Unit 7 Reactions in Solution
Upon completion of this unit, you should be able to
do the following:
• Describe the driving forces for a chemical reaction.
• Use generalizations to predict the products of simple
reactions.
• Describe reactions in solutions by writing molecular,
complete ionic and net ionic equations.
• Identify the key characteristics of the reactions
between strong acids and strong bases
• Classify a reaction as one of five basic types: synthesis,
decomposition, single displacement, double
displacement or combustion.
1
Will a Reaction Occur?
• What causes reactions to occur?
Reactants → Products
• Driving forces that make reactants go in the
direction of the arrow include:
– Formation of a solid
– Formation of a gas
– Formation of water
– Transfer of electrons
2
Reactions which form solids
• One driving force for a chemical reaction is the
formation of a solid, a process called
precipitation.
• The solid formed is called a precipitate
• The reaction is called a precipitation reaction
Example:
Potassium chromate + barium nitrate
react to form a yellow precipitate.
What is the precipitate?
3
Precipitation Reactions
• To predict the identity of the precipitate,
consider the possible products
• First look at the nature of each reactant in an
aqueous solution
• Barium nitrate contains Ba2+ and NO3- ions
• Typically, when a solid containing ions
dissolves in water, the ions separate, moving
around independently
4
Precipitation Reactions
•
•
•
•
Ba(NO3) 2 (aq) does not contain Ba(NO3) 2 units
The solution contains Ba2+ and NO3- ions
There are two NO3- ions for every Ba2+ ion
When each unit of a substance that dissolves
in water produces separated ions, it is called a
strong electrolyte
• Ba(NO3) 2 is a strong electrolyte as is K 2CrO 4
5
Ionic Compounds
• When ionic compounds dissolve, the resulting
solution contains ions.
• We usually write these reactants as
Ba(NO3)2 (aq) + K2CrO4 (aq) → Products
• It is more appropriately shown as
Ba2+ (aq) + 2NO3- (aq) + 2K +(aq) + CrO42- (aq)
→ Products
• The mixed solution contains four types of ions.
6
What products form?
•
•
•
•
A solid compound must have a net zero charge
Must contain cations and anions
K+ and Ba2+ could not combine to form the solid
Most ionic materials contain only two types of
ions – one cation and one anion
• Possible compounds are
K2 CrO4 Ba(NO3)2 KNO3
BaCrO4
7
Predicting Products
• Need more information to determine which of
these is more likely to be the precipitate
KNO3
BaCrO4
• Potassium nitrate is a white solid. The chromate
ion is yellow. So, the likely precipitate is BaCrO4
and the unbalanced equation is
Ba(NO3)2 (aq) + K2CrO4 (aq) →KNO3 (aq) + BaCrO4(s)
8
Solubility Vocabulary
• A soluble solid readily dissolves in water.
• An insoluble solid, or slightly soluble solid,
means only a small amount of the solid
dissolves in water.
• A salt means an ionic compound.
9
Solubility Rules
1.
2.
3.
4.
5.
6.
Table 7.1, Z page 178
Most nitrate (NO3-) salts are soluble.
Most salts of Na+, K+ and NH4+ are soluble.
Most chloride salts are soluble. Notable
exceptions are AgCl, PbCl2 and Hg2Cl2.
Most sulfate salts are soluble. Notable
exceptions are BaSO4, PbSO4 and CaSO4.
Most hydroxide compounds are insoluble.
Notable exceptions are NaOH, KOH, Ba(OH)2
and Ca(OH) 2 .
Most sulfide (S2-), carbonate (CO32-) and
phosphate (PO43-) salts are insoluble.
10
Predicting Products - Z page 180
Step 1 – Write the reactants as they actually
exist before any reaction occurs.
Step 2 – Consider the various solids that could
form. To do this, simply exchange the anions
of the added salts.
Step 3 – Use the solubility rules to decide
whether a solid forms and to predict the
identity of the solid.
11
Predicting Products
Guided example
When an aqueous solution of silver nitrate is
added to an aqueous solution of potassium
chloride, a white precipitate is formed. Identify
the solid and write a balanced equation for the
reaction.
12
Predicting Products
When an aqueous solution of silver nitrate is
added to an aqueous solution of potassium
chloride, a white precipitate is formed.
AgNO3 (aq) + KCl (aq) → AgCl (?) + KNO3 (?)
Solubility rule 1 – NO3- is soluble
Solubility rule 2 – K + is soluble
Solubility rule 3 – AgCl is insoluble
AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3 (aq)
Equation is balanced
13
Predicting Products
In class examples
1. KNO3 (aq) and BaCl2 (aq)
2. Na2SO4 (aq) and Pb(NO3) 2 (aq)
3. KOH (aq) and Fe(NO3) 3 (aq)
Homework
Page 202, problems 18 and 22
14
Reactions in Aqueous Solutions
Returning to this equation, note that it is called a
molecular equation.
Ba(NO3)2 (aq) + K2CrO4 (aq) →2KNO3 (aq) + BaCrO4(s)
It shows the complete formulas of all products and
reactants.
