How Can We Describe
Chemical Reactions?
Chemistry Unit 9
Main Ideas
Chemical reactions are represented by
balanced chemical equations.
There are four main types of chemical
reactions: synthesis, combustion,
decomposition, and replacement reactions.
Double-replacement reactions occur
between substances in aqueous solutions
and produce precipitates, water or gases.
Reactions and
Equations
8.1
Objectives
Recognize evidence of chemical change.
Represent chemical reactions with
equations
Balance chemical equations.
Chemical Reactions
The process by which one or more substances are
rearranged to form different substances is called a
chemical reaction.
Evidence of a chemical reaction is a chemical
change: a process of changing one or more
substances into a new substance.
Evidence: temperature changes, color changes,
odor, gas bubbles or appearance of a precipitate
Representing Chemical
Reactions
Chemical equations – statements that show
chemical reactions by the use of chemical
formulas and conserved matter with the
relative amounts of substances in the
reaction.
Parts of an equation reaction:
Reactants are the starting substances.
Products are the substances formed in the
reaction.
Common Symbols
Representing Reactions
Word Equations: use of words for reactants and
products
aluminum(s) + bromine(l) → aluminum bromide(s)
Skeleton Equations: chemical formulas used for
reactants and products but not balanced
Al(s) + Br(l) → AlBr3(s)
Skeleton equations lack information about how
many atoms are involved in the reaction.
Representing Reactions
Chemical Equation: Is a statement that uses
chemical formulas to show the identities and
relative amounts of the substances involved in
a chemical reaction.
• 2Al(s) +3Br2 2AlBr3(s)
Balancing Chemical
Equations
Balancing Chemical
Equations
A coefficient in a chemical equation is the
number written in front of a reactant or
product, describing the lowest wholenumber ratio of the amounts of all the
reactants and products.
The most fundamental law is the law of
conservation of mass; a balanced
equation shows this law.
Balancing Chemical
Equations
1. Write the skeleton equation:
Make sure chemical formulas are
correct.
Put in symbols and physical states.
liquid sodium carbonate + aqueous calcium
chloride yields solid calcium carbonate +
aqueous sodium chloride
Balancing Chemical
Equations
2. Count the atoms of the elements in the
reactants
Group intact polyatomic ions as a
single substance.
Na2CO3(l) + CaCl2(aq)  CaCO3(s) + NaCl(aq)
Balancing Chemical
Equations
3. Count the atoms of the elements in the
products
Group intact polyatomic ions as a
single substance.
Na2CO3(l) + CaCl2(aq)  CaCO3(s) + NaCl(aq)
Balancing Chemical
Equations
4. Change the coefficients to make the
number of atoms of each element equatl
on both sides of the equation
Never change subscripts
Na2CO3(l) + CaCl2(aq) 
CaCO3(s) +
NaCl(aq)
Balancing Chemical
Equations
5. Write the coefficients in their lowest
possible ratios
Na2CO3(l) + CaCl2(aq) 
CaCO3(s) +
NaCl(aq)
Balancing Chemical
Equations
5. Go back and check math.
Na2CO3(l) + CaCl2(aq)  CaCO3(s) + 2 NaCl(aq)
Balancing Chemical
Equations
Example
aqueous sodium hydroxide + aqueous
calcium bromide yields solid calcium
hydroxide and aqueous sodium bromide
2112
Question 1
Which of the following is NOT a
chemical reaction?
A. a piece of wood burning
B.a car rusting
C.an ice cube melting into water
D. red litmus paper turning blue
Question 1
What is the coefficient of bromine in the
equation 2Al(s) + 3Br2(l) → 2AlBr3(s)?
A. 1
B.2
C.3
D. 6
Practice Problems
Page 287 #4-6; page 288 #7-13
Classifying Chemical
Reactions
8.2
Objectives
Classify chemical reactions.
Identify the characteristics of different
classes of chemical reactions.
Types of Chemical
Reactions
Chemists classify reactions in order to organize
the many types.
