Ch 8.1 Balancing Equations
Indications of a Chemical Reaction
• Evolution of energy as
heat and light.
• Production of a gas
• Formation of a
precipitate
• Color Change
Characteristics of Chemical
Equations
• The equation must represent known facts.
• The equation must contain the correct
formulas for the reactants and products.
• The law of conservation of mass must be
satisfied.
Chemical Reactions
• Chemical equations give information in two major
areas:
1. Reactants and products of the reaction.
2. Coefficients of a balanced chemical equation
tell us the amount of the substances involved.
Example:
2H2 (g) + O2 (g)  2H2O (g)
Review: Reactants are on the left side of the arrow,
and the products are on the right side. The arrow
means “yields”, or “to produce”.
Balancing Chemical Equations
Why do you have to balance a chemical equation?
• Law of Conservation of Matter (or Mass)
• During a chemical reaction, atoms are either joined,
separated, or rearranged. The number and type of each
atom stays the same.
How do you balance a chemical equation?
• Coefficients are placed in front of the substances
involved in the chemical reaction to get the same number
of atoms of each element on both sides of the equation.
Rules for Balancing Chemical Equations
(1) Coefficients can only be placed ___
in _________
front
of a chemical
formula.
Practice Problems: How many atoms of each type are indicated in
the following compounds?
(a)
2 (NH4)3PO4
6
N= ___
24 P= ___
2 O= ___
8
H= ___
(b)
4 KC2H3O2
4 C= ___
8 H= ___
12 O= ___
8
K= ___
(c)
3 Ca(NO3)2
Ca= ___
3 N= ___
6 O= ___
18
Rules for Balancing Chemical Equations
(2) You cannot change a ________________!!
subscript
Example :
2 H2 +
O2  2 H2O
H2 O2
To balance oxygen, you cannot change water’s formula to_________!
(3) You cannot place the coefficient in the ______________
of a
middle
formula!!
Example :
2Al +
N2  2 AlN
To balance nitrogen, you cannot put a 2 in the middle to make _______.
Al2N
whole
# ratio.
(4) Reduce the coefficients to the simplest ____________
___
Example:
4H2 + 2O2  4H2O can be reduced to…
__H
2 2 + __O
1 2  __H
2 2O
Rules for Balancing Chemical Equations
(5) Get rid of any ____________!
Coefficients must be _________
fractions
whole #’s
• You can’t have a _______________
of a molecule or atom!
fraction
Example: 2 x ( 1H2 + ½O2  1H2O )
__
2 H2 + __O
1 2  __H
2 2O
changes to…
Balancing Equations: “Helpful Hints”
a) Balance elements that appear in more than one compound ________.
last
1
___(NH
4)2CO3 
2
___NH
3
+
1
___CO
2
+
1 2O
___H
Polyatomic ions as though it were one item as long as the
b) Balance ________________
ion stays together as a group on each side of the yields arrow.
2
3
___Al
+ ___CuSO
4 
1 2(SO4)3 +
___Al
3
___Cu
start _________
over and begin
c) If you can’t seem to get it balanced, _________
with a different element the next time, or put a “2” somewhere and
then try again.
2
2 2O
___Li
+ ___H

2
1 2
___LiOH
+ ___H
d) This is what I’ll constantly be telling you to do if you are stuck and
you need my help... “Pick an element to balance. How many are on
Fix
it
the left side? How many are on the right side? ________
____!”
2
___Fe(OH)
3 
1 2O3 + ___H
3 2O
___Fe
Example
• Aluminum is a good choice for outdoor
furniture because it reacts with oxygen in
the air to form a thin protective coat of
aluminum oxide. Write word, formula, and
chemical equations for this reaction.
Aluminum + Oxygen  Aluminum Oxide
Al(s) + O2(g)  Al2O3(s)
4Al(s) + 3O2(g)  2Al2O3(s)
Homework
• 8.1 page 274 #1-3 and page 290 #1-2, 13
Ch 8.2: Five Types of Reactions
1) ________________________:
Decomposition
•
one ______________
compound
A reaction that breaks apart ______
into
simpler substances, (usually two elements or an element and a
smaller compound.)
+
General Form:
Examples:
_____
AX
 ___
A + ___
X
H2
O2
H2O  _____
+ _____
O2
KCl + _____
KClO  _____
3
Remember that “H,N,O,F,Cl,Br,I” elements are diatomic when alone!!
•
Remember to balance the equation after you write the products.
General Types of Reactions
2) __________:
(sometimes called Combination)
Synthesis
•
•
two __________________,
substances
A reaction of _____
typically a
one ______________.
compound
metal and a nonmetal to form ______
It is the opposite of decomposition. (The same categories of
reactions from above apply, just in reverse.)
