Chemical Reactions
Section 9.1 Reactions and
Equations
Section 9.2 Classifying Chemical
Reactions
Section 9.3 Reactions in Aqueous
Solutions
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Section 9.1 Reactions and Equations
• Recognize evidence of
chemical change.
• Represent chemical
reactions with equations.
• Balance chemical
equations.
chemical change: a
process involving one or
more substances changing
into a new substance
chemical reaction
reactant
Chemical reactions are
represented by balanced
chemical equations.
product
chemical equation
coefficient
Chemical Reactions
• The process by which one or more
substances are rearranged to form different
substances is called a chemical reaction.
Chemical Reactions (cont.)
• Evidence of a chemical reaction
– Change in temperature
– Change in color
– Odor, gas, or bubbles may form.
Representing Chemical Reactions
• Chemists use statements called equations
to represent chemical reactions.
• Reactants are the
starting substances.
• Products are the
substances formed in
the reaction.
• This table summarizes
the symbols used in
chemical equations.
Representing Chemical Reactions
(cont.)
• In word equations, aluminum(s) +
bromine(l) → aluminum bromide(s) reads
as “aluminum and bromine react to
produce aluminum bromide”.
• Skeleton equations use symbols and
formulas to represent the reactants and
products.
Al(s) + Br(l) → AlBr3(s)
• Skeleton equations lack information about
how many atoms are involved in the reaction.
Representing Chemical Reactions
(cont.)
• A chemical equation is a statement that
uses chemical formulas to show the
identities and relative amounts of the
substances involved in a chemical reaction.
Balancing Chemical Equations
• This figure shows the balanced equation
for the reaction between aluminum and
bromine.
Balancing Chemical Equations
(cont.)
• A coefficient in a chemical equation is the
number written in front of a reactant or
product, describing the lowest whole-number
ratio of the amounts of all the reactants and
products.
Balancing Chemical Equations
(cont.)
Balancing Chemical Equations
(cont.)
Balancing Chemical Equations
(cont.)
Balancing Chemical Equations
(cont.)
• The most fundamental law in chemistry is
the law of conservation of mass.
• Balanced equations show this law.
Balancing Chemical Equations
(cont.)
Section 9.1 Assessment
Which of the following is NOT a chemical
reaction?
A. a piece of wood burning
B. a car rusting
D
A
0%
C
D. red litmus paper turning blue
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. an ice cube melting into water
Section 9.1 Assessment
What is the coefficient of bromine in the
equation 2Al(s) + 3Br2(l) → 2AlBr3(s)?
A. 1
B. 2
D
A
0%
C
D. 6
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. 3
Section 9.2 Classifying Chemical Reactions
• Classify chemical
reactions.
• Identify the
characteristics of
different classes of
chemical reactions.
metal: an element that is
a solid at room
temperature, a good
conductor of heat and
electricity, and is generally
shiny
Section 9.2 Classifying Chemical Reactions
(cont.)
synthesis reaction
combustion reaction
decomposition reaction
double-replacement
reaction
precipitate
single-replacement
reaction
There are four types of chemical
reactions: synthesis, combustion,
decomposition, and replacement
reactions.
Types of Chemical Reactions
• Chemists classify reactions in order to
organize the many types.
• A synthesis reaction is a reaction in which
two or more substances react to produce a
single product.
Types of Chemical Reactions (cont.)
• When two elements react, the reaction is
always a synthesis reaction.
Types of Chemical Reactions (cont.)
• In a combustion reaction, oxygen
combines with a substance and releases
energy in the form of heat and light.
• Heated hydrogen reacts with oxygen to
produce heat and water in a combustion
reaction. This is also a synthesis reaction.
Decomposition Reactions
• 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.
Replacement Reactions
• 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
Replacement Reactions (cont.)
• 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.
Replacement Reactions (cont.)
• Halogens frequently replace other
halogens in replacement reactions.
• Halogens also have different reactivities and
do not always replace each other.
Replacement Reactions (cont.)
• Double replacement reactions occur
when ions exchange between two
compounds.
• This figure shows a generic double
replacement equation.
Replacement Reactions (cont.)
• The solid product produced during a
chemical reaction in a solution is called a
precipitate.
• All double replacement reactions produce
either water, a precipitate, or a gas.
Replacement Reactions (cont.)
• This table shows the steps to write double
replacement reactions.
Replacement Reactions (cont.)
• This table summarizes different ways to
predict the products of a chemical reaction.
Section 9.2 Assessment
Which of the following is NOT one of the
four types of reactions?
A. deconstructive
B. synthesis
D
A
0%
C
D. double replacement
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. single replacement
Section 9.2 Assessment
The following equation is what type of
reaction?
KCN(aq) + HBr(aq) → KBr(aq) + HCN(g)
A. deconstructive
A
0%
D
D. double replacement
C
C. single replacement
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. synthesis
Section 9.3 Reactions in Aqueous Solutions
• 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.
solution: a uniform mixture that might contain
solids, liquids, or gases
Section 9.3 Reactions in Aqueous Solutions
(cont.)
aqueous solution
complete ionic equation
solute
spectator ion
solvent
net ionic equation
Double-replacement reactions occur
between substances in aqueous
solutions and produce precipitates,
water, or gases.
