This document provides information on chemical reactions involving aqueous solutions and precipitation. It discusses: - How ionic compounds dissolve in water to form ions, and how this allows them to conduct electricity. Electrolytes fully or partially dissociate, while non-electrolytes do not. - When solutions of two ionic compounds are mixed, double displacement occurs as ions exchange to form new ionic compounds. If one product is insoluble, it precipitates out as a solid. - The process of predicting products of precipitation reactions using solubility rules to determine which compound will precipitate. - How to write balanced molecular, complete ionic, and net ionic equations for precipitation reactions. Examples

1. 1
Chemical Reactions
Chapter 7
Section 7.5 to 7.7
CHEM 1020 Instructor: Dr. Rekha, MCTC
Water is a polar molecule
▪ Water is a polar covalent
molecule with charges
and dipole moment (like
you learned in Chapter-5)
▪ Oxygen is more
electronegative and pulls
the shared electrons more
towards itself. Hence gets
partial negative charge.
▪ But Hydrogen is least
electronegative, gets
partial positive charge. 2
Electrolytes
• Many substances that are polar, dissolve in
water. They are said to be soluble.
• Because water is a polar molecule, other polar
substances dissolve in it.
• The substance that dissolved is called solute.
• The substance in which things dissolved, is
called solvent.
• Together, they have formed a solution.
• Example, sugar in water when dissolved; salt
in water when dissolved
Ionic compounds Dissociate
▪ For example, NaCl crystals will exist in water as
Na+ and Cl- ions. We represent the dissolved NaCl
as NaCl(aq).
▪ Notice that the suffice (aq) means it is in aqueous
solution, hence the ionic compound is dissolved in
water, and is staying soluble.
▪ Likewise, the dissolved ions will be shown as
Na+(aq) and Cl-(aq).
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Polarity of Water
▪ Ionic compounds are solutes, they dissolve in the
solvent water.
▪ When dissolved, The ionic compounds dissociate
into their respective ions
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Electrolytes
• Many of the substances, when dissolved in water, the
resulting aqueous solution can conduct electricity.
• Those that conduct electricity are called electrolytes.
• Because all electrolytes dissociate into their ions
when dissolved in water. Since ions are charged
particles, they can move in electric field, conducting
an electrical current.
• Substances that do not dissociate into ions when
dissolved, and do not conduct electricity are
considered Non-electrolytes.
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3 4
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2. Electrical conductivity of aqueous solutions. (a) Pure
water does not conduct an electric current. (b) When an
ionic compound is dissolved in water, current flows and
the lamp lights.
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Electrolytes
• In strong electrolytes, virtually all the
molecules are dissociated into ions. Example:
Ionic compounds.
• in weak electrolytes, a small percentage of the
molecules are dissociated into ions
• in nonelectrolytes, none of the molecules are
dissociated into ions
Electrolytes
Example: Ethanol (found in alcoholic beverages) does not
conduct electricity; it is a nonelectrolyte
KCl an ionic compound, all molecules will dissociate into ions
and conduct electricity strongly.
Acetic acid is a weak conductor, does not dissociate completely
into ions.
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Dissociation
• potassium chloride dissociates in water into
potassium cations and chloride anions
KCl(aq) = K+ (aq) + Cl- (aq)
• copper(II) sulfate dissociates in water into
copper(II) cations and sulfate anions
CuSO4(aq) = Cu+2(aq) + SO4
2-(aq)
K+ Cl-
K Cl
Cu+2 SO4
2-
Cu SO4
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When two Solutions of Ionic Comounds are
Mixed
• What happens if two solutions of different ionic
compounds are mixed together?
• Will all of their dissociated ions continue to stay as
separate entities in water?
• When aqueous solutions of two ionic compounds are
mixed, they interact with each other by exchanging
their ions, forming new compounds.
• They displace the ions from one another, and hence
called double displacement.
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Formation of solid: Precipitation Reactions
• Although they are soluble, and clear solutions to begin
with, One of the new compounds formed may NOT
be soluble and can form a solid.
• The solid formed is called a precipitate which results
in a stable new compound.
• These types of reactions usually involve Ionic
Compounds.
• Hence the reactions are known as Ionic Precipitation
Reactions (Your lab experiment is based on this).
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9 10
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3. Process for Predicting the Products
Example: When AgNO3 solution and KCl solutions are
mixed, let us look at the possible combinations (balance
the charges to get the correct formulas)
K+
Ag+
Cl- NO3
-
KCl KNO3
AgCl AgNO3
▪ However, KCl and AgNO3 are already present as reactants
before the reaction. Therefore, they cannot be products.
