Chemical Reactions
Mr. Treanor
Chemical Reaction
A process in which one or more
substances are changed into one or
more different substances
 The different substances have new
properties; they may be more shiny or
a different color
 This does not mean that new elements
have been made.

Chemical Equations


Equations describe Chemical Reactions
and tell you what is happening
There are three steps to writing
correct chemical equations:
1.
2.
3.
Word Equation
Formula Equation
Balance using coefficients
What’s in an equation?
The arrow means ‘yields’ or ‘turns into’
 Substances on the left side are reactants
 Substances on the right side are products
 The law of conservation of mass states that
mass is neither created nor destroyed in
ordinary chemical reactions.


This means that there are the same number
of atoms of each type on BOTH SIDES
Word Equations
To write a word equation, you simply state
the names of the reactants, write the word
“yields,” then state the names of the
products.
 Example:

Hydrogen plus oxygen yields water.
Formula Equations
Uses symbols and formulas
 Shows what happens in a reaction
 Big numbers are coefficients
 Small numbers are subscripts

2H2 + O2
2H2O
Symbols used in equations
Example of a reaction
2H2(g) + O2 (g)
2H2O(g)
Details of a chemical reaction
H2 + Cl2  2HCl
Diatomic elements
(plus 2 friends)
Hydrogen
Nitrogen
Oxygen
Fluorine
Chlorine
Bromine
Iodine
H2
N2
O2
F2
Cl2
Br2
I2
H2 + Cl2  2HCl
In addition to these,
phosphorous is P4
and sulfur is S8.
Law of Conservation of Mass

The law of conservation of mass states that matter
is neither lost nor gained in ordinary chemical
reactions; it simply changes form.
 The total mass of the reactants, 36.04g, is exactly
equal to the total mass of the products, 36.04g
 This holds true for all balanced chemical
equations.
Types of Chemical Reactions
There are clues to tell you a chemical
reaction has occurred and what kind it is
 There are five main types of reactions
 They have different types of equations
 Most chemical reactions happen in water
(called an aqueous solution)

Clue #1 – Color change
(always?)
Clue #2 – Light
(always?)
Clue #3 – Heat
(always?)
Clue #4 – Gas formation
(always?)
Clue #5 – solid formation
(always?)
Solids formed in chemical reactions
are called precipitates.
Five Main Types (in the book):
Synthesis  Decomposition –
 Single Replacement –
 Double Replacement –
 Combustion –


There are other types of reactions!
Five Main Types (in the book):
Synthesis  Decomposition –
 Single Replacement –
 Double Replacement –
 Combustion –


There are other types of reactions!