Synthesis
Combustion
Decomposition
Single Replacement
Double Replacement (Metathesis)
Synthesis
A synthesis reaction is a reaction in which two or
more substances react to produce a single
product.
When two elements react, the reaction is always a
synthesis reaction.
Synthesis
A synthesis reaction is a reaction in which two or
more substances react to produce a single
product.
When two compounds react:
AB + CD  ABCD
AB + BC  ABC
Combustion
In a combustion reaction, oxygen combines with a
substance and releases energy in the form of
heat and light.
Example: Heated hydrogen reacts with oxygen to
produce heat and water in a combustion
reaction. This is also a synthesis reaction.
Combustion
In a combustion reaction, oxygen combines with a
substance and releases energy in the form of
heat and light.
Element and oxygen react: A + O2  AO
Compound and oxygen react: AB + O2  AO + B
Decomposition
A decomposition reaction is one in which
a single compound breaks down into
two or more elements or new
compounds.
Decomposition reactions often require
an energy source, such as heat, light, or
electricity, to occur.
Decomposition
A decomposition reaction is one in which
a single compound breaks down into
two or more elements or new
compounds.
Compound breaks down into two
elements: AB  A + B
Compound breaks down to form new
compounds: ABCD  AC + BD
Replacement/Displacem
ent
A reaction in which the atoms of one element
replace the atoms of another element in a
compound is called a single replacement
reaction.
A + BX → AX + B
Activity Series
A metal will not
always replace a
metal in a compound
dissolved in water
because of differing
reactivities.
An activity series can
be used to predict if
reactions will occur.
Activity Series
Halogens frequently
replace other
halogens in
replacement
reactions.
Halogens also have
different reactivities
and do not always
replace each other.
Activity Series
Metals/Halogens are
listed in order of
reactivity . A less
reactive
metal/halogen will
not replace a more
reactive
metal/halogen
Practice Problems
Page 291 #14-17; page 292 #18-20
Page 295 #21-24
Double Replacement
Double replacement reactions (also called
metathesis) occur when ions exchange
between two compounds.
Double Replacement
Metathesis reactions often form one of
three products:
The solid product produced during a
chemical reaction in a solution is called a
precipitate.
water – is usually formed with the
combination of an acid and a base. A metal
salt is also formed.
gas – formed when a gas is not a reactant.
Steps to Metathesis
Product Prediction
Question 1
Which of the following is NOT one of the
four types of reactions?
A. deconstructive
B. synthesis
C. single replacement
D.double replacement
Question 1
The following equation is what type of
reaction?
KCN(aq) + HBr(aq) → KBr(aq) + HCN(g)
A. deconstructive
B. synthesis
C. single replacement
D.double replacement
Practice Problems
Page 297 #25-28; Page 298 #29-34
Solubility
8.3
Objectives
Identify new possible ionic compounds
in a reaction
Define the terms soluble and insoluble
Predict solids based on solubility rules.
Ionic Compounds in
Solutions
Ionic compounds in aqueous solutions mix and
exchange partners (double replacement).
example: Na2SO4(aq) + CaCl2(aq) 
Some of these products are solids and some
of these products remain aqueous.
Solubility
Solubility rules are used to determine the state
of matter of products in an aqueous solution.
• Soluble means that the compound dissolves
in water.
• Insoluble means that the compound remains
intact in the solid state in water.
Solubility Rules
1. Most nitrate (NO3-) salts are soluble
2. Most salts containing the alkali metal ions (Li+,
Na+, K+, Cs+, Rb+) and the ammonium ion (NH4+)
are soluble.
3. Most chloride, bromide, and iodide salts are
soluble
Exceptions: Ag+, Pb2+, Hg22+
Solubility Rules
4. Most sulfate salts are soluble .
• Exceptions: Bas+, Pb2+, Hg22+, and Ca2+
5. Most hydroxides are only slightly soluble (treat
as insoluble).
• Exceptions: Na+, K+
6. Most sulfide (S2-), carbonate (CO32-), chromate
(CrO42-), and phosphate (PO43-) salts are only
slightly soluble (treat as insoluble).
