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Chapter 8
Balancing Chemical Equations
8.1 Chemical Equations


Chemical equations represent, with symbols and
formulas, the reactants and products in a chemical
reaction.
 reactants

products
Requirements for all chemical equations:



must show all reactants and products
formulas must be correct
Law of Conservation of Mass must be satisfied (equation
must be balanced)
Chemical Equations

Balanced Equations – use coefficients
 H2 + Cl2
 2 HCl


Hydrogen reacts with chlorine to yield hydrochloric acid
Symbols used in equations
 2Na(s) + Cl2(g)

2NaCl(s)

NaCl(aq) - solution of sodium chloride in water
H2O(l) – liquid water


g or ↑ = gas
Chemical Equations

More symbols





 = one way reactions
↔ = reversible reaction
ΔH = heat
cat = catalyst (a substance that speeds up a
reaction without being used up in the reaction)
S or ppt or ↓ = precipitate (solid - only found on
products side)
Chemical Equations

Significance of chemical equations

H2(g) + Cl2(g)  2 HCl (g) means…



Atoms: 2 atoms of hydrogen gas react with 2
atoms of chlorine gas and yields 2 atoms of
hydrogen, 2 atoms of chlorine
Molecules: 1 molecule of hydrogen gas reacts
with 1 molecule of chlorine gas and yields 2
molecules of hydrochloric acid
Molar mass: H = 2.0; Cl = 71.0; HCl = 73.0
Balancing Chemical Equations

Helpful hints to balancing….

1 atom at a time
Balance atoms that appear only 1X per side first
Balance polyatomic ions as whole units
Balance diatomic elements last

Save H + O for last



Balancing Chemical Equations

Examples
 __ H2(g)

__H2O(l)
+__O2(g)

__(NH4)2CO3 (aq)+__CaCl2(s)  __CaCO3(s) +__NH4Cl(aq)
Balancing Chemical Reactions

__Al(s) + __Br2(g) 

__C2H5OH(g) + __O2(g)  __CO2(g) +__H2O(l)
__AlBr3(s)
Balancing Chemical Reactions

NOT IN PACKET!!

_C3H6 + _ O2  _ CO2 + _H2O
Balancing Chemical Equations

Tin (IV) oxide + Carbon  Tin + Carbon monoxide

SnO2(aq) + C(s)  Sn(s) + CO(g)

SnO2(aq) + 2C(s)  Sn(s) + 2CO(g)
Balancing Chemical Equations

Aqueous Iron (III) Chlorate reacts with solid calcium
to yield calcium chlorate and solid iron

Fe(ClO3)3(aq) + Ca (s)  Ca(ClO3)2(aq) + Fe(s)

2Fe(ClO3)3(aq) + 3Ca (s)  3Ca(ClO3)2(aq) + 2Fe(s)
8.2 Types of Chemical Reactions

5 types





synthesis
decomposition
combustion
single replacement
double replacement

Remember: You must first find the correct products,
then balance the equation!
Types of Chemical Reactions

Synthesis (direct combination) - needs
energy to happen (usually heat)

General formula

A
+

2H2 +
B
--->
AB
O2
→
2H2O
Types of Chemical Reactions

Synthesis Examples
 BaS

Ba
+
S

Mg
+
Cl2

Al
+
Cl2
 AlCl
3

Na
+
O2
 Na2O


MgCl2
REMEMBER TO BALANCE!!!!!!!!!!
Types of Chemical Reactions

Decomposition (analysis) - needs energy
to happen (usually heat or electricity)

general formula

AB
--->
A

2H2O → 2H2
+
B
+
O2
Types of Chemical Reactions

Examples

FeCl3  Fe + Cl2

HgO

CuSO4 · 5 H2O
 Hg + O2
CuSO4(s) + H2O(g)
Types of Chemical Reactions

Combustion - the reaction of hydrocarbons
and oxygen
 General Formula

CxH y
+
O2

CO2
+
H2O
Types of Chemical Reactions

CH4 + O2 → CO2 + H2O

C3H8 + O2 → CO2 + H2O

C4H10 + O2 → CO2 + H2O

C2H5OH + O2 → CO2 + H2O
Types of Chemical Reactions

Single replacement - take place in aqueous
solutions - need very little energy to happen
 Two Types


