoichiometry includes all the quantitative relationships involving:
atomic and formula masses
chemical formulas.
Mole ratio is a central conversion factor.
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Example 4-3
Relating the Numbers of Moles of Reactant and Product.
How many moles of H2O are produced by burning 2.72 mol H2 in an excess of O2?
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Example 4-6
Additional Conversion Factors ina Stoichiometric Calculation: Volume, Density, and Percent Composition.
An alloy used in aircraft structures consists of 93.7% Al and 6.3% Cu by mass. The alloy has a density of 2.85 g/cm3. A 0.691 cm3 piece of the alloy reacts with an excess of HCl(aq). If we assume that all the Al but none of the Cu reacts with HCl(aq), what is the mass of H2 obtained?
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Example 4-6
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2 Al + 6 HCl → 2 AlCl3 + 3 H2
Example 4-6
We need 5 conversion factors!
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4-3 Chemical Reactions in Solution
Close contact between atoms, ions and molecules necessary for a reaction to occur.

Solvent
We will usually use aqueous (aq) solution.
Solute
A material dissolved by the solvent.
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Molarity
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Preparation of a Solution
Weigh the solid sample.
Dissolve it in a volumetric flask partially filled with solvent.
Carefully fill to the mark.
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Calculating the mass of Solute in a solution of Known Molarity.
We want to prepare exactly 0.2500 L (250 mL) of an 0.250 M K2CrO4 solution in water. What mass of K2CrO4 should we use?
Plan strategy:
Example 4-6
Volume → moles → mass
We need 2 conversion factors!
Write equation and calculate:
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Solution Dilution
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Preparing a solution by dilution.
A particular analytical chemistry procedure requires 0.0100 M K2CrO4. What volume of 0.250 M K2CrO4 should we use to prepare 0.250 L of 0.0100 M K2CrO4?
Example 4-10
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4-4 Determining Limiting Reagent
The reactant that is completely consumed determines the quantities of the products formed.
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Determining the Limiting Reactant in a Reaction.
Phosphorus trichloride , PCl3, is a commercially important compound used in the manufacture of pesticides, gasoline additives, and a number of other products. It is made by the direct combination of phosphorus and chlorine
P4 (s) + 6 Cl2 (g) → 4 PCl3 (l)
What mass of PCl3 forms in the reaction of 125 g P4 with 323 g Cl2?
Example 4-12
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Example 4-12
nCl2 = 323 g Cl2 × = 4.56 mol Cl2
1 mol Cl2
70.91 g Cl2
actual = 4.55 mol Cl2/mol P4
theoretical = 6.00 mol Cl2/mol P4

Chlorine gas is the limiting reagent.
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4-5 Other Practical Matters in
Reaction Stoichiometry
Theoretical yield is the expected yield from a reactant.
Actual yield is the amount of product actually produced.
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Theoretical, Actual and Percent Yield
When actual yield = % yield the reaction is said to be quantitative.
Side reactions reduce the percent yield.
By-products are formed by side reactions.
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Consecutive Reactions,
Simultaneous Reactions and
Overall Reactions
Multistep synthesis is often unavoidable.
Reactions carried out in sequence are called consecutive reactions.
When substances react independently and at the same time the reaction is a simultaneous reaction.
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Overall Reactions and Intermediates
The Overall Reaction is a chemical equation that expresses all the reactions occurring in a single overall equation.
An intermediate is a substance produced in one step and consumed in another during a multistep synthesis.
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Focus on Industrial Chemistry
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General Chemistry: Chapter 4
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Chapter 4 Questions
1, 6, 12, 25, 39, 45, 53, 65, 69, 75, 84, 94, 83, 112