• Exceptions: any containing Alkali metals and
ammonium.
Solubility Summary
Soluble
Insoluble
1. Nitrates (NO3-)
1. no exceptions
2. Group 1, NH4+
2. no exceptions
3. Halogens
3. Ag+, Pb2+, Hg22+
4. Sulfates (SO42-)
4. Bas+, Pb2+, Hg22+, Ca2+
5. Hydroxides (OH-)
6. S2-, CO32-, CrO42- PO43-
Example Problem
Aqueous silver nitrate mixes with aqueous
sodium chloride; what solid will be produced
from this solution.
Reactions in Aqueous
Solutions
9.3
Objectives
Describe aqueous solutions
Write complete ionic and net ionic
equations for chemical reactions in
aqueous solutions.
Predict whether reactions in aqueous
solutions will produce a precipitate,
water, or a gas.
Aqueous Solutions
An aqueous solution contains one or more
dissolved substances (called solutes) in
water.
Aqueous Solutions
Solution – a homogeneous mixture that
may contain solids, liquids or gases.
Solutes – the substance that is being
dissolved.
Solvent – the substance that is most
plentiful and dissolves the other
substance.
Aqueous Solutions
There are many possible solutes—sugar
and alcohol are molecular compounds that
exist as molecules in aqueous solutions.
Molecules that produce hydrogen ions in
aqueous solutions are acids.
Aqueous Solutions
Ionic compounds can also be solutes in
aqueous solutions.
When ionic compounds dissolve in
water, their ions separate in a process
called dissociation.
Types of Aqueous
Equations
Ionic Equations show ionic detail and dissociation
within reactions.
Formula equation:
2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s)
Types of Aqueous
Equations
2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s)
Ionic equations that show all of the particles in a
solution as they actually exist are called complete
ionic equations.
2Na+(aq) + 2OH–(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+(aq) + 2Cl–(aq) + Cu(OH)2(s)
Ions that do not participate in a reaction are called
spectator ions and are not usually written in ionic
equations.
Types of Aqueous
Reactions
2Na+(aq) + 2OH–(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+(aq) + 2Cl–(aq) + Cu(OH)2(s)
Formulas that include only the particles that
participate in reactions are called net ionic
equations.
2OH–(aq) + Cu2+(aq) → Cu(OH)2(s)
Types of Aqueous
Reactions
Some reactions produce more water
molecules.
HBr(aq) + NaOH(aq) → H2O(l) + NaBr(aq)
Without spectator ions
H+(aq) + OH–(aq) → H2O(l)
Types of Aqueous
Reactions
Gases that are commonly produced are carbon dioxide,
hydrogen cyanide (aq-hydrocyanic acid) , and
hydrogen sulfide (aq-hydrosulfuric acid).
2HI(aq) + Li2S(aq) → H2S(g) + 2LiI(aq)
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
H2CO3(aq) decomposes immediately.
H2CO3(aq) → H2O(l) + CO2(g)
Types of Aqueous
Reactions
Two reactions can be combined and
represented by a single chemical reaction
Reaction 1
Types of Aqueous
Reactions
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
Reaction 2
H2CO3(aq) → H2O(l) + CO2(g)
Combined equation
HCl(aq) + NaHCO3(aq) + H2CO3(aq) → H2CO3(aq) + NaCl(aq) + H2O(l) + CO2(g)
Overall equation
HCl(aq) + NaHCO3(aq) → H2O(l) + CO2(g) + NaCl(aq)
Types of Reactions in
Aqueous Solutions
1. Precipitate is formed- when a compound forms
from ions, an exothermic reaction takes place.
The ions by themselves are less stable and
therefore of higher energy than when combined
in a compound. This change in energy (increase in
stability) ‘drives’ the reaction.
2NaOH(aq) + CuCl2(aq)  2NaCl(aq) + Cu(OH)2(s)
Net:
Types of Reactions in
Aqueous Solutions
2. Water is formed- even though a double reaction
takes place, solutions may look the same since
water will still be the dominant substance.