Aqueous Solutions
• An aqueous solution contains one or
more dissolved substances (called
solutes) in water.
• The solvent is the most plentiful substance in
a solution.
Aqueous Solutions (cont.)
• Water is always the solvent in an aqueous
solution.
• There are many possible solutes—sugar and
alcohol are molecular compounds that exist
as molecules in aqueous solutions.
• Compounds that produce hydrogen ions in
aqueous solutions are acids.
Aqueous Solutions (cont.)
• 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 Reactions in Aqueous Solutions
• When two solutions that contain ions as
solutes are combined, the ions might react.
• If they react, it is always a double
replacement reaction.
• Three products can form: precipitates, water,
or gases.
Types of Reactions in Aqueous Solutions (cont.)
• Aqueous solutions of sodium hydroxide and
copper(II) chloride react to form the
precipitate copper(II) hydroxide.
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)
Types of Reactions in Aqueous Solutions (cont.)
• Ions that do not participate in a reaction are
called spectator ions and are not usually
written in ionic equations.
• 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 Reactions in Aqueous Solutions (cont.)
• Some reactions produce more water
molecules.
• No evidence of a chemical reaction is
observable.
HBr(aq) + NaOH(aq) → H2O(l) + NaBr(aq)
• Without spectator ions
H+(aq) + OH–(aq) → H2O(l).
Types of Reactions in Aqueous Solutions (cont.)
• Gases that are commonly produced are
carbon dioxide, hydrogen cyanide, and
hydrogen sulfide.
2HI(aq) + Li2S(aq) → H2S(g) + 2LiI(aq)
Types of Reactions in Aqueous Solutions (cont.)
• Another example is mixing vinegar and
baking soda, which produces carbon
dioxide gas.
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq)
• H2CO3(aq) decomposes immediately.
H2CO3(aq) → H2O(l) + CO2(g)
Types of Reactions in Aqueous Solutions (cont.)
• Two reactions can be combined and
represented by a single chemical reaction.
Types of Reactions in Aqueous Solutions (cont.)
Reaction 1
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)
Section 9.3 Assessment
What is the solvent in an aqueous
solution?
A. hydrogen
B. sodium ions
D
A
0%
C
D. alcohol
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. water
Section 9.3 Assessment
An equation that includes only the
particles that participate in a reaction is
called:
A. net ionic equation
A
0%
D
D. reduced ionic equation
C
C. complete ionic equation
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. spectator ions
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Section 9.1 Reactions and Equations
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.
• Balancing an equation involves adjusting the
coefficients until the number of atoms of each element
is equal on both sides of the equation.
Section 9.2 Classifying Chemical
Reactions
Key Concepts
• 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.
Section 9.3 Reactions in
Aqueous Solutions
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.
• Reactions that occur in aqueous solutions are doublereplacement reactions.
The law of conservation of mass requires what in
a chemical reaction equation?
A. both sides of the equation to contain
the same substances
A
0%
D
D. the products to have fewer molecules
than the reactants
C
C. both sides to have the same amount
of atoms of each element
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. the reactants to have the same
amount of molecules as the products
A reaction that gives off heat is what type
of reaction?
A. single replacement reaction
B. double replacement reaction
D
A
0%
C
D. combustion reaction
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. synthesis reaction
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
A
0%
D
D. net ions
C
C. products
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. reactants
A double replacement reaction produces
all of the following except ____.
A. gases
B. solids
D
A
0%
C
D. water
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. light
What type of reaction is the following?
2H2O(l) + energy → H2(g) + O2(g)
A. synthesis reaction
B. decomposition reaction
D
A
0%
C
D. replacement reaction
B
C. combustion reaction
A. A
B. B
C. C
0%
0%
0%
D. D
What type of reaction is the following?
2H2(g) + O2(g) → 2H2O(l)
A. replacement reaction
B. synthesis
D
A
0%
C
D. double replacement reaction
B
C. combustion reaction
A. A
B. B
C. C
0%
0%
0%
D. D
A precipitate forms in a double
replacement reaction only if:
A. the reactivities of the compounds
differ
D
D. the new compound is not
soluble in water
A
0%
C
C. the new compound is soluble
in water
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. the new compound is denser
than water
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
A
0%
D
D. balanced equation
C
C. chemical equation
A. A
B. B
C. C
0%
0%
0%
D. D
B
B. skeleton equation
Predict the type of reaction.
LiBr2 (aq) + 2NaOH (aq) → ____
A. synthesis reaction
B. combustion reaction
D
A
0%
C
D. double replacement reaction
B
C. single replacement reaction
A. A
B. B
C. C
0%
0%
0%
D. D
Which reactions are essentially the
opposite of synthesis reactions?
A. single-replacement
B. decomposition
D
A
0%
C
D. double-replacement
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. combustion
Click on an image to enlarge.
Table 9.2
Steps for Balancing Equations
Figure 9.15 The Forming of a Precipitate
Table 9.4
Types of Chemical Reactions
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