Chapter 8:
Chemical
Equations and
Reactions
Chemistry 1-2
Mr. Chumbley
Section 1: Describing
Chemical Reactions
Chemical Reactions

A chemical reaction is a process by which
one or more substances change to produce
one or more different substances

A chemical equation is a representation of a
chemical reaction that uses symbols to show
the relationship between the reactants and
products
(NH4 )2 Cr2 O7 s → N2 g + Cr2 O3 s + 4H2 O(g)
Indicators of a Chemical
Reaction

While absolute proof of a chemical change
can only be found through chemical analysis,
there are certain indicators a chemical
reaction has occurred

Release of energy as heat and light
Color change
Production of a gas
Formation of a precipitate




A precipitate is a solid that is produced as a
result of a chemical reaction in a solution
Chemical Equations
 Chemical
criteria:



equations must satisfy certain
The equation must represent all known facts
The equation must contain the correct
formulas for the reactants and the products
The law of conservation of mass must be
satisfied
A
coefficient is a small whole number that
appears in front of a formula in a chemical
equation
Word Equations

A word equation is an equation in which the
reactants and products in a chemical reaction
are represented with words
methane + oxygen → carbon dioxide + water

An arrow in an equation is read as “reacts to
yield” or “yields”

The above equation would be read as:

Methane and oxygen react to yield carbon
dioxide and water
Formula Equations

A formula equation represents the reactants and
products of a chemical reaction by their symbols
and formulas
CH4 (𝑔) + O2 (𝑔) → CO2 (𝑔) + H2 O(𝑔)

The reaction would be read the same as in the word
equation:


Methane and oxygen react to yield carbon dioxide and
water
Neither the word equation nor the formula equation
give information about the amounts of reactants or
products
Balanced Equations
 While
a formula equations describe the
formulas in a reaction, they don’t
necessarily describe the correct amounts
A
balanced equation has all of the
chemical formulas in the correct ratios to
fully satisfy the law of conservation of
mass
Balancing Equations
CH4 𝑔 + O2 𝑔 → CO2 (𝑔) + H2 O(𝑔)
Symbols Used in Chemical
Equations
“yields”; indicates a
reaction
indicates a reversible
reaction
(s)
solid state
↓
(l)
liquid state
(aq)
(g)
gas state
∆
heat
pressure
MnO2
reaction requires heat
reaction requires pressure
catalyst used to alter
reaction rate
precipitate formed as
product
in aqueous solution
↑
gas formed as product
0˚C
specific temperature at
which reaction was
carried out
2 atm
specific pressure at which
reaction was carried out
Balancing Equations
Lithium metal and aluminum sulfate solution
react to yield a lithium sulfate solution and
aluminum metal.
Li 𝑠 + Al2 SO4
3
𝑎𝑞 → Li2 SO4 (𝑎𝑞) + Al(𝑠)
Section 2: Types of
Chemical Reactions
Synthesis Reaction
 In
a synthesis reaction two or more
substances combine to form a new
compound
2 Mg 𝑠 + O2 𝑔
NH3 𝑔 + H2 O 𝑙
2MgO(𝑠)
pressure
NH4 OH(𝑎𝑞)
Decomposition Reaction
 In
a decomposition reaction a single
compound breaks down into two or more
simpler substances
CaCO3 𝑠
∆
CaO 𝑠 + CO2 (𝑔)
Single Displacement Reaction
 In
a single displacement one element replaces
a similar element in a compound
2Al 𝑠 + 3Pb NO3
2
𝑎𝑞
3Pb(𝑠) + Al NO3 3 (𝑎𝑞)
Double Displacement
Reaction
 In
a double displacement the ions of two
compounds exchange places in an
aqueous solution to form two new
compounds
2KI 𝑎𝑞 + Pb NO3
2
𝑎𝑞
PbI2 𝑠 + 2KNO3 (𝑎𝑞)
Combustion Reaction
A
combustion reaction is the oxidation
reaction of an element or compound in
which energy as heat is released
C3 H8 𝑔 + 5 O2 𝑔
3 CO2 𝑔 + 4 H2 O (𝑔)