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Chapter 7
Reactions in Aqueous
Solutions
Sect. 7.1: Predicting whether a
reaction will occur
• A chemical reaction will proceed if one of the
following “driving forces” is present:
1. a solid wants to form
2. water wants to form
3. electrons from one atom want to transfer to
another atom
4. a gas wants to form
5. heat or cold is applied
• With reactions in aqueous solutions, we will see
solids form and/or water formed
Classifying chemical reactions
• Chemical reactions are classified in order to organize
them
1. Helps us recognize patterns, and predict
products.
• A common method of classification is into 5
categories:
1. synthesis reaction
2. combustion reaction
3. decomposition reaction
4. single replacement reactions
5. double replacement reactions
Classifying chemical reactions
• Synthesis reaction: A chemical reaction in
which 2 or more substances react to produce a
single product.
1. Generically:
A + B  AB
2. Example: Iron metal is reacted with chlorine
gas to produce Iron (III) chloride:
2Fe(s) + 3Cl2(g)  2FeCl3(s)
3. When only 2 elements or compounds react
and only one product is formed, the rxn is always
a synthesis reaction.
Synthesis Reaction Examples
• Sodium metal is reacted with chlorine gas.
2Na(s) + Cl2(g)  2NaCl(s) (2 elements)
• Solid calcium oxide is reacted with water to
produce calcium hydroxide.
CaO(s) + H2O(L)  Ca(OH)2(s) (2 compounds)
• Sulfur dioxide gas is reacted with oxygen gas to
form sulfur trioxide
2SO2(g) + O2(g)  2SO3(g) (element & compound)
Combustion Reactions
• Combustion reaction: A chemical reaction in
which oxygen combines with a substance, and
releases energy in the form of heat and light.
1. Therefore, oxygen (O2) must be a reactant.
• Example: Sulfur dioxide gas is reacted with
oxygen gas to produce sulfur trioxide.
2SO2(g) + O2(g)  2SO3(g) (both a combustion &
synthesis reaction)
Combustion Reactions
• Hydrogen gas is reacted with oxygen gas (the
cause of the Challenger disaster)
2H2(g) + O2(g)  2H2O(L)
1. water is formed
2. a tremendous amount of energy is released.
• Burning of coal, which occurs in power plants to
create electricity, is a reaction between carbon &
oxygen to produce carbon dioxide.
C(s) + O2  CO2(g) (a combustion & synthesis rxn)
Decomposition Reactions
• Decomposition reaction: A chemical reaction in
which a single compound breaks down into 2 or
more elements or new compounds.
1. It is the “opposite” of a synthesis reaction
2. Generically: AB  A + B
3. Example: Ammonium nitrate is decomposed
to produce dinitrogen monoxide and water.
NH4NO3(s)  N2O(g) + 2H2O(L)
4. One reactant will yield 2 or more products.
Replacement reactions: Single &
Double
• Single Replacement Reaction: a reaction in
which the atoms of one element replace the
atoms of another element in a compound.
1. Generically: A + BX  AX + B
2. Example: Lithium metal reacting with water
to produce aqueous lithium hydroxide and
hydrogen gas.
2Li(s) + 2H2O(L)  2LiOH(aq) + H2(g)
In this case, lithium replaces a
hydrogen
Single Replacement Reactions
• When dissolved in water (aqueous), a metal can
replace another metal within a compound.
1. Doesn’t ALWAYS happen: it depends upon the
reactivity of the metal dissolved compared to the
metal within the compound.
2. Example: Copper wire, when placed into an
aqueous solution of silver nitrate, will react to
produce silver metal and aqueous copper (II)
nitrate.
Cu(s) + 2AgNO3(aq)  2Ag(s) + Cu(NO3)2(aq)
But… Ag(s) + Cu(NO3)2(aq)  No Reaction
Metals are listed top to bottom in order of
reactivity: top will replace any that are below it:
Double Replacement Reactions
• Double Replacement Reaction: A chemical
reaction in which there is an “exchange” of ions
between 2 compounds.
1. Generically: AX + BY  AY + BX
a. A & B represent cations (+ charged ions)
b. Y & X represent anions (- charged ions)
2. Will produce one of three types of products:
a. Precipitate (solid)
b. Gas
c. Liquid
Double Replacement Reactions
• Example: Aqueous sodium hydroxide is reacted
with aqueous copper (II) Chloride to produce
aqueous sodium chloride and solid copper (II)
hydroxide.
2NaOH(aq) + CuCl2(aq)  2NaCl(aq) + Cu(OH)2(s)
1. The above resulting product is a solid
precipitate
• Precipitate: a solid, produced during a chemical
reaction in a solution.
