Chapter 4
Reactions in Aqueous Solutions
Types of Chemical Reactions
• Chemical Reactions discussed in College
Chemistry can be broken down into 3 main
categories
– Precipitation reactions
– Acid-Base reactions
– Oxidation-Reduction (redox) reactions
Types of Chemical Reactions
• Precipitation Reactions: A process in
which an insoluble solid (precipitate) drops
out of the solution.
– Clear solutions of two ionic compounds when
mixed form a cloudy solution (cloudiness
indicates solid)
Types of Reactions
• Acid–Base Neutralization: A process in
which an acid reacts with a base to yield
water plus an ionic compound called a
salt.
– The driving force of this reaction is the
formation of the stable water molecule.
Types of Reaction
• Metathesis Reactions (Double
Displacement Reaction): These are
reactions where two reactants just
exchange parts.
AX + BY  AY + BX
Types of Reactions
• Oxidation–Reduction (Redox) Reaction:
A process in which one or more electrons
are transferred between reaction partners.
– The driving force of this reaction is the
decrease in electrical potential.
Precipitation Reactions
• Develop the reaction equation
• Balance the reaction equation
• Predict the state of matter of each species
present
Precipitation Reactions and
Solubility Rules
• To predict whether a precipitation reaction
will occur must be able to predict whether
a compound is soluble or not
– Solubility rules
Solubility Rules
• Salts - soluble:
• All alkali metal and ammonium ion salts
• All salts of the NO3–, ClO3–, ClO4–, C2H3O2–,
and HCO3– ions
Solubility Rules
• Salts which are soluble with exceptions:
• Cl–, Br–, I– ion salts except with Ag+, Pb2+, &
Hg22+
• SO42– ion salts except with Ag+, Pb2+, Hg22+,
Ca2+, Sr2+, & Ba2+
Solubility Rules
• Salts which are insoluble with exceptions:
• O2– & OH– ion salts except with the alkali metal ions,
and Ca2+, Sr2+, & Ba2+ ions
• CO32–, PO43–, S2–, CrO42–, & SO32– ion salts except
with the alkali metal ions and the ammonium ion
Precipitation Reactions and
Solubility Rules
• Predict the solubility of:
– (a) CdCO3 (b) MgO
(d) PbSO4
(e) (NH4)3PO4
(c) Na2S
(f) HgCl2
Precipitation Reaction
• Precipitation reactions only occur if a solid
is produced as a product.
• If all products are aqueous compounds
then no reaction has taken place.
Electrolytes in Aqueous Solution
• To discuss aqueous reactions, must understand
how different compounds interact with water
• Electrolyte – substance that when added to
water dissociates to form ions allowing for
conductivity
– Strong electrolytes – completely dissociate
– Weak electrolytes – partially dissociate
– Non-electrolyte – no dissociation in water
Aqueous Reactions and Net Ionic
Equations
• Molecular equations – substances involved in chemical
reaction have been written using full formulas
• Complete ionic equation – shows every species as it
truly appears in the reaction vessel (aqueous strong
electrolytes are shown as ions)
• Net ionic equation – shows what is truly taking place in
the reaction (all spectators are removed)
– Must have a net ionic reaction for the reaction to occur as written
Complete Ionic Equation
• AgNO3(aq) + KI(aq) → KNO3(aq) + AgI(s)
Net Ionic Equation
• Shows species that undergo a change
during the reaction
– Obtained by removal of spectator ions.
Additional Examples
• Reaction of magnesium acetate and
sodium hydroxide in solution.
• Reaction of lead nitrate with potassium
dichromate in solution.
