Chapter 5
Chemical Reactions and Quantities
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Physical Change
In a physical change,
• the identity and
composition of the
substance do not
change.
• the state can change or
the material can be torn
into smaller pieces.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Chemical Change
In a chemical change,
• reacting substances
form new substances
with different
compositions and
properties.
• a chemical reaction
takes place.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Some Examples of Chemical
and Physical Changes
Learning Check
Classify each of the following as a
1) physical change or 2) chemical change.
2
A. ____Burning
a candle.
1
B. ____Ice
melting on the street.
2
C. ____Toasting
a marshmallow.
1
D. ____Cutting
a pizza.
2
E. ____Polishing
a silver bowl.
Chemical Reaction
In a chemical reaction
• a chemical change
produces one or more
new substances.
• there is a change in the
composition of one or
more substances.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Chemical Reaction
In a chemical reaction,
• old bonds are broken and
new bonds are formed.
• atoms in the reactants are
rearranged to form one or
more different substances.
• Fe and O2 form rust
(Fe2O3).
5.2
Chemical Equations
Chemical Equations
A chemical equation gives the chemical formulas of the
reactants on the left of the arrow and the products on the
right.
Reactants
O2 (g)
C(s)
Product
CO2 (g)
Symbols Used in Equations
Symbols used in
chemical
equations show
• the states of the
reactants.
• the states of the
products.
• the reaction
conditions.
TABLE 5.2
Chemical Equations are
Balanced
In a balanced
chemical reaction
• atoms are not
gained or lost.
• the number of
reactant atoms
is equal to the
number of
product atoms.
A Balanced Chemical Equation
In a balanced chemical equation,
• there must be the same number of each type of atom on
the reactant side and on the product side of a balanced
equation.
• numbers called coefficients are used in front of one or
more formulas.
S
 Al2S3
Not Balanced
+ 3S
 Al2S3
Balanced
Al +
2Al
2 Al
=
2 Al
3S
=
3S
Learning Check
State the number of atoms of each element on the
reactant side and the product side for each of the
following balanced equations.
A.
P4(s) + 6Br2(l)  4 PBr3(g)
4P
4P
12 Br
12 Br
B. 2Al(s) + Fe2O3(s)  2Fe(s) + Al2O3(s)
2 Al
2 Al
2 Fe
2 Fe
3O
3O
Learning Check
Determine if each equation is balanced or not.
A. Na(s) + N2(g)  Na3N(s)
No. 2 N on reactant side, 1 N on product side.
1 Na on reactant side, 3 Na on product side.
B. C2H4(g) + H2O(l)  C2H5OH(l)
Yes. 2 C
= 2C
6H
= 6H
1O
= 1O
Steps in Balancing an Equation
To balance the following equation,
Fe3O4(s) + H2(g)  Fe(s) + H2O(l)
• work on one element at a time.
• use only coefficients in front of formulas.
• do not change any subscripts.
Fe:
Fe3O4(s) + H2(g)  3Fe(s) + H2O(l)
O:
Fe3O4(s) + H2(g)
 3Fe(s)
+ 4H2O(l)
H:
Fe3O4(s) + 4H2(g)  3Fe(s)
+ 4H2O(l)
Balancing Chemical Equations
1. Write the equation with the correct formulas.
NH3(g) + O2(g)
 NO(g) + H2O(g)
2. Determine if the equation is balanced.
No, not all atoms are balanced.
3. Balance with coefficients in front of formulas.
4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)
4. Check that atoms of each element are equal in reactants
and products.
4 N (4 x 1 N)
=
4 N (4 x 1 N)
12 H (4 x 3 H)
=
12 H (6 x 2 H)
10 O (5 x 2 O)
=
10 O (4 O + 6 O)
Balancing with Polyatomic Ions
MgCl2(aq) + Na3PO4(aq)  NaCl(aq) + Mg3(PO4)2(s)
Balance PO43- as a unit
MgCl2(aq) + 2Na3PO4(aq)  NaCl(aq) + Mg3(PO4)2(s)
2 PO43=
2 PO43-
Balance Mg and Cl
3MgCl2(aq) + 2Na3PO4(aq)  6NaCl(aq) + Mg3(PO4)2(s)
3 Mg2+
=
3 Mg2+
6 Na+
=
6 Na+
6 Cl=
6 Cl-
Types of Reactions
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Type of Reactions
Chemical reactions can be classified as
• combination reactions.
• decomposition reactions.
• single replacement reactions.
• double replacement reactions.
Combination
In a combination reaction,
• two or more elements form one product.
• or simple compounds combine to form one product.
A
+
B
A
B
2Mg(s) + O2(g)
2MgO(s)
2Na(s) + Cl2(g)
2NaCl(s)
SO3(g) + H2O(l)
H2SO4(aq)
Decomposition
In a decomposition reaction,
• one substance splits into two or more simpler
substances.