General Form:
Examples:
+
___
A + ___
X  _____
AX
Al
+
AlCl3
Cl2  _______
Pb(OH)2
PbO + H2O  ______
General Types of Reactions (Continued)
3) _________________:
Combustion
Reacts with oxygen gas!!!
O2 always
• A reaction between a Carbon/Hydrogen with _____
CO2
H2O
produces the same… ________
+ ________
•
This reaction is too easy!! Don’t miss it!
General Form:
Examples:
2C2H2
CO2 + ____
H2O
CxHy + O2  ____
4CO2 + _______
2H2O
+ 5O2  _______
C7H6O +
7CO2 + _______
3H2O
8O2  _______
General Types of Reactions (Continued)
4) _____________
Displacement:
Single
•
one ______________
compound
one ____________
element
A reaction between ____
and ___
that produces a different _____________
and ______________.
compound
element
General Forms: ____
AX + __
Y  ____
AY
+
+ __
X
____
AX + __
B  ____
BX + __
A
+
General Types of Reactions (Continued)
5) _______________
Double
Displacement:
two ________________
compounds
• A reaction between _____
that are dissolved in
two ________________
compounds
water that produces _____
, one of which is
insoluble
________________.
•
Water or a gas may be one of the two compounds being produced.
BX(s)
(aq) + BY
(aq)  AY
(aq) + ____
General Form: AX
____
____
____
+
+
A solid produced during a chemical reaction is called a precipitate.
Examples: CaCl2 (aq) +
Ca(NO3)2 (aq)+ ________
AgCl (s)
AgNO3 (aq)  _________
NaCl (aq) + ________
H2O (l)
NaOH (aq) + HCl (aq)  ________
Double Displacement Reaction
Homework
8.2 pg 284 #1-2
pg 291 #27 (write formulas, balance, and
then classify!)
Ch 8.2 and 8.3 Notes
Types of Reactions and Activity
Series
• Standard 3.a.: Students know how to
describe chemical equations by writing
balanced equations.
• Content Objective: We will know how to
determine products for the five types of
reactions.
• Language Objective: I will write the
products of the chemical reaction and then
balance the chemical reaction.
Making Synthesis Reactions
• Metals with nonmetals  ionic compounds
Mg + O2  MgO
• Nonmetals w/ nonmetals  covalent compounds
C + O2  CO2
• Metal Oxides react with water to form Hydroxides
CaO + H2O  Ca(OH)2
• Nonmetal Oxides react with water to form acids
SO2 + H2O  H2SO3
Decomposition
• Decomposition of Binary Compounds
HgO  Hg + O2
• Decomposition of Metal Carbonates
CaCO3  CaO + CO2
• Decomposition of Metal Hydroxides
Ca(OH)2  CaO + H2O
• Decomposition of Metal Chlorates
KClO3  KCl + O2
• Decomposition of Acids
H2CO3  CO2 + H2O
Combustion
Remember, these
ALWAYS produce
CO2 and H2O!
Single Displacement
• The element that is trying to replace the other
must be more reactive than the one it is
replacing.
• You must use the Activity Series to see if the
reaction will happen. Page 286 Table 3
• The Higher up it is = the more reactive it is.
• Elements from Li to Na can displace hydrogen
in water to form a metallic hydroxide and H2
gas.
Single Replacement Reactions
Examples:
NaCl
NaF + _____
Cl2
+ F2  _____
FeCl2 +
KCl + _____
Fe
K  _____
HCl
+
H2
Zn  ZnCl
_____
2 + _____
HCl
+
no reaction
Au  _____
+ _____
Na 
H2
_____
NaOH + _____
Fe 
no reaction
_____
+ _____
AgNO3 + Cu 
CuNO
Ag
_____3 + _____
H2O +
H(OH)
H2O +
Single Displacement
•
•
•
•
Remember the Activity Series!
Metals Replace Metals
Al + Pb(NO3)2  Pb + Al(NO3)3
Metals Replace Hydrogen in Water
Na + H2O  NaOH + H2
Metals Replace Hydrogen in Acid
Mg + HCl  H2 + MgCl2
Halogens replace halogens below them
Double Displacement
• Switch the compounds and make sure the
new compounds are balanced.
KI + Pb(NO3)2  PbI2 + KNO3
• They will always react and switch, no
activity series needed!
Once you have solved
for the products of the
reaction, you must
then BALANCE the
reaction!
• Standard 3.a.: Students know how to
describe chemical equations by writing
balanced equations.
• Content Objective: We will know how to
determine products for the five types of
reactions.
• Language Objective: I will write the
products of the chemical reaction and then
balance the chemical reaction.
Homework
8.2 page 284 #4
(solve for products, balance, and classify the
reactions.)
8.3 page 287 #2-3
(In #2, predict which reactions will occur
using activity series, then in #3, solve for
the products for the ones that will occur
and then balance.)