Chapter 8
Chemical Equations and
Reactions
8.1
Describing Chemical Reactions
Chemical Reaction
 Chemical
Reaction – the process by
which one or more substances
change to produce one or more
different substances
 Evidence of chemical change
–Formation of a gas, E change,
solid formed, color change in
system
Chemical Reactions
A
new substance results from a
chem rxn. to prove a rxn has
occurred
Reactants  Products
 Energy changes
 Collisions must occur
 Law of Conservation of Mass
Chemical Equation
 Chemical
Equation – A
representation of a chemical
reaction that uses symbols to
show the relationship between
the reactants and products
 Word equations: Ethanol +
oxygen  carbon dioxide + water
Chemical Equations
 Tells
you…
–Number and type of atoms
–State of reactants and products
(s,l,g)
–If it is dissolved in water (aq)
–If heat or catalyst are used
Equations
C2H6 (g)
800 K
Cr2O3
C2H4 (g) + H2 (g)
Physical Catalyst
State
Reactants
Products
Assignment
 Do
the section review on page
266 for homework. Do only
numbers 1-5, This will be due
on Monday.
Balancing Chemical
Equations
8.2
Equations
 _CH3CH2OH + _O2  _CO2 + _H2O
 This
eqn needs balanced
 Coefficients – integer/numeral
used in a chemical eqn to indicate
relative amts of reactants or
products.
 Never change subscripts
Balancing an Equation
1.
2.
3.
4.
5.
Write the word eqn
Write the unbalanced formula
eqn.
Count the # of each element
Insert coefficients for atoms
Check to make sure both sides
are the same.
Tips for Balancing Equations
Delay balancing elements (like H
and O) that occur in several
reactants or products.
2. If the same polyatomic ions
appear on both sides of the
equation, treat them as single
units, like monatomic ions.
1.
Tips for Balancing Equations
3. After considering 1&2, balance
elements left to right.
4. Remember, balancing 1 element
may unbalance others.
5. For ionic equations, be sure
charges are balanced.
Practice Balancing
 Ammonia
plus oxygen yields
nitrogen monoxide plus water
plus heat.
 Ionic equations
 Need to make sure ionic charges
are balanced
 HCl + H2O  Cl- + H3O+