Section 2.6—Chemical
Reactions
Objectives:
•Define the components of a chemical reaction
•Convert word equations to formula equations
•Define and predict products for double replacement and
neutralization reactions
Chemical Reactions
When substances react, bonds are
broken, atoms are rearranged, and bond
are re-formed to create new compounds.
2 H2 + O2  2 H2O
O
O
Watch as 2 H2 and 1 O2 undergo a chemical reaction
Chemical Reactions
Bonds and atoms are rearranged to
form new compounds.
2 H2 + O2  2 H2O
O
O
The substances in the end are different from those in the beginning.
Bonds are broken and re-formed between different atoms
Chemical Reactions
In chemistry, the “sentence” (showing
the starting materials and the final
products of a chemical reaction) is
called a chemical equation.
Examples of Chemical Equations
Word equation: magnesium metal is reacted with
aqueous hydrochloric acid to produce aqueous
magnesium chloride and hydrogen gas
Formula equation: Mg (s) + 2 HCl (aq)
MgCl2 (aq) + H2 (g)
Visualization
H
Cl
Mg
Cl
H
Cl
Mg
Cl
H H
Parts of a Chemical Equation
The starting materials
for the reaction—each
compound is
separated by a “+”
Formed in the
reaction
Products
Reactants
NaCl (aq) + AgNO3 (aq)  AgCl(s) + NaNO3 (aq)
States of matter
Arrow
s = solid
l = liquid
g = gas
aq = aqueous
(dissolved in water)
Read as:
Yields
Produces
Forms
Makes
etc.
Writing Chemical Equations
You must write each chemical formula
correctly first! (Recall how to write formulas
from names - Section 2.2)
“and”, “is mixed with” or “reacts with” = +
“yield”, “produces” and “forms” = 
Then after they are written, chemical
equations have to be balanced – we’ll do
that in Sec. 2.7!
Let’s Practice #1
Example:
Write the word
equation into
symbol form
Aluminum metal is reacted with
hydrochloric acid to form
aluminum chloride and hydrogen
gas
Let’s Practice #1
Example:
Write the word
equation into
symbol form
Aluminum metal is reacted with
hydrochloric acid to form
aluminum chloride and hydrogen
gas
Al + HCl  AlCl3 + H2
Let’s Practice #2
Example:
Write the word
equation into
symbol form
Copper (II) nitrate and sodium
hydroxide form copper (II)
hydroxide and sodium nitrate
Let’s Practice #2
Example:
Write the word
equation into
symbol form
Copper (II) nitrate and sodium
hydroxide form copper (II)
hydroxide and sodium nitrate
Cu(NO3)2 + NaOH  Cu(OH)2 + NaNO3
Double Replacement Reactions
The cations from two compounds replace
each other.
NaCl + AgNO3  AgCl + NaNO3
Two ionic compounds switch ions
Double Replacement Reactions
The general format of a double replacement reaction:
A
X
B
Z
A
Z
B
X
Products of a Double Replacement
1
Combine the cation of the first reactant with the
anion of the second reactant
CaCl2 + AgNO3 
Products of a Double Replacement
2
Combine the cation of the second reactant with
the anion of the first reactant
CaCl2 + AgNO3 
Products of a Double Replacement
Remember to write cations first …
3 & balance charges with subscripts when
writing formulas
Remember NOT the change the subscripts that are part of
any polyatomic ion!
CaCl2
CaCl2
+ AgNO3
+
AgNO3
Ca(NO3)2
+
AgCl
Neutralization Reactions
Neutralization reactions are double replacement reactions
where one cation is “H” and one anion is “OH” and water
is formed.
General format of a neutralization reaction:
H
X
ACID
B
O
BASE
H
B
X
IONIC
COMPOUND
O
H
H
WATER
Products of a Neutralization Reaction
Combine the cation of the first reactant (the
1 acid) with the anion of the second reactant (the
base). This gives you the water.
H2SO4 + NaOH 
Products of a Neutralization Reaction
2
Combine the cation of the second reactant with
the anion of the first reactant. This gives you
an ionic compound.
H2SO4 + NaOH 
Products of a Neutralization Reaction
3
Remember to write cations first …
& balance charges with subscripts when writing formulas
Only leave subscripts that are in the original compound
there if they are a part of a polyatomic ion!
H2 SO4
H2 SO4
+ Na OH
+
Na OH
Na2SO4
+
HOH
You can write the water as “HOH” when you combine the cation & anion
or you can change it to “H2O”
Let’s Practice #3
Example:
Write the
products for
this reaction
Sr(OH)2 + HBr 
Let’s Practice #3
Example:
Write the
products for
this reaction
Sr(OH)2 + HBr 
SrBr2 + HOH
Let’s Practice #4
Example:
Write the
products for
this reaction
HCl + Ca(OH)2 
Let’s Practice #4
Example:
Write the
products for
this reaction
HCl + Ca(OH)2 
CaCl2 + HOH