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Modern Chemistry
Chapter 8
Chemical Equations
and Reactions
1
Section 1
Describing Chemical Reactions
2
Indications of a Chemical Reaction
Proof: one or more substances has changed
identity – chemical change
1. Evolution of energy as heat and/or light
2. Production of a gas
3. Formation of a precipitate
4. Color change
3
4
Indications of a Reaction
Characteristics of a Chemical Reaction
1. The equation must represent known facts.
2. The equation must contain the correct
formulas for the reactants and products.
• Diatomic elements: I2 Br2 Cl2 F2 O2 N2 H2
• Molecular elements: S8 P4 (page 263)
3. The law of conservation of mass must be
satisfied.
• Coefficients – a whole number that
appears in front of a formula in a
chemical equation.
5
Diatomic Elements
p. 263
6
Nitrogen gas molecules
A Collection of Argon Atoms
Word and Formula Equations
• Word equations – reactants & products
expressed in words
methane + oxygen  carbon dioxide + water
PRODUCTS
REACTANTS
YIELDS
9
Word and Formula Equations
• Formula equations – reactants & products
expressed with formulas – not balanced
methane
+ Hwater
CH4 (g) + oxygen
O2 (g)  carbon
COdioxide
+
2 (g)
2O(g)
STATE OF MATTER SYMBOLS
Solid
Liquid
Gas
Aqueous
(s)
(l)
(g)
(aq)
 Dissolved in water
10
Methane Combustion
11
Additional Symbols
p. 226
12
Additional Symbols
13
p. 226
Balancing Chemical Equations
1. If the reaction is described by a
paragraph, write the word equation.
2. Write the formula for each reactant
and product to get the formula
equation.
3. Balance the equation.
4. Insert state of matter symbols and
other additional symbols.
14
Balancing Chemical Equations
GOAL OF THE GAME:
To get the same number of atom of each
element in the reactant and the
product.
To obey the law of conservation of mass.
15
Balancing Chemical Equations
RULES OF THE GAME:
Only coefficients can be added or
changed.
Once formulas are written subscripts can
not be changed.
16
Balancing Chemical Equations
TIPS FOR PLAY:
Balance the different types of atoms one
at a time.
First balance elements that appear only
once on each side.
Balance polyatomic ions that appear on
both sides as a single unit.
Balance H and O last.
page 271
17
Balancing Chemical Equations
TIPS FOR PLAY:
Try keeping a tally for each element on
each side below the equation.
If it could be balanced by a coefficient of
1½ - use it- multiply all coefficients in
the equation by 2.
18
Sample Problem
The reaction of zinc with aqueous
hydrochloric acid produces a solution of
zinc chloride and hydrogen gas. Write a
balanced equation for the reaction
19
Sample Problem p.272
zinc + hydrochloric acid  zinc chloride + hydrogen
Zn
+
HH Cl
Cl
Zn
Zn
2 HCl  ZnCl2
(s)
+ 2HCl
(aq)
Cl Zn Cl
+
H2
H H
 ZnCl2 (aq) + H2 (g)
20
Writing Equations
Two atoms of aluminum react with
three units of aqueous copper(II)
chloride to produce three atoms of
copper and two units of aqueous
aluminum chloride.
• How many?
• Of what?
• In what state?
Writing Equations
2 Al (s) + 3 CuCl2 (aq) 
3Cu (s) + 2 AlCl3 (aq)
Sample Problem
Solid aluminum carbide, reacts with
water to produce methane gas, CH4,
and solid aluminum hydroxide. Write a
balanced chemical equation for this
reaction.
23
Sample Problem p.273
aluminum carbide + water  methane + aluminum hydroxide
Al4C3 + 12 H2O  3 CH4 +
4 Al(OH)3
Al = 4
Al = 4
1
C= 3
C= 3
1
H = 24
2
H = 24
7
16
O = 12
1
O = 12
3
Al4C3
(s)
+ 12H2O
(l)
 3CH4 (g) + 4Al(OH)3 (s)
24
Sample Problem
Aluminum sulfate and calcium hydroxide are
used in water-purification process. When
added to water, they dissolve and react to
produce two insoluble products, aluminum
hydroxide and calcium sulfate. These
products settle out, taking suspended solid
impurities with them. Write a balanced
chemical equation for the reaction.
