Zumdahl • Zumdahl • DeCoste
World of
CHEMISTRY
Chapter 8
Reactions in
Aqueous
Solutions
An aqueous solution is a solution in which water is
the solvent (the dissolving compound).
• Reactions that keep us alive:
• Oxygen dissolves in blood, associates with
hemoglobin
• Oxygen reacts with fuel (food) to provide
energy
• Hydrochloric Acid reacts with food
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8-3
Goals of Chapter 8
• Factors that cause reactions to occur
• Identify solid formed in precipitation reaction
• Describe reactions in solutions by writing
molecular and ionic equations
• Characteristics of reactions between strong
acids and strong bases
• Characteristics of reactions between metals
and nonmetals
• Electron transfer – driving force for chemical
reaction
• Classification schemes for reactions
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Why does a chemical reaction occur?
What causes reactants to “want” to form products?
• Most common driving forces:
•
•
•
•
Formation of solid
Formation of water
Transfer of electrons
Formation of a gas
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Driving force: Formation of solid
Reactions in Which a Solid Forms
• Precipitation: process where solid is
formed
• Precipitation reaction: chemical reaction
where solid forms
• Precipitate: solid that forms
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What happens when ionic compound
dissolves in water?
• Almost always, the ions separate and move
around independently
• Ions of solid dissociate when solid dissolves
in water
• Strong electrolyte: every molecule separates
into ions when dissolved in water
• How do we know this really happens?
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Figure 8.2: Pure water does not conduct an electric
current.
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Figure 8.2: When an ionic compound is dissolved in
water, current flows.
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Deciding what products form
• Not easy question to answer
• Experienced chemists do not know what
will happen in new reaction
• Think of possibilities → consider
likelihood → make prediction →
determine experimentally
• Knowledge of facts
• Knowledge of concepts
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Solubility Rules
• Soluble solid: readily dissolves in water;
solid “disappears” as ions dispersed in
water
• Insoluble solid (slightly soluble solid):
tiny amount dissolves in water,
undetectable to human eye
• Solubility Table: based on observations
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Table 8.1
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Figure 8.3:
Solubilities
of common
compounds.
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KCl(aq) + AgNO3 (aq)→ KNO3(aq) + AgCl(s)
Solid AgCl
Precipitates
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KCl & AgNO3 dissociate
AgCl is insoluble in water, forms precipitate
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How can you predict
precipitates when
solutions of two ionic
compounds are
mixed?
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8-16
Types of Equations for Reactions in
Aqueous Solutions
• Molecular Equation: shows overall reaction,
not necessarily actual forms of reactants and
products in solution
• Complete Ionic Equation: shows all reactants
& products that are strong electrolytes as
ions. Includes all reactants & products.
• Net Ionic Equation: includes only components
that undergo change. Spectator ions not
included.
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8-17
Molecular Equation
Aqueous potassium chromate is mixed with aqueous
barium nitrate, a reaction occurs to form solid barium
chromate and dissolved potassium nitrate.
K2CrO4(aq) + Ba(NO3)2(aq) → BaCrO4(s) + 2KNO3(aq)
Equation shows complete formulas of all reactants
and products.
