Balancing Chemical Equations

1
Balancing Chemical Equations
2
Balancing by Inspection

• Most chemical equations we will encounter can be
  balanced by inspection.
• This involves a step-by-step approach.
• We will use the decomposition of water as our
  example.

3
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
In our example reaction, water is being
decomposed into hydrogen and oxygen.
water
hydrogen

4
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
Write a skeleton equation by substituting correct
formulas for the names of the reactants and the
products.
2.
5
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
H2O(l)
H2(g)

O2(g)
2.
Write a skeleton equation by substituting correct
formulas for the names of the reactants and the
products.
3.
Balance the skeleton equation according to the
law of conservation of mass.
a.
Balance the different types of atoms one at a
time.
b.
First balance the atoms of elements that are
combined and that appear only once on each side
of the equation.
c.
Balance polyatomic ions that appear on both sides
of the equation as single units.
d.
Balance H atoms and O atoms after atoms of all
other elements have been balanced.
Lets look at the Hs first.
6
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
H2O(l)
H2(g)

O2(g)
2.
Write a skeleton equation by substituting correct
formulas for the names of the reactants and the
products.
3.
Balance the skeleton equation according to the
law of conservation of mass.
a.
Balance the different types of atoms one at a
time.
b.
First balance the atoms of elements that are
combined and that appear only once on each side
of the equation.
c.
Balance polyatomic ions that appear on both sides
of the equation as single units.
d.
Balance H atoms and O atoms after atoms of all
other elements have been balanced.
There are two Hs on the left and two Hs on the
right - balanced.
7
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
H2O(l)
H2(g)

O2(g)
2.
Write a skeleton equation by substituting correct
formulas for the names of the reactants and the
products.
3.
Balance the skeleton equation according to the
law of conservation of mass.
a.
Balance the different types of atoms one at a
time.
b.
First balance the atoms of elements that are
combined and that appear only once on each side
of the equation.
c.
Balance polyatomic ions that appear on both sides
of the equation as single units.
d.
Balance H atoms and O atoms after atoms of all
other elements have been balanced.
There is 1 O on the left and 2 Os on the right -
unbalanced
8
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
H2O(l)
H2(g)

O2(g)
2
2.
Write a skeleton equation by substituting correct
formulas for the names of the reactants and the
products.
3.
Balance the skeleton equation according to the
law of conservation of mass.
a.
Balance the different types of atoms one at a
time.
b.
First balance the atoms of elements that are
combined and that appear only once on each side
of the equation.
c.
Balance polyatomic ions that appear on both sides
of the equation as single units.
d.
Balance H atoms and O atoms after atoms of all
other elements have been balanced.
We put a 2 in front of the H2O to balance the
Os.
9
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
H2O(l)
H2(g)

O2(g)
2
2.
Write a skeleton equation by substituting correct
formulas for the names of the reactants and the
products.
3.
Balance the skeleton equation according to the
law of conservation of mass.
a.
Balance the different types of atoms one at a
time.
b.
First balance the atoms of elements that are
combined and that appear only once on each side
of the equation.
c.
Balance polyatomic ions that appear on both sides
of the equation as single units.
d.
Balance H atoms and O atoms after atoms of all
other elements have been balanced.
We now need to rebalance the Hs.
10
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
H2O(l)
H2(g)

O2(g)
2
2
2.
Write a skeleton equation by substituting correct
formulas for the names of the reactants and the
products.
3.
Balance the skeleton equation according to the
law of conservation of mass.
a.
Balance the different types of atoms one at a
time.
b.
First balance the atoms of elements that are
combined and that appear only once on each side
of the equation.
c.
Balance polyatomic ions that appear on both sides
of the equation as single units.
d.
Balance H atoms and O atoms after atoms of all
other elements have been balanced.
We place a 2 in front of the H2 to balance the
Hs.
11
Balancing by Inspection
1.
Identify the names of the reactants and the
products, and write a word equation.
H2O(l)
H2(g)

O2(g)
2
2
2.
Write a skeleton equation by substituting correct
formulas for the names of the reactants and the
products.
3.
Balance the skeleton equation according to the
law of conservation of mass.
4.
Count atoms to be sure that the equation is
balanced.
On the right -
On the left -
Its balanced!
4 Hs
4 Hs
2 Os
2 Os
12
Sample Problem
The reaction of zinc with aqueous hydrochloric
acid produces a solution of zinc chloride and
hydrogen gas. Write a balanced chemical equation
for the reaction.
First, we write the word equation
zinc

hydrochloric acid
?
zinc chloride

hydrogen
Next, we write the skeleton equation
Zn(s)

HCl(aq)
?
ZnCl2(aq)

H2(g)
Next, we balance the skeleton equation.
Zinc is balanced.
13
Sample Problem
The reaction of zinc with aqueous hydrochloric
acid produces a solution of zinc chloride and
hydrogen gas. Write a balanced chemical equation
for the reaction.
First, we write the word equation
zinc

hydrochloric acid
?
zinc chloride

hydrogen
Next, we write the skeleton equation
Zn(s)

HCl(aq)
?
ZnCl2(aq)

H2(g)
Next, we balance the skeleton equation.
We have 2 Cls on the right and only 1 Cl on the
left.
14
Sample Problem
The reaction of zinc with aqueous hydrochloric
acid produces a solution of zinc chloride and
hydrogen gas. Write a balanced chemical equation
for the reaction.
First, we write the word equation
zinc

hydrochloric acid
?
zinc chloride

hydrogen
Next, we write the skeleton equation
Zn(s)