25
Sample Problem p.273
aluminum sulfate + calcium hydroxide  aluminum hydroxide + calcium sulfate
Al2(SO4 )3 + 3 Ca(OH)2  2 Al(OH)3 + 3 CaSO4
Al = 2
Al = 22
1
SO4 =3
SO4 =13
1
Ca = 3
1
Ca = 13
1
OH = 6
2
OH = 66
3
Al2(SO4)3(aq) + 3Ca(OH)2(aq)  2Al(OH)3(s) + 3CaSO4(s)
26
Practice Problems
1. Write word, formula, and balanced
chemical equations for each of the following
reactions:
a. Magnesium and hydrochloric acid react to
produc magnesium chloride and
hydrogen.
b. Aqueous nitric acid reacts with solid
magnesium hydroxide to produce aqueous
magnesium nitrate and water
27
Practice Problems
2. Solid calcium metal reacts with water to form
aqueous calcium hydroxide and hydrogen
gas. Write a balanced chemical equation for
this reaction.
28
Practice Problems
1. Write balanced chemical equations
for each of the following reactions:
a. Solid sodium combines with chlorine gas
to produce solid sodium chloride.
b. When solid copper reacts with aqueous
silver nitrate, the products are aqueous
copper (II) nitrate and solid silver
c. In a blast furnace, the reaction between
solid iron (III) oxide and carbon monoxide
gas produces solid iron and carbon
dioxide gas.
29
Describing Equations
• Describing Coefficients:
– individual atom = “atom”
– covalent substance = “molecule”
– ionic substance = “ formula unit”
3CO2  3 molecules of carbon dioxide
2Mg  2 atoms of magnesium
4MgO  4 formula units of magnesium
oxide
Describing Equations
• Describing Coefficients:
– Also moles for EVERY type
3CO2  3 moles of carbon dioxide
2Mg  2 moles of magnesium
4MgO  4 moles of magnesium oxide
Significance of a Equation
4 Fe
(s)
+ 3O2
(g)
 2Fe2O3
(s)
4 ATOMS 3 MOLECULES 2 FORMULA UNITS
4 MOLES 3 MOLES
2 MOLES
223.4g + 96.00 g
=
319.40g
Convert
moles=to(or
grams
Coefficients
Coefficients
= molecules
moles
formula units
4mol
x 55.85g/1mole
= 223.4g
for Fe
ionic
or atoms for elements)
32
Types of Chemical RxN’s
Section 2
Types of RxN’s
•
•
•
•
•
•
There are millions of reactions.
Can’t remember them all
Fall into several categories.
We will learn 5 types.
Will be able to predict the products.
For some we will be able to predict
whether they will happen at all.
• Will recognize them by the reactants
Types of RxN’s
• The classification scheme described in
this section provides an introduction to
five basic types of reactions:
• Synthesis (Combination)
• Decomposition
• Single-displacement (replacement)
• Double-displacement (replacement)
• Combustion reactions
Synthesis Reactions
• In a synthesis reaction, also known as a
composition reaction or combination reaction,
two or more substances combine to form a
new compound.
• This type of reaction is represented by the
following general equation.
A+X
AX
• A and X can be elements or compounds.
• AX is a compound
Synthesis Reactions
• 2 elements, or compounds combine to
make one compound.
• Ca +O2 CaO
• SO3 + H2O  H2SO4
• We can predict the products if they are
two elements.
• Mg + N2 
Reactions of Elements with Oxygen
• One simple type of synthesis reaction is the
combination of an element with oxygen to produce an
oxide of the element.
• Almost all metals react with oxygen to form oxides.
• example: 2Mg(s) + O2(g)
2MgO(s)
• Group 2 elements react in a similar manner, forming
oxides with the formula MO, where M represents
the metal.
Synthesis
H2(g) + Cl2(g)  2 HCl(g)
Your Turn III
Ca + Cl2 
Fe + O2  iron (II) oxide
Al + O2 
Remember that the first step is to write
the formula
• Then balance
•
•
•
•
Decomposition Reactions
• In a decomposition reaction, a single compound
undergoes a reaction that produces two or more
simpler substances. s or compounds.