Double Replacement Reactions
2. Example (production of a liquid): Aqueous
calcium hydroxide is reacted with hydrochloric
acid to produce aqueous calcium chloride and
water.
Ca(OH)2(aq) + 2HCL(aq)  CaCl2(aq) + 2H2O(L)
3. Example (production of a gas): Aqueous
potassium cyanide is reacted with hydrobromic
acid, to produce aqueous potassium bromide
and hydrocyanic gas.
KCN(aq) + HBr(aq)  KBr(aq) + HCN(g)
Double Replacement Reactions
• Steps for predicting products in double
replacement reactions:
1. Step 1: Write out the chemical formulas for
the reactants.
a. Al(NO3)3(aq) + H2SO4(aq)
2. Step 2: Identify the cations and anions in the
formulas.
a. Al(NO3)3 has Al+3 and NO3b. H2SO4
has H+ and SO4-2
Double Replacement Reactions
3. Step 3: Pair up each cation with the anion that is
found in the other compound.
a. Al+3 will pair with SO4-2 = Al2(SO4)3
b. H+ will pair with NO3= HNO3
4. Step 4: Rewrite the chemical formulas for the
reactants and products from above step, into a
complete skeletal equation:
Al(NO3)3(aq) + H2SO4(aq)  Al2(SO4)3(aq) + HNO3(g)
5. Step 5: Balance the equation:
2Al(NO3)3(aq) + 3H2SO4(aq)  Al2(SO4)3(aq) + 6HNO3(g)
Reactions in Aqueous Solutions
• Aqueous Solution: A solution in which the solvent
is water
1. Solution = homogenous mixture
a. Contains:
1). solutes: one or more substances that are
dissolved into the solvent.
2). solvent: the most plentiful substance in
the solution; the substance doing the dissolving.
2. In an aqueous solution, the solvent is always
water.
Reactions in Aqueous Solutions
• Some solutes when dissolved in water will form
ions and create an acid
1. Example: HCl(g) dissolved into water
HCl(g)  H+(aq) + Cl-(aq)
a. Any compounds that produce
hydrogen ions in an aqueous solution = acids
• Other ionic compounds will dissolve in water,
separating cations & anions
1. Example: NaOH(s)  Na+(aq) + OH-(aq)
Reactions in Aqueous Solutions
• When 2 different aqueous solutions are combined
that contain ions as solutes, the ions will react
1. The type of reaction will be a double
replacement reaction
2. produces one of 3 products:
a. Solid precipitate
b. Liquid (water)
c. Gas
3. The solvent molecules (water) will not react
4. The ions in solution are known as electrolytes
Sect. 7.2: Reactions that form
precipitates
• In order to identify which ions will react in an
aqueous double replacement reaction (and form a
solid precipitate), solubility rules/guidelines need to
be reviewed.
• Steps in predicting the precipitate that will form:
1. Write the reactants as they will exist upon
dissolving (show their charges) for both compounds
2. Exchange the anions.
3. Use solubility rules to identify which cation and
anion will form a precipitate, and which will remain
in solution (remain aqueous).
Solubility Rules
• Solubility: the property of a substance to dissolve in a
solvent
1. Can be affected by:
a. Temperature
b. Pressure
2. a substance that is “soluble” will easily dissolve
into solution, and remain in solution
3. **a substance that is “insoluble” will not want to
remain in solution, but will come out of solution as a
solid, liquid or gas.
4. In order to determine if a substance is soluble or
insoluble, you MUST use the “solubility rules”.
Solubility Rules
Solubility Rules
Reactions that form precipitates
• Example: An aqueous solution of silver (I) nitrate is
reacted with aqueous potassium chloride. What
precipitate will form, and what will remain as ions in
aqueous solution?
1. AgNO3(aq) + KCl(aq)  ?
2. Checking solubility rules shows that KNO3 is
soluble, which means it will remain as separate ions
in the aqueous solution.
3. Solubility rules state that AgCl is insoluble, which
means it will come “out” of solution, and form a
precipitate.
4. Therefore: AgNO3(aq) + KCl(aq)  AgCl(s) + K+ + NO3-
Reactions that form precipitates
• Example: Aqueous sodium hydroxide is reacted
with aqueous copper (II) chloride.
1. NaOH(aq) + CuCl2(aq)  ?
2. Solubility rules show that NaCl is soluble,
which means it will remain as ions in solution
3. Solubility rules show that Cu(OH)2 is insoluble,
which means it will come “out” of solution and
form a precipitate.