HBr(aq) + NaOH(aq)  H2O(l) + NaBr(aq)
Net:
Types of Reactions in
Aqueous Solutions
3 Gas is formed- for the same reasons that a
precipitate is formed, a gas will also form.
Common gases in double replacement reactions
are: carbon dioxide, hydrogen cyanide, and
hydrogen sulfide.
2HI(aq) + Li2S(aq)  H2S(g) + 2LiI(aq)
Net:
Question 1
What is the solvent in an aqueous
solution?
A. hydrogen
B. sodium ions
C. water
D.alcohol
Question 2
An equation that includes only the
particles that participate in a reaction is
called:
A. net ionic equation
B. spectator ions
C. complete ionic equation
D.reduced ionic equation
Practice Problems
Page 302 #35-39; Page 304 #40-44;
Page 306 #45-49; Page #50-56
Accumulating Content
8.5
Accumulating Content
Objective: Apply knowledge and skills from previous units
to content learned in this unit.
• Why do polyatomic ions stay intact in an aqueous
solution?
Accumulating Content
Objective: Apply knowledge and skills from previous units
to content learned in this unit.
• How do naming rules change when working with
gases vs. aqueous acids?
Accumulating Content
Objective: Apply knowledge and skills from previous units
to content learned in this unit.
How does lattice energy relate to solubility rules?
Key Concepts
Some physical changes are evidence that indicate a
chemical reaction has occurred.
Word equations and skeleton equations provide
important information about a chemical reaction.
A chemical equation gives the identities and
relative amounts of the reactants and products that
are involved in a chemical reaction.
Key Concepts
Balancing an equation involves adjusting the
coefficients until the number of atoms of each
element is equal on both sides of the equation.
Classifying chemical reactions makes them easier
to understand, remember, and recognize.
Activity series of metals and halogens can be used
to predict if single-replacement reactions will
occur.
Key Concepts
In aqueous solutions, the solvent is always water.
There are many possible solutes.
Many molecular compounds form ions when they
dissolve in water. When some ionic compounds
dissolve in water, their ions separate.
When two aqueous solutions that contain ions as
solutes are combined, the ions might react with
one another. The solvent molecules do not usually
react.
Key Concepts
Reactions that occur in aqueous solutions
are double-replacement reactions.
Question 1
The law of conservation of mass requires what in a chemical
reaction equation?
A. both sides of the equation to contain
the same substances
B. the reactants to have the same
amount of molecules as the products
C. both sides to have the same amount
of atoms of each element
D. the products to have fewer molecules
than the reactant
Question 1
A reaction that gives off heat is what type of
reaction?
A. single replacement reaction
B. double replacement reaction
C. synthesis reaction
D. combustion reaction
Question 1
Ions that are present in a solution and do not
participate in a chemical reaction when another
substance is added are called ____.
A. spectator ions
B. reactants
C. products
D. net ions
Question 1
A double replacement reaction produces all of the
following except ____.
A. gases
B. solids
C. light
D. water
Question 1
What type of reaction is the following?
2H2O(l) + energy → H2(g) + O2(g)
A. synthesis reaction
B. decomposition reaction
C. combustion reaction
D. replacement reaction
Question 1
What type of reaction is the following?
2H2(g) + O2(g) → 2H2O(l)
A. replacement reaction
B. synthesis
C. combustion reaction
D. double replacement reaction
Question 1
A precipitate forms in a double replacement reaction
only if:
A. the reactivities of the compounds differ
B. the new compound is denser than water
C. the new compound is soluble in water
D. the new compound is not
soluble in water
Question 1
A ____ is a statement that uses chemical formulas to
show the identities and relative amounts of the
substances involved in a chemical reaction.
A. word equation
B. skeleton equation
C. chemical equation
D. balanced equation
Question 1
Predict the type of reaction.
LiBr2 (aq) + 2NaOH (aq) → ____
A. synthesis reaction
B. combustion reaction
C. single replacement reaction
D. double replacement reaction
Question 1
Which reactions are essentially the opposite of
synthesis reactions?
A. single-replacement
B. decomposition
C. combustion
D. double-replacement
The End