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Unit 6:
Chemical
Equations and
Reactions
Chemistry 1-2
Mr. Chumbley
Describing Chemical
Reactions
Chapter 8 – Section 1
EQ: How can chemical reactions be identified and
described?
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
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
Symbols Used in Chemical
Equations (p. 252)
“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
Diatomic Elements (p. 249)


Some elements do
not exist as individual
atoms in nature.
These non-metals are
found in elemental
form as two atoms
bonded together
Element
Molecular
Formula
State of
Matter
Hydrogen H2
Gas
Nitrogen
N2
Gas
Oxygen
O2
Gas
Fluorine
F2
Gas
Chlorine
Cl2
Gas
Bromine
Br2
Liquid
Iodine
I2
Liquid
Sample Problem A
Write word and formula equations for the
chemical reaction that occurs when solid
sodium oxide is added to water at room
temperature and forms sodium hydroxide
(dissolved in water)
𝑠𝑜𝑑𝑖𝑢𝑚 𝑜𝑥𝑖𝑑𝑒 + 𝑤𝑎𝑡𝑒𝑟
Na2 O(𝑠) + H2 O(𝑙)
𝑠𝑜𝑑𝑖𝑢𝑚 ℎ𝑦𝑑𝑟𝑜𝑥𝑖𝑑𝑒
NaOH(𝑎𝑞)
Homework!
 p.
260 #2
 P. 277 #11 & 12
Balanced Chemical
Equations
EQ: How can chemical equations satisfy the law of
conservation of mass?
The Mole

Because atoms are so small, they are very
difficult to count and measure

A mole is the amount of a substance that
contains as many particles as there are atoms
in exactly 12g of carbon-12

A mole is abbreviated as mol

The mole is the base unit of amount in SI
Avogadro’s Number

Avogadro’s number is the number of particles in
exactly one mole of a pure substance

This number has been experimentally determined
to be 6.022 141 79 × 1023

We will define Avogadro’s number as 6.022 × 1023

This number is named after 19th century scientist,
Amedeo Avogadro, who did not actually
determine this number
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

A coefficient is a small whole number that appears in
front of a formula in a chemical equation
Balancing Equations
CH4 𝑔 + O2 𝑔 → CO2 (𝑔) + H2 O(𝑔)
Balancing Equations
Lithium metal and aluminum sulfate solution react to
yield a lithium sulfate solution and aluminum metal.
𝑙𝑖𝑡ℎ𝑖𝑢𝑚 + 𝑎𝑙𝑢𝑚𝑖𝑛𝑢𝑚 𝑠𝑢𝑙𝑓𝑎𝑡𝑒 → 𝑙𝑖𝑡ℎ𝑖𝑢𝑚 𝑠𝑢𝑙𝑓𝑎𝑡𝑒 + 𝑎𝑙𝑢𝑚𝑖𝑛𝑢𝑚
Li 𝑠 + Al2 SO4
3
𝑎𝑞 → Li2 SO4 (𝑎𝑞) + Al(𝑠)
Reflection!!!
 For

MONDAY…
Balance the equations from the Describing
Chemical Reactions 2 worksheet
Section 2: Types of
Chemical Reactions
EQ: How can chemical equations be used to classify
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)