CHAPTER 9 CHEMICAL
EQUATIONS
CHEMICAL CHANGES AND HOW CHEMICAL
REACTIONS ARE WRITTEN
WHAT IS A CHEMICAL REACTION?
Chemical reactions are CHEMICAL CHANGES.
Evidence of a chemical change:
• Light
• Heat
• Formation of a gas
• change in color
• Formation of a precipitate
**Particles must collide for a chemical reaction to
occur
WHAT IS A CHEMICAL REACTION?
Physical change: when only the physical structure of
the material has changed.
Chemical change: where the chemical structure of
the material AND physical structure of the material
has changed.
REACTIONS INVOLVE
REARRANGEMENTS OF ATOMS
Law of conservation of mass states: mass cannot be
created or destroyed!
Start and end with the same number of atoms, just in
a different arrangement
In a sense its like musical chairs, but with atoms.
CHEMICAL REACTIONS RELEASE OR
ABSORB ENERGY
Exothermic Reactions: heat is released
Endothermic Reactions: heat is absorbed
HOW ARE CHEMICAL EQUATIONS FOR
REACTIONS WRITTEN?
A word equation describes a reaction and translates
into a formula equation.
Ethanol reacts with oxygen to produce carbon
dioxide and water.
Word equation:
Ethanol + oxygen  carbon dioxide + water
Formula equation:
CH2CH3OH + O2  CO2 + H2O
KEY WORDS TO LOOK FOR!!
•
•
•
•
Reacts with = +
Forms, produces = 
Combustion = + O2
Any HONCLBrIF
element stated is
DIATOMIC!!!
PRACTICING WRITING CHEMICAL
EQUATIONS
1. Iron reacts with oxygen to form iron (III) oxide and
releases 824.8 kJ of energy
1. Heat is added to dinitrogen pentoxide to produce
dinitrogen tetroxide and oxygen gas.
BALANCING CHEMICAL
EQUATIONS
BALANCING CHEMICAL EQUATIONS
Coefficients: show how much of each compound
(either reactants or products) is present.
Subscripts: show the number of each type of atom in
a formula
Superscripts: tell the type of charge on an ion
2CH3
SO42-
GENERAL GUIDELINES FOR
BALANCING CHEMICAL REACTIONS
1. Balance elements that are found in more than
one reactant or product LAST!!
2. If the same polyatomic ions appear on both sides
of the equation, treat them as a single unit
3. Balance elements, starting with the reactants
BALANCING CHEMICAL REACTIONS
PRACTICE
Example 1The first step in the commercial production of nitric
acid is the combustion of ammonia in the presence
of a catalyst. The exothermic reaction produces
nitrogen monoxide and water vapor. Write and
balance the equation for the reaction.
ANSWER
• Word equation:
Ammonia + oxygen  nitrogen monoxide + water +heat
• Unbalanced formula equation:
__NH3 + __O2  __NO + __H2O + heat
• Balanced formula:
4NH3 + 5O2  4NO + 6H2O + heat
BALANCING CHEMICAL REACTIONS
PRACTICE
Example 2-
Hydrogen reacts with chlorine to produce
hydrochloric acid (hydrogen chloride). What is the
balanced chemical equation?
• Word equation:
Hydrogen + chlorine  hydrogen chloride
• Unbalanced formula equation:
__H2 + __Cl2  __HCl
• Balanced formula:
H2 + Cl2  2HCl
HOMEWORK
Page 316- #1
Page 317- #’s 6, 7, 8, 9
DUE NEXT TIME!!