2HgO(s)
2KClO3(s)
2Hg(l) + O2(g)
2KCl(s) + 3 O2(g)
Learning Check
Classify the following reactions as
1) combination or 2) decomposition.
___A. H2(g) + Br2(g)
2HBr(l)
___B. Al2(CO3)3(s)
Al2O3(s) + 3CO2(g)
___C. 4Al(s) + 3C(s)
Al4C3(s)
Single Replacement
In a single replacement reaction,
• one element takes the place of a different element in a
reacting compound.
Zn(s) + 2HCl(aq)
ZnCl2(aq) + H2(g)
Fe(s) + CuSO4(aq)
FeSO4(aq) + Cu(s)
Zn and HCl is a Single
Replacement Reaction
Double Replacement
In a double replacement,
• two elements in the reactants exchange places.
AgNO3(aq) + NaCl(aq)
AgCl(s) + NaNO3(aq)
ZnS(s)
ZnCl2(aq) + H2S(g)
+ 2HCl(aq)
Example of a Double
Replacement
Learning Check
Classify the following reactions as
1) single replacement 2) double replacement.
A. 2Al(s) + 3H2SO4(aq)
1
Al2(SO4)3(s) + 3H2(g)
A. 2Al(s) + 3H2SO4(aq)
Al2(SO4)3(s) + 3H2(g)
B. Na2SO4(aq) + 2AgNO3(aq)
Ag2SO4(s) + 2NaNO3(aq)
2
Ag2SO4(s) + 2NaNO3(aq)
B. Na2SO4(aq) + 2AgNO3(aq)
C. 3C(s) + Fe2O3(s)
2Fe(s) + 3CO(g)
1
2Fe(s) + 3CO(g)
C. 3C(s) + Fe2O3(s)
Learning Check
Identify each reaction as
1) combination
3) single replacement
A. 3Ba(s) + N2(g)
B. 2Ag(s) + H2S(aq)
2) decomposition
4) double replacement
1
3
C. SiO2(s) + 4HF(aq)
D. PbCl2(aq) + K2SO4(aq)
2
E. K2CO3(s)
Ba3N2(s)
Ag2S(s) + H2(g)
4
SiF4(s) + 2H2O(l)
4
2KCl(aq) + PbSO4(s)
K2O(aq) + CO2(g)
Oxidation-Reduction Reactions
Oxidation and Reduction
An oxidation-reduction reaction
• provides us with energy from food.
• provides electrical energy in
batteries.
• occurs when iron rusts.
4Fe(s) + 3O2(g)
2Fe2O3(s)
Electron Loss and Gain
An oxidation-reduction reaction
• transfers electrons from one reactant to another.
• loses electrons in oxidation.
(LEO)
Zn(s)
Zn2+(aq) + 2e- (loss of electrons)
• gains electrons in reduction.
(GER)
Cu2+(aq) + 2eCu(s) (gain of electrons)
Oxidation and Reduction
Zn and Cu2+
Zn(s)
Silvery metal
Cu2+(aq) + 2eBlue
Zn2+(aq) + 2e-
oxidation
Cu(s)
orange
reduction
Electron Transfer from Zn to Cu2+
Oxidation: electron loss
Reduction: electron gain
Learning Check
Identify each of the following as
1) oxidation or 2) reduction.
A.
Sn(s)
Sn4+(aq) + 4e−
Oxidation
B.
Fe3+(aq) + 1e−
Fe2+(aq)
Reduction
C.
Cl2(g) + 2e−
Reduction
2Cl-(aq)
Writing Oxidation and Reduction
Reactions
Write the separate oxidation and reduction reactions
for the following equation.
2Cs(s) + F2(g)
2CsF(s)
A cesium atom loses an electron to form cesium ion.
Cs(s)
Cs+(s) + 1e−
oxidation
Fluorine atoms gain electrons to form fluoride ions.
F2(s) + 2e2F−(s)
reduction
Learning Check
In light-sensitive sunglasses, UV light initiates
an oxidation-reduction reaction.
uv light
Ag+ + Cl−
Ag +
Cl
A.
Which reactant is oxidized?
Cl−
Cl + 1e−
B.
Which reactant is reduced?
Ag+ + 1e−
Ag
Learning Check
Identify the substances that are oxidized and reduced in
each of the following reactions.
A.
Mg(s) + 2H+(aq)
Mg2+(aq) + H2(g)
Mg is oxidized Mg(s)
Mg2+(aq) + 2e−
H+ is reduced 2H+ + 2e−
H2
B.
2Al(s) + 3Br2(g)
2AlBr3(s)
Al is oxidized Al
Al3+ + 3e−
Br is reduced Br + e−
Br −