Chemical Reactions
A chemical change: any change in which a new substance is formed.
Evidence of a Chemical Change:
 Release of energy as heat
 Release of energy as light
 Change in color
 Formation of a gas
 Change in odor…
Types of Reactions
•
There are SIX types of chemical reactions we will talk
about (FOCUS ON THE first 5!)
1. Synthesis reactions (S)
2. Decomposition reactions (D)
3. Single replacement/ displacement reactions (SR)
4. Double replacement/displacement reactions (DR)
5. Combustion reactions (C )
6. Neutralization (Acid –base)
You need to be able to identify the type of reaction
and predict the product(s)
1. Synthesis: A + B  AB
2. Decomposition: AB  A + B
3. Single Replacement/displacement:
A + BC  AC + B
4. Double Replacement/ displacement:
AB + CD  AD + CB
5. Combustion: AB + oxygen  CO2
+
H2O
6. Neutralization: Acid(H) + Base(OH)  salt + H(OH)
Steps to Writing Reactions
•
Some steps for doing reactions
1. Identify the type of reaction
2. Predict the product(s) using the type of
reaction as a model
3. Balance it
Don’t forget about the diatomic elements!
(BrINClHOF)
For example, Oxygen is O2 (NOTE: oxygen is not
an element anymore, it’s a compound!)
The law of conservation of
mass
• French chemist Antoine Lavoisier In 1774
found that if nothing is allowed to enter or
leave a reaction vessel , the total mass is the
same after as it was before
• Atoms are not created or destroyed in
chemical equations they are simply
rearranged…… so equations must balance!
Balanced chemical equations
1) Tell us the reactants and products in a reaction, and the
relative amounts involved.
2) Equation is balanced using coefficients so that there are equal
numbers of each type of atom on both sides the arrow.
• CH4 (g) + 2O2 (g)  CO2 (g) + 2H20 (l)
• State symbols
• State (phase) symbols are included in chemical equations to
show the physical state of the reactants and products
• (g) Gas
(l)Liquid
(s)Solid
• (aq) aqueous solution
Steps for balancing equations
• The reaction between solid Ca and H20 produces calcium
hydroxide in solutions and hydrogen gas.
STEP 1 Determine what the reactants (starting substances) and
products (resulting substances)are:
• calcium(s) + water(l) calcium hydroxide(aq) + hydrogen(g)
STEP 2 Write formulas for the substances and state symbols
• Ca (s) + H20 (l)  Ca (OH) 2 (aq) + H2 (g)
STEP 3 Use coefficients to balance the equation so there are the
same number of each kind of atom on both sides of the arrow
• Ca (s) + 2H20 (l)  Ca (OH) 2 (aq) + H2 (g)
Equations can only be balanced by putting numbers
(coefficients) in front of the formulas. You cannot balance them
by altering the formulas themselves because you would then
create different substances.
Tips for balancing:
• Start by balancing the substance with the
largest number of elements.
• If O2 (or any BrINClHOF) is in the equation,
save it for last.
• If a polyatomic ion stays the same on both
sides of the arrow, count it as a unit when
counting atoms.
• When finished, make sure you have the
smallest whole number ratio of
coefficients.
Make a RIP chart
 6 CO2 (g) + 6 H2O (l)
C6H1206 (aq) + 6 O2 (g)
R (reactants)
6C
12 H
18 O
I
P (Products)
6C
12 H
18 O
• (You will Rest In Peace in chemistry if you do this!)
Make a RIP chart
Pb(NO3)2 (aq) + KI (aq)  KNO3 (aq) +PbI2
R (reactants)
I
P (Products)
• (You will Rest In Peace in chemistry if you do this!)
1. Synthesis reactions
•Synthesis reactions occur when two substances
(generally elements) combine and form a
compound. (Sometimes these are called
combination or addition reactions.)
reactant + reactant  1 product
•Basically: A + B  AB
•Example: 2H2 + O2  2H2O
•Example: C + O2  CO2
2. Decomposition Reactions
• Decomposition reactions occur when a compound breaks up into
the elements or in a few to simpler compounds
• 1 Reactant  Product + Product
• In general: AB  A + B
• Example: 2 H2O  2H2 + O2
• Example: 2 HgO  2Hg + O2
3. Single Replacement Reactions
• Single Replacement Reactions occur when one element replaces another
in a compound.
• A metal can replace a metal (+) OR
a nonmetal can replace a nonmetal (-).
• element + compound element + compound
A + BC  AC + B (if A is a metal) OR
A + BC  BA + C (if A is a nonmetal)
(remember the cation always goes first!)
NOTE: When H2O splits into ions, it splits into
H+ and OH- (not H+ and O-2 !!)
1. When A is a metal:
• aluminum foil in a solution of copper(II)
chloride produces solid copper and
aluminum chloride.
• Al + CuCl2 
Cu + AlCl3
• BALANCE: 2Al + 3CuCl2  3Cu + 2AlCl3
2. When A is a non-metal:
• When fluorine is bubbled through a sodium
iodide solution, iodine and sodium fluoride
are produced.
• F2 + NaI  I2 + NaF
• BALANCE: F2 + 2NaI  I2 + 2NaF
4. Double Replacement Reactions
• Double Replacement Reactions occur when a metal replaces a metal in a
compound and a nonmetal replaces a nonmetal in a compound
• Compound + compound  compound+ compound
• AB + CD  AD + CB
Types: Double displacement
Example: MgO + CaS
Mg
O
+
Ca
S
General:

Mg
+
Ca
S
AB + CD  AD + CB
O
5. Combustion Reactions
• Combustion reactions occur
when a hydrocarbon reacts
with oxygen gas.