• Decomposition reactions are the opposite of
synthesis reactions.
• They are represented by the following general
equation.
AX
A+X
• AX is a compound.
• A and X can be elements or compounds.
Decomposition of Binary
Compounds
• The decomposition of a substance by an electric
current is called electrolysis.
• example: 2H O(l) electricity

 2H2 (g) + O2 (g)
2
• Oxides of the less-active metals, which are located in
the lower center of the periodic table, decompose into
their elements when heated.
• example: 2HgO(s) 
 2Hg(l) + O2 (g)
Decomposition Reactions
Decomposition of Metal Carbonates

CaCO3 (s) 
 CaO(s) + CO2 (g)
Decomposition of Metal Hydroxides

Ca(OH)2 (s) 
 CaO(s) + H2O(g)
Decomposition of Metal Chlorates

2KClO3 (s) MnO

 2KCl(s) + 3O2 (g)
(s)
2
Decomposition
2 H2O(l)  2 H2(g) + O2(g)
Your Turn IV

• NiCO3  
• H2CO3(aq)
Single-Replacement Reactions
• In a single-replacement reaction, also known as a
displacement reaction, one element replaces a similar
element in a compound.
• Many single-replacement reactions take place in
aqueous solution.
• Single-replacement reactions can be represented by
the following general equations.
A + BX
AX + B or Y + BX
BY + X
• A, B, X, and Y are elements. AX, BX, and BY are compounds.
Single-Replacement Reactions
• One element replaces another
• Reactants must be an element and a
compound.
• Products will be a different element and a
different compound.
• Na + KCl  K + NaCl
• F2 + LiCl  LiF + Cl2
Single Replacement
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Single-Replacement Reactions
•
•
•
•
•
Metals replace metals (and hydrogen)
K + AlN 
Zn + HCl 
Think of water as HOH
Metals replace one of the H, combine
with hydroxide.
• Na + HOH 
Activity Series
•
•
•
•
•
•
We can tell whether a reaction will happen
Some are more active than other
More active replaces less active
There is a list on page 286
Higher on the list replaces lower.
If the element by itself is higher, it
happens, if lower it doesn’t
Activity Series
• H can be replaced in acids by everything
higher
• Only the first 6 (Li - Na) react with water.
• Fe + CuSO4 
• Pb + KCl 
• Al + HCl 
Activity Series
• What does it mean that Au And Ag are on the
bottom of the list?
• Nonmetals can replace other nonmetals
• Limited to F2 , Cl2 , Br2 , I2
• The order of activity is that on the table.
• Higher replaces lower.
• F2 + HCl 
• Br2 + KCl 
Double-Replacement Reactions
• In double-replacement reactions, the ions of two
compounds exchange places in an aqueous solution to
form two new compounds.
• One of the compounds formed is usually a precipitate,
an insoluble gas that bubbles out of the solution, or a
molecular compound, usually water.
• The other compound is often soluble and remains
dissolved in solution.
Double-Replacement Reactions
• A double-displacement reaction is represented by the
following general equation.
AX + BY
AY + BX
• A, X, B, and Y in the reactants represent ions.
• AY and BX represent ionic or molecular
compounds.
Double Replacement
Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq)
Double-Replacement Reactions
• The formation of a precipitate occurs when the
cations of one reactant combine with the anions of
another reactant to form an insoluble or slightly
soluble compound.
• example:
2KI(aq) + Pb(NO3)2(aq)
PbI2(s) + 2KNO3(aq)
• The precipitate forms as a result of the very strong attractive
forces between the Pb2+ cations and the I anions.
Double-Replacement Reactions
• Two things replace each other.
• Reactants must be two ionic compounds or
acids.
• Usually in aqueous solution
• NaOH + FeCl3 
• The positive ions change place.
• NaOH + FeCl3 Fe+3 OH- + Na+1Cl-1
• NaOH + FeCl3 Fe(OH)3 + NaCl
Double-Replacement Reactions
• Will only happen if one of the products
– doesn’t dissolve in water and forms a solid
– or is a gas that bubbles out.
