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Chemical Reactions
Chemical Reactions
A.
Definition

Chemical reaction- process where one or
more substances are rearranged to form
new substances
Precipitate Formation

If you combine some
substances, a
reaction can occur
forming a solid
called a precipitate
Gas formation

Sometimes when
two substances
come in contact a
reaction occurs
producing a gas.
Permanent color change

Some substances,
when combined,
will turn another
color
Energy is produced in the form of
light

When sodium is
placed in water,
the reaction is so
violent a fire
results
Temperature Change

When some chemicals react they either:
give off energy, exothermic reaction (feel
warm to the touch)
OR
use energy, endothermic reaction, and get
cooler
C. Representing Chemical Reactions
a)
b)
c)
d)
We use chemical equations as a way to represent
reactions
Substances that start the reaction are called
reactants
Substances that are formed are called the products
Reactant 1 + Reactant 2  Product 1 + Product 2
(the number of reactants and products will vary)
II. Describing Chemical Changes
A.
Writing equations

Equations are more practical than words to
express a chemical reaction

It is very important that you note that some
elements do not occur as single atoms when by
themselves. (Ex. Cl2 & NaCl not NaCl2)
A. Writing Equations


If they are not combined with another element,
they will bond with themselves, forming a
diatomic molecule.
In any chemical reaction, when you see these
elements alone, they must be shown with a
diatomic formula.

Diatomic elements are always written as X2 when
isolated in a reaction
2Mg + O2  2MgO
Diatomic Elements

Remember: 7 elements that NEVER occur
as singular atoms (always paired with an
the same or different element)
H2 N2 O2 F2 Cl2 Br2 I2
-BrINClHOF-
Ex:
2 HCl + K
 2 KCl + H2
Br I N Cl HO F
Bromine
Iodine
Nitrogen
Chlorine
Hydrogen
Oxygen
Fluorine
Diatomics
Br I N Cl HO F
Anytime you are writing one
of the Brinclhof elements in a
reaction by itself, you must
add the 2 subscript.
Symbols used in reactions
These symbols must be
memorized!
Symbol
Meaning
+
combine with, reacts with

yields, forms
reversible reaction
(s)
solid
(l)
liquid
(g)
gas
(aq)
aqueous solution (dissolved in
water)
NOTE: All acids will be
aqueous unless stated
otherwise
heat is supplied to the reaction
precipitate is formed
Precipitate = solid formed from
two liquids
gas is released
pressure
pressure applied
Formula of the catalyst
Catalyst
Not in your
notes!
Word Equation

equation expressed in words
Iron (s) + chlorine (g)  iron (III) chloride (s)
Ex 1:
Solid sodium hydrogen carbonate (bicarbonate)
reacts with hydrochloric acid to produce an aqueous
solution of sodium chloride, water and carbon dioxide
gas.
NaHCO3 (s) + HCl (aq)  NaCl (aq) + H2O (l) + CO2

Equations that only show formulas of substances
involved, but are not balanced are called skeletal
equations
Ex 2: Turn this into a sentence
BaCl2 (aq) + Na2SO4 (aq)  BaSO4 (s) + 2NaCl (aq)
An aqueous solution containing 1 mole of Barium
Chloride reacts with 1 mole of aqueous sodium
sulfate to yield 1 mole of solid barium sulfate and 2
moles of aqueous sodium chloride.
Practice
Practice
1. Write equations for each chemical reaction.
a. Sulfur burns in oxygen to form sulfur dioxide.
S + O2  SO2
b. Heating potassium chlorate solid in the presence of
the catalyst manganese (IV) oxide produces oxygen
gas. Potassium chloride is left as a solid.
KClO3(s)
MnO2
O2 (g) + KCl(s)
Practice
Practice
2. Write a sentence that describes each chemical reaction.
a. KOH(aq) + H2SO4(aq)  H2O(l) + K2SO4(aq)
An aqueous solution containing 1 mole of
potassium hydroxide reacts with 1 mole of
sulfuric acid to yield 1 mole of water and 1 mole
of aqueous potassium sulfate.
b. Na(s) + H2O(l)  NaOH(aq) + H2(g)
One mole of solid sodium reacts with 1 mole of
water to yield 1 mole of aqueous sodium
hydroxide and 1 mole of hydrogen gas.
III. Balancing
Chemical Equations
A.
Law of Conservation of Mass



Mass can not be created or destroyed
Since atoms have mass, the number of each
type of atom must be equal on both sides of
a reaction
The sum of the mass of the reactant must
equal the sum of the mass of products
Rules for balancing equations




Only coefficients are changed in
balancing reactions
Never change subscript during balancing
The coefficient also tells you the number
of atoms used in each compound
When no coefficient is written it is
assumed to be 1
Not in your notes!