Acids, Bases and Neutralization
Reactions
• Acid / Base Definitions
– Arrhenius
• Acid – donates a H+ (H3O+)
• Base – donates an OH-
– Bronsted-Lowry
• Acid – donates a H+
• Base – H+ acceptor
Acids, Bases and Neutralization
Reactions
Strong Acids (all)
Strong Bases (all)
Hydrochloric acid – HCl (aq)
Lithium hydroxide – LiOH (aq)
Hydrobromic acid – HBr (aq)
Sodium hydroxide – NaOH (aq)
Hydroiodic acid – HI (aq)
Potassium hydroxide – KOH (aq)
Sulfuric acid – H2SO4 (aq)
Calcium hydroxide – Ca(OH)2 (aq)
Nitric acid – HNO3 (aq)
Strontium hydroxide – Sr(OH)2 (aq)
Perchloric acid – HClO4 (aq)
Barium hydroxide – Ba(OH)2 (aq)
Weak Acids (examples)
Acetic acid – HC2H3O2 (aq)
Cyanic acid – HCN (aq)
Phosphoric acid – H3PO4 (aq)
Organic acids – contain ending group
–COOH
Weak Bases (examples)
Ammonia (ammonium hydroxide) – NH3
(aq) actually NH4OH (aq)
Organic amines – contain ending group
-NHx
Electrolytes in Aqueous Solution
• To discuss aqueous reactions, must understand
how different compounds interact with water
• Electrolyte – substance that when added to
water dissociates to form ions allowing for
conductivity
– Strong electrolytes – completely dissociate
– Weak electrolytes – partially dissociate
– Non-electrolyte – no dissociation in water
Acids, Bases and Neutralization
Reactions
• Neutralization Reactions
– Complete ionic reactions
• Strong electrolytes (strong acids and bases) will
dissociate entirely
• Weak electrolytes are not shown as dissociated
– Net ionic for strong acids reacting with strong
bases will always be for the formation of water
Acids, Bases and Neutralization
Reactions
• Neutralization Reaction: produces salt & water.
– HA(aq) + MOH(aq)  H2O(l) + MA(aq)
• Write ionic and net ionic equations for the
following:
– (a) HBr(aq) + Ba(OH)2(aq) 
– (b) HCl(aq) + NH3(aq) 
Neutralization Reactions
• 2HBr (aq) + Ba(OH)2 (aq) → BaBr2(aq) + 2 H2O (l)
Neutralization Reactions
• HCN (aq) + NaOH (aq) → NaCN (aq) + H2O (l)
Problem
• Select the correct set of products for the
following reaction.
Ba(OH)2(aq) + HNO3(aq) →
–
–
–
–
–
A.
B.
C.
D.
E.
BaN2(s) + H2O(l)
Ba(NO3)2(aq) + H2O(l)
Ba(s) + H2(g) + NO2(g)
Ba2O(s) + NO2(g) + H2O(l)
No reaction occurs
Problem
• Select the net ionic equation for the reaction
between lithium hydroxide and hydrobromic acid.
LiOH(aq) + HBr(aq) → H2O(l) + LiBr(aq)
–
–
–
–
–
a.
b.
c.
d.
e.
LiOH(aq) → Li+(aq) + OH-(aq)
HBr(aq) → H+(aq) + Br-(aq)
H+(aq) + OH-(aq) → H2O(l)
Li+(aq) + Br-(aq) → LiBr(aq)
Li+(aq) + OH-(aq)+ H+(aq) + Br-(aq) → H2O(l) + LiBr(aq)
Oxidation-Reduction Reactions
• Redox reactions are those involving the
oxidation and reduction of species (element or
ion of an element).
• Oxidation and reduction must occur together.
They cannot exist alone.
• Two important types
– Single displacement reactions (activity series)
– Combustions – reaction of a substance with O2
Oxidation Reduction Reactions
Oxidation
Is
Loss (of electrons)
Anode Oxidation
Reducing Agent
Oxidation Reduction Reactions
Reduction
Is
Gain (of electrons)
Cathode Reduction
Oxidizing Agent
Redox Reactions
• Assigning Oxidation Numbers: All atoms have an
“oxidation number” regardless of whether it carries an
ionic charge.
1. An atom in its elemental state has an oxidation number
of zero.
2. An atom in a monatomic ion has an oxidation number
identical to its charge.
Redox Reactions
3. An atom in a polyatomic ion or in a molecular
compound usually has the same oxidation
number it would have if it were a monatomic ion.
