Chemical Reactions
Chapter 8
Chemical Reactions
• Chemical reactions represent the formation of
a new substance (remember chemical
changes).
• We know a chemical change has occurred if:
• Give some examples--• Some can be found on pages 261-262.
Chemical Equations
• A chemical equation is a representation of a
chemical reaction.
• The starting material(s) are the reactants and
they react to form products.
• Reactants  Products
• We can use words to describe the reaction or
use the formulas of the substances involved.
• YOU MUST KNOW THE CORRECT FORMULAS
TO WRITE THE CORRECT EQUATION
Chemical Equations
• For example: methane combines with oxygen
to produce carbon dioxide and water.
• Methane + oxygen  carbon dioxide + water
• CH4 + O2  CO2 + H2O
• This is sometimes called a skeleton equation
• We have written the correct formulas for the
substances involved. Note oxygen gas is
diatomic-what are the other diatomic
molecules?
Chemical Equations
• If we know the physical state of the reactants
and products we can add them to our
equation.
• s-l-g-aq are commonly used symbols.
• Since methane is a gas we can rewrite our
skeleton equation as:
• CH4(g) + O2(g)  CO2(g) + H2O(l)
Chemical Equations
• Equations must conform to the Law of
Conservation of Mass which we already know
(?)-In writing equations the atoms of the
reactants must be balanced by the atoms of
the products.
• Lets look at our equation again
• CH4 + O2  CO2 + H2O
• On the reactant side there are: 4-H,1C and 2O
CH4 + O2  CO2 + H2O
• On the product side 2H,1C and 3O. Things are
not in balance! We know the formulas are
correct so we can’t change them but we can
use coefficients before the formula .
• Consider the H-4 on the left and 2 on the
right. So
• CH4 + O2  CO2 + 2H2O
• Now we need to balance the O’s-4 on the right
and 2 on the left, so
• CH4 + 2O2  CO2 + 2H2O
• Now things are balanced, so our final equation
would be:
• CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
Chemical equations
• Some other examples:
• Iron(III) oxide reacts with hydrogen gas to
produce metallic iron and water.
Another example
• Ammonia reacts with oxygen gas to produce
nitrogen monoxide gas and water vapor.
Types of Reactions
• 1. Combustion reactions occur when
substances react with oxygen and usually
produce a flame in the process. We will
restrict our discussion for reactions of oxygen
with hydrocarbons (compounds containing
carbon and hydrogen only). The previous
reaction of methane fits this description.
Combustion of hydrocarbons always produce
carbon dioxide and water.
An example:
• Combustion of butane C4H10
• Hint: when you have an even number of
atoms on one side of the equation and an
odd number on the other-multiply
everything by 2
Reaction Types
• Combination or synthesis reactions:
• You do not have to predict products of these
reactions, but you must be able to identify
them and be able to balance them!
•
•
Combination or Synthesis
Reactions
Combination reactions occur when two or more
substances combine to form a compound.
There are three basic types of combination
reactions.
1. Two elements react to form a new compound
2. An element and a compound react to form one new
compound
3. Two compounds react to form one compound
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Combination Reactions
1. Element + Element  Compound
A. Metal + Nonmetal  Binary Ionic Compound
Na s   Cl 2g   NaCls 
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Combination Reactions
1. Element + Element  Compound
A. Metal + Nonmetal  Binary Ionic Compound
Als   Br2   AlBr3s 
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Combination Reactions
1. Element + Element  Compound
B. Nonmetal + Nonmetal  Covalent Binary
Compound
P4s   Cl 2g   PCl 3 
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Combination Reactions
2. Compound + Element  Compound
AsCl3s   Cl 2 g   AsCl5s 
SF4 s   F2 g   SF6 g 
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Combination Reactions
3. Compound + Compound  Compound
–
gaseous ammonia and hydrogen chloride
NH 3g   HClg   NH 4Cls 
– lithium oxide and sulfur dioxide
Li2O  SO2  Li2SO3
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Decomposition Reactions
•
Decomposition reactions occur when one
compound decomposes to form:
1. Two elements
2. One or more elements and one or more
compounds
3. Two or more compounds
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Decomposition Reactions
1. Compound  Element + Element
–
decomposition of dinitrogen oxide

2 N 2O g  
 2 N 2 g   O2 g 
• decomposition of calcium chloride
CaCl 2   
 Ca    Cl 2g 
electricity
 decomposition of silver halides
h
2 AgBrs   2 Ag s   Br2  
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Decomposition Reactions
2. Compound  One Element +
Compound(s)
–
decomposition of hydrogen peroxide
hν or Fe3 or Mn
2 H 2O2 aq      2 H 2O   O2 g 
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Decomposition Reactions
3. Compound  Compound + Compound
–
decomposition of ammonium hydrogen carbonate