▪ Hence KNO3 and AgCl are the products formed by Double
Displacement. 12
Reactions of Aqueous Ionic Compounds
To determine if there is a new substance formed or
not:
▪ If the two aqueous solutions that were clear to
begin with, but when combined can form an
insoluble solid, this means a new substance is
formed, and there is a chemical reaction called
Precipitation reaction (or Ionic Precipitation
reaction)
▪ The new insoluble solid formed is called
Precipitate. (Pre-ci-pi-tate)
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Reactions of Aqueous Ionic Compounds
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Process for Predicting the Products of
a Precipitation Reaction
Now, how do we know which product among the
two is the solid (precipitate)?
Determine Solubility of Each Product in Water,
using solubility rules.
Solubility Rules: There is no need to memorize
this. But you should learn how to use these rules.
 if product is mentioned as insoluble or slightly soluble,
it means it will precipitate out
Solubility Rules
1) Most compounds that contain NO3
- ions are soluble
2) Most compounds that contain Na+, K+, or NH4
+ ions
are soluble
3) Most compounds that contain Cl- ions are soluble,
except AgCl, PbCl2, and Hg2Cl2
4) Most compounds that contain SO4
2- ions are soluble,
except BaSO4, PbSO4, CaSO4
5) Most compounds that contain OH- ions are not soluble
(will precipitate), except NaOH, KOH, are soluble
6) Most compounds that contain S2-, CO3
2-, or PO4
3- ions
are not soluble (will precipitate)
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Which one of the Precipitate?
• In the previous example, when KCl and AgNO3
solutions were mixed together,
• you noticed that the KNO3 and AgCl are the products.
• If you look at the solubility rules,
Rule#1: Most compounds that contain NO3
- ions are
soluble
Rule#2: Most compounds that contain Na+, K+, or NH4
+
ions are soluble
Hence, in the products, KNO3 is soluble, it is NOT our
precipitate.
Rule#3: Most compounds that contain Cl- ions are
soluble, except AgCl, PbCl2, and Hg2Cl2
Based on this rule, AgCl is our precipitate.
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4. Solubilities
of common
compounds.
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Ionic Precipitation Reactions are Double
Displacement Reactions
• In all precipitation reactions, the ions of one
substance are exchanged with the ions of another
substance when their aqueous solutions are mixed.
• In other words, there is double exchange. Hence
these reactions can also be classified as Double
Displacement Reactions.
K+
Cl-
Ag+
NO3
-
K+
NO3
-
Ag Cl
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Chemical Equations for Precipitation Reactions
• Now let us write four different steps of chemical
equations for this reaction and balance them:
• 1) Word Equation showing the names of reactants and
products in word, along with their physical states
• 2) Balanced Molecular equation. Equations which
describe the formulas of reactants and products along
with their physical state of existence.
• 3) Balanced Complete ionic equations which describe
the actual dissociated ions and solid molecules (if any)
• 4) Balanced Net Ionic Equation: which only shows the
ions that actually were responsible for the solid formation
and also the solid compound that is formed.
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Describing Ionic Equations
• Write the Word equation, by doing double
displacement. Make sure to show the physical states.
• Potassium chloride (aq) + Silver nitrate (aq)
→Potassium nitrate (aq) + Silver chloride(s)
Start writing the formulas correctly, before balacig.
KCl(aq) + AgNO3(aq) → KNO3(aq) + AgCl(s)
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Describing Ionic Equations
• Check if it is a balanced molecular equation; If not
balance it.
KCl(aq) + AgNO3(aq) → KNO3(aq) + AgCl(s)
Reactant side: 1 K, 1 Cl, 1 Ag, and 1 NO3
Product side: 1K, 1 Cl, 1 Ag, and 1 NO3’
Therefore, the number of ions of each are already same on
both sides, already balanced.
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5. 25
Describing Ionic Equations
KCl(aq) + AgNO3(aq) → KNO3(aq) + AgCl(s)
• Next, Write the complete ionic equation which
describe the actual dissociated ions and the Precipitate
(undissociated). Use the skills learned in chapter-5 for
showing the correct charges. Include physical states.
K+
(aq)+ Cl-
(aq)+ Ag+
(aq)+ NO3
-
(aq) →
K+
(aq) + NO3
-
(aq) + AgCl(s)
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Net Ionic Equations
• Ions that are not involved in the precipitate formation are
called spectator ions. In our example, K+
(aq) , NO3
-
(aq)
are the spectator ions.
• Net Ionic Equation: An ionic equation in which only
the ions that participated in forming the solid
precipitate, is called a net ionic equation. Include
physical states.
• In our example,
Cl-
(aq) + Ag+
(aq) → AgCl(s)
Another Example
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Example-2: Write all the equations for the following Ionic
Precipitation Reaction:
Lead(II)nitrate(aq) mixed with Ammonium Sulfate(aq)
1) Word equation: (switch the ions to get the products))
Lead(II)nitrate(aq) mixed with Ammonium Sulfate(aq)
Lead (II)Sulfate(s) + Ammonium nitrate (aq)
2) Write Molecular equation and later worry about balancing
it. Keep your periodic table and the polyatomic ions list ready for
reference. Use the skills you learned in chapter-5 for writing
correct formulas.