HCl(aq)
?
ZnCl2(aq)

H2(g)
2
Next, we balance the skeleton equation.
We balance the Cls by putting a 2 in front of
the HCl.
15
Sample Problem
The reaction of zinc with aqueous hydrochloric
acid produces a solution of zinc chloride and
hydrogen gas. Write a balanced chemical equation
for the reaction.
First, we write the word equation
zinc

hydrochloric acid
?
zinc chloride

hydrogen
Next, we write the skeleton equation
Zn(s)

HCl(aq)
?
ZnCl2(aq)

H2(g)
2
Next, we balance the skeleton equation.
This also balances the Hs.
16
Sample Problem
The reaction of zinc with aqueous hydrochloric
acid produces a solution of zinc chloride and
hydrogen gas. Write a balanced chemical equation
for the reaction.
First, we write the word equation
zinc

hydrochloric acid
?
zinc chloride

hydrogen
Next, we write the skeleton equation
Zn(s)

HCl(aq)
?
ZnCl2(aq)

H2(g)
2
Next, we balance the skeleton equation.
Finally, we count atoms.
on the left -
on the right -
1 Zn
1 Zn
Its balanced
2 Hs
2 Hs
2 Cls
2 Cls
17
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
Next, we balance the skeleton equation.
We have 4 Als on the left and 1 Al on the right.
18
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
4
Next, we balance the skeleton equation.
We put a 4 in front of the Al(OH)3 to balance
the Als.
19
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
4
Next, we balance the skeleton equation.
We have 3 Cs on the left and only 1 C on the
right.
20
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
4
3
Next, we balance the skeleton equation.
We put a 3 in front of the CH4 to balance the
Cs.
21
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
4
3
Next, we balance the skeleton equation.
We have 2 Hs on the left and (12 12 24) Hs
on the right.
22
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
4
3
12
Next, we balance the skeleton equation.
We put a 12 in front of the H2O to balance the
Hs.
23
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
4
3
12
Next, we balance the skeleton equation.
We have 12 Os on the left and (4x3 12) Os on
the right.
24
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
4
3
12
Next, we balance the skeleton equation.
The Os are balanced.
25
Sample Problem
Solid aluminum carbide, Al4C3, reacts with water
to produce methane gas and solid aluminum
hydroxide. Write a balanced chemical equation for
this reaction.
First, we write the word equation
aluminum carbide

water
?
methane

aluminum hydroxide
Next, we write the skeleton equation
Al4C3(s)

H2O(l)
?
CH4(g)

Al(OH)3(s)
4
3
12
Next, we balance the skeleton equation.
Finally, we count atoms to check the balance.
on the left -
on the right -
4 Als
24 Hs
4 Als
24 Hs
Its balanced
3 Cs
3 Cs
12 Os
12 Os
26
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
Next, we balance the skeleton equation.
We have 2 Als on the left and 1 Al on the right.
27
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
2
Next, we balance the skeleton equation.
We put a 2 in front of the Al(OH)3 to balance
the Als.
28
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
2
Next, we balance the skeleton equation.
We have 3 SO4s on the left and only 1 SO4 on the
right.
29
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
2
3
Next, we balance the skeleton equation.
We put a 3 in front of the CaSO4 to balance the
SO4s.
30
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
2
3
Next, we balance the skeleton equation.
We have 3 Cas on the right and 1 Ca on the left.
31
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
2
3
3
Next, we balance the skeleton equation.
We put a 3 in front of the Ca(OH)2 to balance
the Cas.
32
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
2
3
3
Next, we balance the skeleton equation.
We have 6 OHs on the left and 6 OHs on the
right.
33
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
2
3
3
Next, we balance the skeleton equation.
The OHs are balanced.
34
Sample Problem
Aluminum sulfate and calcium hydroxide are used
in a 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.
First, we write the word equation
aluminum sulfate

calcium hydroxide
?
calcium sulfate

aluminum hydroxide
Next, we write the skeleton equation
Al2(SO4)3(aq)

Ca(OH)2(aq)
?
CaSO4(s)

Al(OH)3(s)
2
3
3
Next, we balance the skeleton equation.
Finally, we count atoms to check the balance.
on the left -
on the right -
2 Als
3 SO4s
2 Als
3 SO4s
Its balanced
3 Cas
3 Cas
6 OHs
6 OHs
35
Practice Problems

• Balance each of the following chemical equations.
• Pb(s) AgNO3(aq) ? Ag(s) Pb(NO3)2(aq)
• Pb(NO3)2(aq) KI(aq) ? PbI2(s) KNO3(aq)
• C3H8(g) O2(g) ? CO2(g) H2O(l)
• H2SO4(aq) NaOH(aq) ? Na2SO4(aq) H2O(l)
• HgO(s) ? Hg(l) O2(g)

Pb(s) 2 AgNO3(aq) ? 2 Ag(s)
Pb(NO3)2(aq) Pb(NO3)2(aq) 2 KI(aq) ? PbI2(s)
2 KNO3(aq) C3H8(g) 5 O2(g) ? 3 CO2(g) 4
H2O(l) H2SO4(aq) 2 NaOH(aq) ? Na2SO4(aq) 2
H2O(aq) 2 HgO(s) ? 2 Hg(s) O2(g)