– A. Hydrogen can be either +1 or –1.
– B. Oxygen usually has an oxidation number of –2.
• In peroxides, oxygen is –1.
– C. Halogens usually have an oxidation number of –1.
• When bonded to oxygen, chlorine, bromine, and iodine have
positive oxidation numbers.
Redox Reactions
4. The sum of the oxidation numbers must be zero for a
neutral compound and must be equal to the net charge
for a polyatomic ion.
– A. H2SO4
2(+1) + (?) + 4(–2) = 0 net charge
? = 0 – 2(+1) – 4(–2) = +6
– B. ClO4–
(?) + 4(–2) = –1 net charge
? = –1 – 4(–2) = +7
Problem
• Sodium tripolyphosphate is used in detergents
to make them effective in hard water. Calculate
the oxidation number of phosphorus in
Na5P3O10.
–
–
–
–
–
A.
B.
C.
D.
E.
+3
+5
+10
+15
none of these is the correct oxidation number
Problem
• The oxidation numbers of P, S and Cl in
H2PO2-, H2S and KClO4 are, respectively
– A.
– B.
– C.
– D.
– E.
-1, -1, +3
+1, -2, +7
+1, +2, +7
-1, -2, +7
-1, -2, +3
Redox Reactions
5. Whenever one atom loses electrons (is
oxidized), another atom must gain those
electrons (be reduced).
– A substance which loses electrons (oxidized) is called a
reducing agent. Its oxidation number increases.
– A substance which gains electrons (reduced) is called the
oxidizing agent. Its oxidation number decreases.
Redox Reactions
• For each of the following, identify which
species is the reducing agent and which is
the oxidizing agent.
• Ca(s) + 2 H+(aq)  Ca2+(aq) + H2(g)
• 2 Fe2+(aq) + Cl2(aq)  2 Fe3+(aq) + 2 Cl–(aq)
• SnO2(s) + 2 C(s)  Sn(s) + 2 CO(g)
• Sn2+(aq) + 2 Fe3+(aq)  Sn4+(aq) + 2 Fe2+(aq)
Problem
• Identify the oxidizing agent in the following redox
reaction.
Hg2+(aq) + Cu(s) → Cu2+(aq) + Hg(l)
–
–
–
–
–
A.
B.
C.
D.
E.
Hg2+(aq)
Cu(s)
Cu2+(aq)
Hg(l)
Hg2+(aq) and Cu2+(aq)
Problem
• Sodium thiosulfate, Na2S2O3, is used as a "fixer"
in black and white photography. Identify the
reducing agent in the reaction of thiosulfate with
iodine.
2S2O32-(aq) + I2(aq) → S4O62-(aq) + 2I-(aq)
–
–
–
–
–
a.
b.
c.
d.
e.
I2(aq)
I-(aq)
S2O32-(aq)
S4O62-(aq)
S2O32-(aq) and I-(aq)
The Activity Series of the Elements
The Activity Series of the Elements
• Activity series looks at the relative
reactivity of a free metal with an
aqueous cation.
– Fe(s) + Cu2+(aq)  Fe2+(aq) + Cu(s)
– Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)
– Cu(s) + 2 Ag+(aq)  2 Ag(s) + Cu2+(aq)
– Mg(s) + 2 H+(aq)  Mg2+(aq) + H2(g)
The Activity Series of the Elements
Given the following three
reactions, determine the
activity series for Cu, Zn, &
Fe.
Fe(s) + Cu2+(aq)  Fe2+(aq) +
Cu(s)
Zn(s) + Cu2+(aq)  Zn2+(aq) +
Optional Homework
• Text – 4.24, 4.30, 4.32, 4.34, 4.36, 4.40,
4.42, 4.46, 4.48, 4.52, 4.54, 4.58, 4.60,
4.62, 4.64, 4.66, 4.68, 4.92, 4.100, 4.102,
4.108
• Chapter 4 Homework
Required Homework
• Assignment 4