Pb(NO3)2(aq) + (NH4)2SO4 (aq) PbSO4 (s) + NH4NO3 (aq)
Solubility Rules
How did I know, PbSO4 is the solid precipitate? By
referring to solubility rules #1, 2 and 4. (You check it).
1) Most compounds that contain NO3
- ions are soluble
2) Most compounds that contain Na+, K+, or NH4
+ ions
are soluble
3) Most compounds that contain Cl- ions are soluble,
except AgCl, PbCl2, and Hg2Cl2
4) Most compounds that contain SO4
2- ions are soluble,
except BaSO4, PbSO4, CaSO4
5) Most compounds that contain OH- ions are not soluble
(will precipitate), except NaOH, KOH, are soluble
6) Most compounds that contain S2-, CO3
2-, or PO4
3- ions
are not soluble (will precipitate)
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Balance the Molecular Equation
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Pb(NO3)2(aq) + (NH4)2SO4 (aq) PbSO4 (s) + NH4NO3 (aq)
• We should now balance the equation= ensure the number of
atoms and the number of polyatomic units are all same on both
sides of the equation.
• The Pb and the (SO4) units are same (1 each) on both sides.
• However, the (NO3) and (NH4) units are 2 units on reactant
side, but only 1 on product side.
• Therefore, if we add Coefficient (or prefix) of 2 on the right
side, for NH4NO3 we will make both (NO3) and (NH4)
balanced. (Don’t change the formula or the subscripts).
Pb(NO3)2(aq) + (NH4)2SO4 (aq) PbSO4 (s) + 2NH4NO3 (aq)
Balanced complete Ionic Equation
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Pb(NO3)2(aq) + (NH4)2SO4 (aq) PbSO4 (s) + 2NH4NO3 (aq)
How to write the Complete balanced Ionic Equation? Write all
the ions with their appropriate charges and number of units by
looking at the balanced molecular equation.
Do not show ions for the precipitate. As it cannot dissociate.
Pb2+
(aq) + 2(NO3)-
(aq) +2(NH4)(aq) + SO4
2-
(aq)
PbSO4 (s) + 2(NH4)(aq) + 2(NO3)-
(aq)
How to write the balanced Net Ionic Equation? Show only the
substances that were involved in precipitate formation. In this case
it is already balanced.
Pb2+
(aq) + SO4
2-
(aq) PbSO4 (s)
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6. • More Examples
Example-3:
• When potassium hydroxide and Iron(III)nitrate
aqueous solutions combine, the balanced molecular
equation:
• 3KOH(aq) + Fe(NO3)3(aq) 3KNO3(aq) + Fe(OH)3(s)
• Note, Fe(OH)3 is insoluble according to solubility rules.
• The complete balanced ionic equation is:
• 3K+(aq) + 3OH-(aq) + Fe3+(aq) +3NO3
-(aq)
3K+(aq) + NO3
-(aq) + Fe(OH)3(s)
• Therefore, the balanced Net Ionic Equation is:
• Fe3+(aq)+ 3OH-(aq) Fe(OH)3(s)
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• More Examples
• Practice more examples from your text-book.
• What if you come across an ionic reaction
where the both products formed are also soluble
and therefore, no precipitate? How do you
represent that?
• Let us see one example in the next slide.
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• More Examples
• When potassium chloride and copper(II) sulfate
aqueous solutions combine, the balanced molecular
equation is:
• 2KCl(aq) + CuSO4(aq) K2SO4(aq) + CuCl2(aq)
• As per solublity rules, both products are soluble.
• The Complete ionic equation is:
2K+(aq) + 2Cl-(aq) + Cu2+(aq) + SO4
2-(aq)
2K+(aq) + SO4
2-(aq) + Cu2+(aq) + 2Cl-(aq)
• There are no solids formed. Therefore, there is no Net
Ionic Equation. No Precipitation reaction.
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Developing a photo involves several aqueous
chemical reactions.
Source: Dan McCoy/Rainbow
34
The precipitation
reaction that occurs
when yellow
potassium
chromate,
K2CrO4(aq), is
mixed with a
colorless barium
nitrate solution,
Ba(NO3)2(aq).
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Precipitation of
silver chloride occurs
when solutions of
silver nitrate and
potassium chloride
are mixed.
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7. 37
Predicting Whether a Reaction Will Occur
“Forces” that drive a reaction are:
• formation of a solid (as in the case of ionic
precipitation reactions)
• formation of water
• transfer of electrons
• formation of a gas
• when chemicals are mixed and if one of these
4 things can occur, it is an indication that the
reaction has occurred.
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