This does not give a clear picture of what occurs in
solution.
15
Reactions in Aqueous Solutions
• Aqueous solutions of barium nitrate,
potassium chromate and potassium nitrate
contain the individual ions, not the complete
molecules implied by the molecular equation.
• The complete ionic equation better
represents the actual forms of reactants and
products in solution.
Ba2+ (aq) + 2NO3- (aq) + 2K +(aq) + CrO42- (aq)
→ BaCrO4 (s) + 2K +(aq) + 2NO3- (aq)
16
Reactions in Aqueous Solutions
• The complete ionic equation reveals that only
some of the ions participate in the reaction.
• Ions which do not participate in the reaction,
such as K + and NO3- , are called spectator ions.
• The ions that participate in the reaction are
Ba2+ (aq) + CrO42- (aq) → BaCrO4 (s)
• This equation, called the net ionic equation,
includes only those components that are
directly involved on the reaction.
17
Molecular, Complete Ionic and
Net Ionic Equations
In class
Z page 184 examples 7.3a and 7.3b
18
Equations
7.3a) Molecular equation
NaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)
Complete ionic equation
Na+ (aq) + Cl- (aq) + Ag + (aq) + NO3- (aq) →
AgCl (s) + Na + (aq) + NO3- (aq)
Net ionic equation
Cl- (aq) + Ag + (aq) → AgCl (s)
19
Equations
7.3b ) Molecular equation
3KOH(aq) +Fe(NO3) 3(aq) → Fe(OH) 3(s) + 3KNO3(aq)
Complete ionic equation
3K + (aq) + 3OH- (aq) + Fe 3+ (aq) + NO3- (aq) →
Fe(OH) 3(s) + 3K + (aq) + 3NO3- (aq)
Net ionic equation
3OH- (aq) + Fe 3+ (aq) → Fe(OH) 3(s)
20
Molecular, Complete Ionic and
Net Ionic Equations
Homework
Page 202 and 203, problem 26
21
Reactions that Form Water
• Acids – first associated with the sour taste of
citrus fruits. Derived from Latin word acidus
meaning sour
• Essential nature of acids discovered by Svante
Arrhenius in the late 1800’s
• Arrhenius proposed that an acid is a substance
that produces H+ ions when it is dissolved in
water
• Studies show that when HCl, HNO3 and H2SO4 are
placed in water nearly every molecule dissociates
to give ions
• Because these substances are strong electrolytes
that produce H + ions they are called strong acids
22
Reactions that Form Water
• Bases – characterized by bitter taste and
slippery feel
• Arrhenius found that aqueous solutions that
exhibit basic behavior always contain
hydroxide ions (OH-). He defined a base as a
substance that produces OH- in water.
• Most common base in the lab is NaOH which
dissolves in water to form Na+ and OH- ions
23
Reactions that Form Water
• When strong acids and strong bases are
mixed, the fundamental change that occurs is
that H+ and OH- ions react to form water
H+ (aq) + OH- (aq) → H2O (l)
• The tendency to form water is one of the
driving forces for chemical reactions
24
Reactions that Form Water
The reaction between hydrochloric acid and
sodium hydroxide shown as a molecular
equation is:
HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
25
Reactions that Form Water
Because HCl and NaOH exist as completely
separated ions in water, the complete ionic
equation can be written as:
H+ (aq) + Cl- (aq) + Na + (aq) + OH- (aq) →
Na + (aq) + Cl- (aq) + H2O (l)
26
Reactions that Form Water
Notice that Cl- and Na + are spectator ions so
the net ionic equation is:
H+ (aq) + OH- (aq) → H2O (l)
27
Acid-Base Reactions
In class
Z Page 187 – Example 7.4
28
Acid-Base Reactions
In class
Page 187 – Example 7.4H
Molecular equation:
HNO3 (aq) + KOH (aq) → H2O (l) + KNO3 (aq)
Complete ionic equation:
H+ (aq) + NO3- (aq) + K + (aq) + OH- (aq) →
H2O (l) + NO3- (aq) + K + (aq)
Net ionic equation:
H+ (aq) + OH- (aq) → H2O (l)
29
Acid-Base Reactions
Note two things when looking at the reaction
between HCl and NaOH and the reaction between
HNO3 and KOH:
1. The net ionic equation is the same
H+ (aq) + OH- (aq) → H2O (l)
2. Besides water, the second product formed is an
ionic compound called a salt. The salt may
precipitate or remain in solution, depending on
solubility. How could you recover the salt if it is
soluble? If it is insoluble?
30
Acid-Base Reactions
Homework
Page 203
problems 39a, 39c, 39d, 40a, 40c, 40d
31
Classifying Reactions
• Formation of a solid (precipitation reaction)
• Formation of water (acid-base reaction)
• Transfer of electrons (oxidation-reduction reaction)
-single replacement
-double replacement
Other types of reactions
• Combustion reaction
• Synthesis or Combination reaction
• Decomposition reaction
32