A chemical reaction is a change in
composition.
• Is represented by a
chemical equation which
uses symbols and
formulas.
• Reactant(s)-the original
substance(s) undergoing
change
• Product(s)- the new
substance(s) produced
A silver spoon tarnishes. The silver
reacts with sulfur in the air to make
silver sulfide, the black material we
call tarnish.
2 Ag + S  Ag2S
Reactants
Product
2 H2 (g) + O2 (g) --> 2 H2O (l)
Symbols
Meaning
Reactants
what is written on the left-hand-side of the arrow.
ex. H2 and O2
Products
what is written on the right-hand-side of the arrow.
ex. H2O
-->
separates the reactants from the products
(s),(l), (g)
indicates the physical states of each species in the chemical
reaction. For example,
•
(s) a solid substance ,
•
(l) a liquid substance,
•
(g) a gaseous substance,
•
(aq) an aqueous solution (substance dissolved in water)
Types of Reactions
•
•
•
•
•
Synthesis
Decomposition
Single Displacement
Double Displacement
Combustion
SYNTHESIS REACTIONS
A + B  AB
Two materials, elements or compounds, come together to make a
single product.
Some examples of synthesis reactions are:
• Hydrogen gas and oxygen gas burn to produce water.
2 H 2 + O2  2 H 2 O
• sulfur trioxide reacts with water to make sulfuric acid.
H2O + SO3  H2SO4
DECOMPOSITION REACTIONS
XZ  X + Z
One reactant only
Some examples of decomposition reactions are:
potassium chlorate when heated comes apart into oxygen
gas and potassium chloride
• 2 KClO3  2 KCl + 3 O2
and heating sodium bicarbonate releases water and carbon
dioxide and sodium carbonate.
• 6 NaHCO3  3 Na2CO3 + 3 H2O + 3 CO2
SINGLE REPLACEMENT
REACTIONS
ALSO CALLED SINGLE DISPLACEMENT
• 2 KI + Cl2  2 KCl + I2
• Positive ions will replace positive ionsnegative ions will replace negative ions
DOUBLE REPLACEMENT
REACTIONS
ALSO CALLED DOUBLE DISPLACEMENT
• AgNO3 + KCl  AgCl(s) + KNO3
• Positive and negative ions in an equation
switch partners.
• Remember…the positive ion is always
written 1st
Combustion Reactions
• Occurs between a hydrocarbon and
oxygen
• Products are always carbon dioxide
and water
• CH4 +2 O2  CO2 + 2H2O
Balancing Chemical Equations
• An equation is balanced by
adding whole number
COEFFICIENTS in front of a
compound’s formula or
element’s symbol.
Tips!!!!
• Delay the balancing of
hydrogen and oxygen
• If a polyatomic appears on both
sides of an equation, treat them
as a single unit
• Remember… balancing one
element may unbalance
another!!!
Predicting Products of Reactions
For many reactions, the products can be
predicted by classifying the reaction and
looking at the reactants.
Double Replacement
• Check solubility rules!
• A basic knowledge of which compounds
are soluble in aqueous solutions
(dissolved in water) is essential for
predicting whether a given reaction might
involve formation of a precipitate (an
insoluble solid that comes from a liquid
solution).
Single Replacement
• Activity Series- the
ability to react - a list
of elements organized
according to the ease
with which they
undergo single
replacement
reactions.
• The most active elements
are at the top-the least
reactive at the bottom
• Elements can replace those
that are below them (i.e.
those that are less reactive)
but NOT those above them
(i.e. those that are more
reactive)
• Elements can also do
everything in it’s group and
everything below it but
NOT above.
Diatomics
F2
Cl2
Br2
H2
I2
N2
O2
Examples:
• Al + ZnCl2 
This reaction would go to completion
because aluminum is above zinc in the
series and can therefore replace it.
• Co + NaCl  NR
Cobalt can not replace Na because Na is
above it in the series and is therefore the
more reactive element.
More Examples:
• NaCl + I2  NR
Iodine is below Cl in the halogen activity
series and therefore can’t replace it.
• Pt + O2  NR
Pt forms oxides only indirectly.
More Examples:
• Al + H2O (l)  NR
Al can only react with water that is steam
(a.k.a. gas)
• Li + HCl 
This reaction would go to completion
because Li can react with acids
(remember.. they can do anything in their
group and the groups below them)
Synthesis and Decomposition:
• Use “cheat sheet” to know how the elements
or compounds combine or decompose.
• Use gas ion chart
Combustion:
• Products should always be CO2 and H2O