• This is also called burning!!!
In order to burn something
you need the 3 things in the
“fire triangle”:
1) A Fuel (hydrocarbon)
2) Oxygen to burn it with
3) Something to ignite the
reaction (spark)
Combustion
Reactions
Edgar Allen Poe’s
drooping eyes and
mouth are potential
signs of CO
poisoning.
• Combustion reactions occur when a compound or element
react with oxygen to release energy and produce an oxide.
• Also sometimes referred to as hydrocarbon combustion.
CXHY + O2  CO2 + H2O
where X and Y represent integers
See page 264
(c) McGraw Hill Ryerson 2007
• Neutralization reactions occur when an acid (most
compounds starting with H) and a base (most
compounds ending in OH) react to form a salt and
water.
• Neutralization reactions are a type of double
replacement (DR).
• Acid + base  salt + water
HX + MOH  MX + H2O
where X and M are elements
See page 263
(c) McGraw Hill Ryerson 2007
1. Sulfuric acid is used to neutralize calcium hydroxide 
• H2SO4 + Ca(OH) 2  CaSO4 + 2H2O
2. Phosphoric acid helps to neutralize the compounds that cause
rust, such as iron(II) hydroxide 
• 2H3PO4 + 3Fe(OH)2  Fe3(PO4)2 + 6H2O
(c) McGraw Hill Ryerson 2007
Some 2 special products
•If H2CO3 is formed, it
immediately decomposes into
H2O and CO2
•If H2SO3 is formed, it
immediately decomposes into
H2O and SO2
Class of reaction
Reactants
Possible products
General equation
Example
Synthesis
2 or more substances
one new
compound
A + B  AB
2 Mg + O2
 2 MgO
Decomposition
One compound
Two or more
substance
AB  A + B
2 H2O 
2 H 2 + O2
Single
Replacement
An element and a
compound
A different element
& compound
A+BCB+AC
or
A+BCC+BA
Zn + 2HCl 
ZnCl2 + H2
Double
Replacement
Two compounds
(exchange parts)
Two different
compounds
AB + CD 
AD + CB
2KI+Pb(NO3)2

PbI2 + 2KNO3
Combustion
Substance (usually a
fuel w/ C & H) +
oxygen
oxides, light, heat
Fuel + O2 
CO2 + H2O
CH3OH + O2 CO2
+ H2O
Acid/Base or
Neutralization (a
type of Double
Replacement)
Acid and Base
Salt and water
HX + MOH  salt +
H2O
HCl + NaOH 
NaCl + H2O
(c) McGraw Hill Ryerson 2007
Reactions in Aqueous Solutions
• Molecular Equations: shows complete formulas for
reactants and products
• Does not show what happens on the molecular level
• Total (or Complete) Ionic Equations: All substances that
are strong electrolytes (are soluble and dissociate) are written
as their ions.
• Some ions participate in the reaction
• Some ions do NOT participate in the reaction-called spectator ions.
• Net Ionic Equations (NIE): show only the ions that
participate in the reaction
Total Ionic Equations
• Once you write the molecular equation
(synthesis, decomposition, etc.), you should
check for reactants and products that are soluble
or insoluble.
• We usually assume the reaction is in water
• We can use a solubility table to tell us what
compounds dissolve in water.
• If the compound is soluble (does dissolve in
water), then splits the compound into its
component ions
• If the compound is insoluble (does NOT dissolve
in water), then it remains as a compound
Use the solubility rules – See
periodic table!!
• Learn how to read and apply the rules based on your periodic
table
Total Ionic Equations
Molecular Equation:
K2CrO4(aq)+
Pb(NO3)2(aq)  PbCrO4 (S)
Soluble
Soluble
Insoluble
+ 2KNO3(aq)
Soluble
Total Ionic Equation:
2 K+(aq)+ CrO4 2-(aq) + Pb2+(aq)+ 2 NO3-(aq) 
PbCrO4 (s) + 2 K+(aq)+ 2 NO3-(aq)
Net Ionic Equations
• These are the same as total ionic equations, but you
should cancel out ions that appear on BOTH sides of
the equation
Total Ionic Equation:
2 K+ (aq)+ CrO42- (aq)+ Pb2+ (aq)+ 2 NO3- (aq)+ 
PbCrO4 (s) + 2 K+ (aq)+ 2 NO3- (aq)
(Spectator ions)
Net Ionic Equation:
CrO4 -2 + Pb+2  PbCrO4 (s)
Net Ionic Equations
• Try this one! Write the molecular, total ionic, and net
ionic equations for this reaction: silver nitrate reacts with
lead (II) chloride in hot water.
Molecular:
Total Ionic:
Net Ionic:
Net Ionic Equations
• Try this one! Write the molecular, total ionic, and net ionic
equations for this reaction: silver nitrate reacts with lead
(II) chloride in hot water.
AgNO3 (aq)+ PbCl2 (aq) 
Molecular:
2 AgNO3 (aq) + PbCl2 (aq)  2 AgCl (s) + Pb(NO3)2 (aq)
Total Ionic:
2 Ag+ + 2 NO3- + Pb+2 + 2 Cl-  2 AgCl (s) + Pb+2 + 2 NO3Net Ionic:
Ag+ (aq) + Cl- (aq)  AgCl (s)