Chemical
Reactions
Chapter 10
Representing Chemical
Changes

Chemical equations are used to
represent chemical reactions (the
process by which one or more
substances are changed into one
or more different substances).
Evidence of chemical
reactions
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Pg. 277 list types of evidence that
a chemical reaction has occurred
Temperature change
Energy released in form of heat
and light
Color change
Odor
Gas bubbles
Appearance of a solid ( precipitate)
A correct chemical
equation shows:
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what changes take place
the relative amounts of the
elements
compounds that take part in the
changes
Representing Chemical Changes
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Reactants are the starting
substances in a chemical reaction.
Products are the substances
formed by the chemical reaction.
Representing Chemical Changes
Reactants yield Products
C2H2(g) + O2(g)
yield
reactants
CO2 (g) + H2O(l)
products
C2H2(g) + O2(g)  CO2 (g) + H2O(l)
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The letters in parenthesis represent the physical
states.
g = gases
l = liquids
s= crystalline solid (also represented “cr”).
aq = substance dissolved in water
Representing reactions
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Word equations
Skeleton equations
-Formulas and states are
represented, but not amounts –
equation not balanced
Left pg. entry practice
problems
1.
2.

Hydrogen bromide gas is
produced from the reaction
between hydrogen gas and
bromine gas
Write a skeleton equation.
Pg. 279 practice problems
Balancing Equations

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Reactants are written on the left
side of the chemical equations.
Products are written on the right.
Balancing Equations

Use the following steps to balance
equations:
1. Determine the reactants and the products.
2. Assemble the parts of the chemical equation.
The symbols and the formulas must be correct.
3. Balance the equation. Balanced equations
have the same kind and number of atoms on
each side. In a chemical reaction no mass is
lost or gained. Law of conservation of mass
Balancing Equations

Balance the equation(continued)
The same amount of matter is
present before and after the
reactions. Coefficients, not
subscripts, may be changed to
balance an equation. Never
change a subscript to balance
an equation.
Balancing Equations

Balance the equation(continued)
The same amount of matter is
present before and after the
reactions. Coefficients, not
subscripts, may be changed to
Never
change a subscript to
balance an equation.
balance an equation.
EVER!
Don’t do it !!!
Balancing Equations
CO2 +
H2O  C6H12O6 +
O2
Notice one C in carbon dioxide and six carbons in
glucose.
CO2
C6H12O6
Balancing Equations
6 CO2 +
H2O  C6H12O6 +
O2
Multiply the carbon dioxide by six to equal the six
carbons in glucose.
CO2
C6H12O6
Balancing Equations
6 CO2 + 6 H2O  C6H12O6 +
O2
Next look at the water molecule.
Notice that there are 12 H in glucose. Therefore
you’ll need six waters to balance.
Balancing Equations
6 CO2 + 6 H2O  C6H12O6 +
(6x2)
=18
+ (6x1)
O2
6
18 - 6=
12  2 =
Next, count the total oxygen atoms on the reactant
side and compare that to the product side.
Balancing Equations
6 CO2 + 6 H2O  C6H12O6 + 6 O2
Balancing Equations
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Finally, Check all atoms,
comparing both sides.
6CO2 + 6H2O
C6H12O6 + 6O2
6C
=
6C
 12 H
=
12 H
 18 O
=
18 O
End of Part I
Practice problems
pg.282 #’s 4-6
4. In water, iron(III) chloride
reacts with sodium hydroxide,
producing solid iron(III)
hydroxide and sodium chloride.
Types of Chemical Reactions
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Single Replacement (Displacement)
Double Replacement (Displacement)
Decomposition
Synthesis
Combustion
Single Displacement
Re
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element + compound
Cl2 + 2KBr
compound + element
2KCl + Br2
Single Displacement
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Cl2 + 2KBr
Cl-
2KCl
+ Br2
The anion(in this case)
replaces
Double Displacement
Re
compound AB + compound CD  compound AD +
compound CB
PbCl2
+ Li2SO4
Anions swap places

PbSO4 + 2LiCl
Decomposition
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compound
compounds
Pb(OH)2
See Pg 260
two or more elements or
PbO + H2O
Synthesis
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element/compound + element/compound
compound
2H2(g) + O2(g)
2H2O(g)
Combustion
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hydrocarbon + oxygen
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CH4 + 2O2
carbon dioxide + water
CO2 + 2H2O