Doesn’t give clear picture of what occurs in solution
Complete Ionic Equation
2K+(aq) + CrO42-(aq) + Ba2+(aq) + 2NO3-(aq) →
BaCrO4(s) + 2K+(aq) + 2NO3-(aq)
Better representation of reactants & products in solution
All strong electrolytes represented as ions
BaCrO4 present as solid – not dissolved
K+ and NO3- on both sides of equation = spectator ions
Net Ionic Equation
Ba2+(aq) + CrO42-(aq) → BaCrO4(s)
Only includes ions that participate in reaction
Spectator ions do not participate directly in
reaction in solution – eliminated from equation
Only includes species that undergoes a change
Acids
First associated with sour taste of citrus fruits
From Latin acidus meaning “sour”
Vinegar – acetic acid; citric acid – lemons
Substance that produces H+ ions (protons)
when it is dissolved in water
When HCl, HNO3, and H2SO4 are placed in
water, virtually every molecule dissociates to
give ions = strong acids
Bases (alkalis)
Bitter taste and slippery feel (like wet soap)
Drano (other commercial products for
unclogging drains)
Substance that produces hydroxide (OH-)
ions in water
Sodium hydroxide (NaOH) is most common
base used in chemical laboratory
When NaOH and KOH are placed in water
they completely dissociate = strong bases
Reaction That Form Water
Acids & Bases
• When strong acids and strong bases
mix: H+ ions react with OH- ions to form
water
• Water very stable (large abundance on
earth is evidence), so when substances
that can form water are mixed there is a
strong tendency for the reaction to
occur
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H+ (aq) + OH- (aq) → H2O (l)
Hydroxide ion has strong
+
affinity for H ion to produce
water
Strong Acid – Strong Base Reactions
• Water is always a product
• Second product is ionic compound
which may precipitate or remain
dissolved depending on solubility (called
a salt – do not confuse with NaCl) – can
obtain salt be evaporating the water
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Reactions Involving Transfer of Electrons
aka Reactions of Metals with Nonmetals
aka Oxidation-Reduction Reactions
Example: Sodium Chloride is formed
from reaction of sodium metal and
chlorine gas
2Na (s) + Cl2 (g) → 2NaCl (s)
Sodium metal – net charge of zero
Chlorine molecule – net charge of zero
Sodium chloride is made up of Na+ and Cl-
How is this possible?
One electron is transferred from each
sodium atom to each chlorine atom
(see page 231)
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Characteristics of Oxidation-Reduction
Reactions
• Metal-Nonmetal Reaction: Metal
becomes Cation, Non-metal becomes
Anion
• Two non-metals can undergo oxidationreduction reaction, we will discuss later.
Compound formed in nonionic. O2
present as reactant or product.
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Classifying
Reactions
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Precipitation Reactions
• Formation of solid when 2 solutions are
mixed
• Double-displacement reaction:
• AB + CD → AD + CB
• A,B,C,D are ions
• Example: reaction of potassium chromate
with barium nitrate
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Acid-Base Reactions
• Involves H+ ion that ends up in the
product water
• Net ionic equations similar (see slide
24)
• Example: Hydrochloric Acid reacting
with Potassium Hydroxide
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Formation of a Gas
• Usually classified as acid-base,
oxidation-reduction, etc.
• Have to look at chemical reaction to
determine
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Single-Replacement Reactions
• A + BC → B + AC
• Two types of anion are exchanged
• See example on page 239
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Oxidation-Reduction
Reactions
(see page 238)
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Figure 8.8: The space shuttle Discovery.
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Combustion Reactions
•
•
•
•
•
Oxygen involved
Produces heat so rapidly, flame results
Special class of oxidation-reduction
See examples pp. 239-240
Most provide heat or electricity for
homes/businesses or energy for
transportation
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Synthesis Reactions
• Compound is formed from simpler
materials
• Plastic, polyester, aspirin
• Subclass of oxidation-reduction
reactions
• Example: Synthesis of water
• 2H2 (g) + O2 (g) → 2 H2O (l)
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Decomposition Reactions
• Compound broken down to simpler
compounds or to component elements
• Heating
• Application of electric current
• The opposite of synthesis
• Example: Decomposition of water
• 2H2O (l) → 2H2 (g) + O2 (g)
• Subclass of oxidation-reduction
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Figure 8.11: Classes of reactions.
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Figure 8.12: Summary of classes of
reactions.
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What’s in your water?
1.
2.
3.
4.
Obtain various samples of
water (bottled, tap, distilled,
drinking fountain, etc.)
Add a few drops of silver nitrate
solution (0.1 M)
Write down observations,
compare with classmates.
Write net ionic equation(s).