NH 4 HCO3s  
 NH 3g   H 2Og   CO2 g 
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Disassociation Reactions
• These are not true chemical reactions (no new
substance is produced) , but they are
important to understand.
• When an ionic compound dissolves in water it
does so by breaking up or dissociating into its
component ions. So….
• NaCl(aq)  Na+(aq) + Cl-(aq)
• Some other examples:
• CaBr2 
• Mg3(PO4)2 
• Al2(SO4)3 
Displacement Reactions
• Displacement reactions occur when one
element displaces another element from a
compound.
– These are reactions in which the more active
metal displaces the less active metal or hydrogen
from a compound in aqueous solution.
– Activity series is given on page 281. The most
active metals are at the top and will displace those
lower on the list.
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Displacement Reactions
[More Active Metal + Salt of Less Active Metal]  [Less Active
Metal + Salt of More Active Metal]
1.
–
Molecular or formula unit equation
AgNO3aq  + Cu (s)  CuNO 3aq   Ag (s)
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• Note that the silver nitrate and copper(I)
nitrate are in solution, so we can write an
ionic equation by showing these compounds
in their dissociated form:
• Did any compound or ion remain the same
during this reaction?
• An ion that does not undergo a change is
called a spectator ion.
• These ions can be eliminated from the
equation producing a net ionic equation.
Displacement Reactions
[Active Metal + Nonoxidizing Acid]  [Hydrogen +
Salt of Acid]
2.
–
•
Common method for preparing hydrogen in the laboratory.
Molecular or formula unit equation
Al(s) + H 2SO 4aq   Al2 (SO 4 )3aq  + H 2g 
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Displacement Reactions
• Total ionic equation
You do it!
• Net ionic equation
You do it!
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Displacement Reactions
• The following metals are active enough to
displace hydrogen
– K, Ca, Na, Mg, Al, Zn, Fe, Sn, & Pb
Non-metals can displace less active non-metals, we
will just consider the halogens (?). The activity
series for halogens is simple-its their order on the
periodic table. Fluorine is the most active, iodine
the least.
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Displacement Reactions
3.
[Active Nonmetal + Salt of Less Active Nonmetal]  [Less Active
Nonmetal + Salt of More Active
Nonmetal]
•
Molecular equation
Cl 2g  + 2 NaIaq   I 2s   2 NaCl(aq)
Total
ionic equation
Cl 2g  + 2 Na

aq 
+ 2 I aq   I 2s   2 Na
-

aq 
-
+ 2 Cl aq 
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Displacement Reactions
• Net ionic equation
Cl 2g  + 2 I aq   I 2s   2 Cl aq 
-
-
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•
Double-Displacement or
Metathesis
Reactions
Metathesis reactions occur when two ionic
aqueous solutions are mixed and the ions switch
partners.
AX + BY  AY + BX
•
Metathesis reactions remove ions from solution in
two ways:
1. form predominantly unionized molecules like H2O
2. form an insoluble solid
•
Ion removal is the driving force of metathesis
reactions.
35
Metathesis Reactions
1. Acid-Base (neutralization) Reactions
– Formation of the nonelectrolyte H2O
– acid + base  salt + water
– a nonelectrolyte is a compound that does not
produce ions-With the exception of organic acids
and some other organic compounds , all other
molecular compounds are nonelectrolytes.
36
Metathesis Reactions
• Molecular equation
HBr(aq) + KOH (aq)  KBr(aq) + H 2 O (  )
Total
Net
ionic equation
ionic equation
37
Metathesis Reactions
2. Precipitation reactions are metathesis
reactions in which an insoluble compound is
formed.
– The solid precipitates out of the solution much
like rain or snow precipitates out of the air.
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Metathesis Reactions
• Precipitation Reactions
• Molecular or formula unit equation
Ca(NO3 ) 2 (aq) + K 2 CO 3( aq)  2 KNO3( aq ) + CaCO 3(s)
Total
ionic reaction
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Metathesis Reactions
• Net ionic reaction
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Metathesis Reactions
• Molecular or formula unit equation
CaCl 2 (aq) + Na 3PO 4 ( aq)  NaCl( aq ) + Ca 3 PO 4 2(s)
Total
ionic reaction
41
Metathesis Reactions
• Net ionic reaction
42