Positive Ions Switch
 AB +
M  MB + A
Negative Ions Switch
 MB +
X  MX +
B
Types of Chemical Reactions

Positive Ions Switch

HI(aq) + Mg(s) 

AlCl3(aq) + Ca(s) 

Ca(s) +
MgI2(aq) +
I2(s)
CaCl2(aq) + Al(s)
HOH(l)  Ca(OH)2(aq) + H2(g)
Types of Chemical Reactions

Negative Ions Switch
 NaCl +
F2


BaS +
O2

NaF + Cl2
BaO +
S
Types of Chemical Reactions

Double Replacement - aqueous solution - little
energy - usually forms one soluble ionic product
(aka - aqueous) and either a ppt, water, or a
gas that bubbles out of water

General Formula (molecule + molecule)


AB +
CD

AgNO3 + NaCl →
CB
AgCl +
+
AD
NaNO3
Types of Chemical Reactions

FeCl3 +
NaOH  Fe(OH)3 + NaCl

H2SO4 +
NaOH  HOH + Na2SO4

NH4Cl +
NaOH  NH4OH + NaCl
8.3 Activity Series of the
Elements

Another Lie!! : )

Some reactions happen and some don’t!


Assume all synthesis, decomposition, and
combustions happen
Not all single or double displacement occur


Single Replacement

Use activity series
Double Replacement

Use solubility table
Activity Series of the Elements

Rules for the single replacement activity
series:
 Any single element above an element in a
compound will replace it.
 The top 5 elements react with water.
 Metals above H react with acids (molecules
that start with H – not water).
 The nonmetal reactivity series is F> Cl >
Br...
Activity Series of the Elements

Ca + H2O →

Al
+ H2O →

Al
+ HI
→
Ca(OH)2 +
No Rxn
AlI3 +
H2
H2
Activity Series of the Elements

Cu
+
HI
 No Rxn

NaCl
+
F2
 NaF + Cl2

NaF
+
Cl2 → No Rxn
Solubility Table of the
Elements

Rules for double replacement
reactions using a solubility table:



If one of the products formed is water, the
reaction happens.
If a gas is formed, the reaction happens.
If an insoluble product forms (I or Ss), the
reaction happens
Solubility Table of the
Elements

Na2CrO4 + KCl →

FeCl3 + KOH

HCl
+
No Rxn
→ Fe(OH)3(ppt) + KCl(aq)
NaOH →
H2O(l) + NaCl(s)
Solubility Trends

Cations



Anions 




very soluble - Na, K, ammonium
very insoluble- Ag, Pb, Hg, transitions
very soluble – nitrate
for monatomicsF>Cl>Br ....
very insoluble - carbonate, hydroxide, phosphate, sulfate
sulfides - decompose
General trend 
As size decreases, solubility increases OR
Heavier stuff doesn’t dissolve well
Showing Energy Changes in
Equations

endothermic 
A + B + heat
C



ΔH is positive
exothermic 
A + B


ΔH is negative
C + heat
Some helpful notes on writing phases in
chemical reactions







Metals are solids (except Hg, Br)
SR and DR reactions, reactants that are compounds are
always aqueous.
SR and DR reactions, products that are compounds
should have their phases identified using a solubility
chart (aqueous vs. precipitate)
S and D reactions, ionic compounds are solids.
In C reactions, the water, CO2, and O2 are gases. The
hydrocarbon is hard to tell, but is usually a liquid after
C=6 or higher.
Most other covalent compounds are gases.
Acids (chemicals starting with hydrogen) are always
aqueous