2NaOH(aq) + CuCl2(aq)  Cu(OH)2(s) + 2Na+ + 2Cl-
7.3: Describing Reactions in Aqueous
Solutions
• All reactions that take place in aqueous solutions can
be shown in a different type of chemical equation
called an “Ionic Equation”
Ionic Equation: a chemical equation that shows
ions that remain as ions in aqueous solution after a
chemical reaction has taken place.
• Example: Balanced chemical (molecular) equation:
2NaOH(aq) + CuCl2(aq)  Cu(OH)2(s) + 2NaCl(aq)
Same, but as a complete ionic equation:
2Na+(aq) 2OH-(aq)+ Cu+2(aq) + 2Cl-(aq)  Cu(OH)2(s) + 2Na+(aq) + 2Cl-(aq)
Ionic Equations
• Two types of ionic equations exist:
1. complete ionic equation
2. net ionic equation
• Complete ionic Equation: an ionic equation that
shows all particles in a solution as they realistically
exist
1. will show “spectator ions”
2. Spectator ions: those ions that do not actually
participate in the reaction, but remain in solution
a. They are usually removed from the equation,
to form a “net ionic equation”
Ionic Equations
• Net ionic equation: ionic equations that include
only the particles that participate in the reaction
1. a net ionic equation is created by crossing out
all the spectator ions (the ions that are aqueous
on both sides of the equation), and re-writing
only those that remain.
2. Therefore, the net ionic equation includes only
those components that undergo a change.
a. Spectator ions are eliminated.
Ionic Equations
• Example: Aqueous sodium hydroxide is reacted
with aqueous copper (II) chloride.
As a balanced chemical (molecular) equation:
2NaOH(aq) + CuCl2(aq)  Cu(OH)2(s) + 2NaCl(aq)
Same, but as a complete ionic equation:
2Na+(aq) 2OH-(aq)+ Cu+2(aq) + 2Cl-(aq)  Cu(OH)2(s) + 2Na+(aq) + 2Cl-(aq)
Same, but as a net ionic equation:
2OH-(aq) + Cu+2(aq)  Cu(OH)2(s)
Ionic Equations
• Example: Hydroiodic acid is reacted with aqueous
lithium sulfide.
As a balanced chemical (molecular) equation:
2HI(aq) + Li2S(aq)  H2S(g) + 2LiI(aq)
Same, but as a complete ionic equation:
2H+(aq) 2I-(aq)+ 2Li+ (aq) + S-2(aq)  H2S(g) + 2Li+(aq) + 2I-(aq)
Same, but as a net ionic equation:
2H+(aq) + S-2(aq) H2S(g)
Sect. 7.4: Reactions that form water
• When an acid is mixed with a base, water will always
be the product.
• Read about the history of acids & bases, pgs 191 to
192.
• Acid: a Latin word meaning “sour”
1. it gives the sour taste to citrus fruits
2. they are substances that produce H+ ions when
dissolved in water.
3. acids that are considered “strong” acids are those
that easily dissolve when placed into water, forming
electrolytes (charged ion particles) in the water
a. Examples of strong acids: HCl, HNO3, and H2SO4
Reactions that form water
• Base: substances that have a bitter taste, and are
slippery to the touch (like soap)
1. are also called “alkaline” solutions
2. they are substances that produce hydroxide ions
(OH-) when dissolved into water.
3. bases are considered “strong” bases are those
that easily dissolve when placed into water, forming
electrolytes (charged ion particles) in the water
a. Examples: NaOH and KOH
4. There will also be a “salt” which will remain
aqueous on the product side
Writing equations for acid-base
reactions
• Reaction: Aqueous nitric acid is reacted with
potassium hydroxide.
1. Molecular equation:
HNO3(aq) + KOH(aq)  H2O(L) + KNO3(aq)
2. Complete ionic equation:
H+(aq) + NO3-(aq) + K+(aq) + OH-(aq)  H2O(L) + K+(aq) + NO3-(aq)
**Eliminate the spectator ions
3. Net ionic equation:
H+(aq) + OH-(aq)  H2O(L)
Writing equations for acid-base
reactions summary:
1. Strong acids dissolve completely into H+ ions
and an anion. (ex: HCL, HNO3, and H2SO4)
2. Strong bases (metal hydroxides) will dissolve
completely into OH- ions and a metallic cation.
(ex: NaOH and KOH)
3. The net ionic equation for a reaction between a
base and an acid is: H+(aq) + OH-(aq)  H2O(L)
4. There will also be a salt produced, which will
remain aqueous, unless it is evaporated out of
the aqueous solution.