– or is a covalent compound usually
water.
Your Turn V
•
•
•
•
•
assume all of the reactions take place.
CaCl2 + NaOH 
CuCl2 + K2S 
KOH + Fe(NO3)3 
(NH4)2SO4 + BaF2 
Combustion Reactions
• In a combustion reaction, a substance combines with
oxygen, releasing a large amount of energy in the form
of light and heat.
• example: combustion of hydrogen
2H2(g) + O2(g)
2H2O(g)
• example: combustion of propane
C3H8(g) + 5O2(g)
3CO2(g) + 4H2O(g)
Combustion Reactions
• Combustion
• A compound composed of only C H and
maybe O is reacted with oxygen
• If the combustion is complete, the
products will be CO2 and H2O.
• If the combustion is incomplete, the
products will be CO and H2O.
Your Turn VI
• C4H10 + O2  (complete)
• C4H10 + O2  (incomplete)
• C6H12O6 + O2  (complete)
• C8H8 +O2  (incomplete)
Your Turn VI Part II
Distinguish between complete an incomplete combustion.
Write a balanced equation for the complete combustion of
each of these compounds.
a) acetic acid, HC2H3O2 c) glycerol, C3H8O3
b) decane, C10H22
d) sucrose, C12H22O11
Write a balanced equation for the incomplete combustion
of each of these compounds.
a) glycerol, C3H8O3
c) acetic acid, HC2H3O2
b) glucose, C6H12O6
d) acetylene, C2H2
Your Turn VII
• Determine the type of reaction and the products
and balance.
• H2 + O2 
• H2O 
• Zn + H2SO4 
• HgO 
• KBr +Cl2 
• AgNO3 + NaCl 
• Mg(OH)2 + H2SO3 
Let’s Review
• Write out the balanced equation for Copper
reacts with chlorine to form copper (II) chloride.
• Write out the balanced equation for Zinc reacts
with Hydrochloric acid to form Zinc chloride and
hydrogen.
• Write out the balanced equation for Calcium
carbonate reacts with Zinc nitride to form
Calcium nitride and Zinc carbonate.
Let’s Review
• Balance the following equations
– C7H16 + O2 ---> CO2 + H2O
– Na2O2 + H2O ---> NaOH + O2
– Al2(SO4)3+Ca(OH)2----> Al(OH)3+CaSO4
– NH3 +HCl ----> NH4Cl
– Ca3(PO4)2 +SiO2 + C ----> CaSiO3 + CO + P
Let’s Review
• What type of reaction are the following
–
–
–
–
–
–
Rb + S8 ---> Rb2S
NH3 + O2 ---> N2 + H2O
C + SO2 ---> CS2 + CO
C3H11 + O2 ---> CO2 + H2O
C5H12 +O2 ---> CO + H2O
FeCl3 + NH4OH --->Fe(OH)3 + NH4Cl
Let’s Review
• Balance them
–
–
–
–
–
–
Rb + S8 ---> Rb2S
NH3 + O2 ---> N2 + H2O
C + SO2 ---> CS2 + CO
C3H11 + O2 ---> CO2 + H2O
C5H12 +O2 ---> CO + H2O
FeCl3 + NH4OH --->Fe(OH)3 + NH4Cl
Let’s Review
• Predict the products and balance
– Sodium phosphate reacts with Lithium
metal.
– Butane (C4H10) is completely combusted.
– Sodium Chloride reacts with Silver
Hydroxide
– Aluminum Bromate reacts with Copper
– Potassium reacts with Sulfur
– Octane (C8H18) is incompletely combusted
Let’s Review
• Predict the products and balance
–
–
–
–
–
–
–
H2 + O2 
Al2Te3 
Zn + H2SO4 
HgO 
KBr +Cl2 
AgNO3 + NaCl 
Mg(OH)2 + H2SO3 
Let’s Review
• Using the table on Pg 286. Predict if these
reactions will occur
–
–
–
–
–
–
–
–
Li + BaS 
Fe + KCl 
Au + HgS 
Mn + CuS 
Ba + LiS 
Na + KCl 
HCl + Pb 
PbCl2 + H2 