How many grams of solid silver are produced
Cu(s) + AgNO3(aq)  CuNO3(aq) + Ag(s)
88.4g
63.5g
148.3g
123.4g
?
Ex 1:

Write a balanced equation for the reaction of copper
metal and an aqueous solution of silver nitrate to form
aqueous copper (I) nitrate and silver.
__
1 Cu(s) + __
1 AgNO3(aq)  __
1 CuNO3(aq) + __
1 Ag
Correct Formulas for:
silver nitrate & copper (II) nitrate
Ag+
NO3-
Ag (NO3 )
Parentheses not needed
Cu+2
NO3-
Cu(NO3)2
Parentheses needed
Cu(s) + AgNO3(aq) 
Cu(NO3)2 (aq) + Ag(s)
Example 1
2
2
__ Cu(s)+ __ AgNO3(aq)  __ Cu(NO3)2(aq)+ __ Ag
1 Cu 1
2 1 Ag 1 2
21 N 2
63 O 6
Balanced!
Example 1 again

2
2
__ Cu(s)+ __ AgNO3(aq)  __ Cu(NO3)2(aq)+ __ Ag
1 Cu 1
2 1 Ag 1 2
2 1 NO3 2
Balanced!
Ex 2:
Sodium and chlorine combine to form
sodium chloride.
2 Na + 1 Cl2  2 NaCl
Molar Ratio between sodium and chlorine is
2:1, it takes 2 moles of sodium to completely
react with 1 mole of Chlorine (2 moles of
sodium chloride are produced)
Ex 2:

Balance the following reaction:
Na(s) +
Cl2(g) 
2
2 NaCl(s)
2 1 Na 1 2
2 Cl 1 2
Balanced!
Molar Ratio between sodium and chlorine is 2:1, it takes
2 moles of sodium to completely react with 1 mole of
Chlorine (2 moles of sodium chloride are produced)
Ex 3:
Sodium hydroxide and calcium bromide
react to produce calcium hydroxide and
sodium bromide.
2 NaOH + 1 CaBr2  1 Ca(OH)2 + 2 NaBr
What is the molar ratio between sodium
hydroxide and calcium bromide in this equation?
2:1

Tips for Balancing Chemical Reactions
1.
Write a correct skeletal chemical equation (use
oxidation number to make products)
2.
Identify and Count the number of atoms of each
element in the reactants and products.
3.
Balance the elements one at a time by using
coefficients. (NEVER CHANGE SUBSCRIPTS)



it’s usually easier to begin with elements that
appear only once
save lone elements (i.e. O2) to balance last
many times you can treat polyatomics as one unit
Check each atom or polyatomic ion to be sure the
equation is balanced
4.
•
•
Sometimes it is helpful to think of water as HOH
Finally, make sure all the coefficients are in the
lowest possible whole number ratio that balances.
Practice
Practice
1. Write balanced chemical equations.
a. Sodium and water form aqueous sodium hydroxide and
hydrogen gas.
Na + H2O(l)  NaOH(aq) + H2(g)
b. Aqueous calcium hydroxide and sulfuric acid react to
form aqueous calcium sulfate and water.
Ca(OH)2(aq) + H2SO4(aq)  CaSO4(aq) + H2O(aq)
Practice
Practice
2. Balance the following reactions.
a. 2SO2 +
b.
O2  2SO3
Fe2O3 + 3H2  2Fe + 3H2O
c. 4P + 5O2 
d. 2
1Al +
P4O10
6HCl  2 AlCl3 + 3H2
3
e. 2C2H6 + 7O2  4CO2 + 6H20
IV. Types of Chemical Reactions
A.
Classifying Chemical Reactions
There are 5 basic types:
 Synthesis
 Decomposition
 Single replacement
 Double Replacement
 Combustion
1. Synthesis



Two or more substances combine to
create a more complex substance
(synthesis means “to make”)
Two parts combine and make ONE
product
General Form:
A + B  AB
Hey baby let’s get jiggy.
Synthesis

reaction of two elements

Ex:
2 Al + ___
3 Cl2  ___
2 AlCl3
___
Al
3+
1Cl
2. Decomposition



A complex substance is broken down
into two ore more simpler substances
(decompose means “to break down”)
one substance breaks down into
more than one simpler products
General Form:
AB  A + B
Break yoself fool!
Decomposition
Ex:
2 NaCl  Na + Cl2
3. Single Replacement Reactions


one element replaces another
element in a compound to form new
compound
General Form:
A + BX  AX + B
I’m gon’ dance with yo’ lady
3. Single Replacement Reactions


Ex:
aluminum + iron (III) oxide
2 Al
+ 1 Fe2O3
2 Fe + 1 Al2O3
4. Double Replacement



Ions from two ionic compounds switch places
Basically: an exchange of cations between two ionic
compounds
When a solution of sodium sulfate and barium
chloride are mixed,
the sulfate anion combines with the barium cation
and this combination precipitates as barium sulfate.
The newly formed sodium chloride remains soluble.
Na2SO4(aq) + BaCl2(aq) 
BaSO4(s) + 2 NaCl(aq)
4. Double Replacement

General Form:
AB + CD  AD + CB
switch
4. Double Replacement
4. Double Replacement

Ex:
Li1+

I1-
Ag1+ NO31-
lithium iodide and aqueous silver nitrate
react
LiI + AgNO3
LiNO3 + AgI(s)
5. Combustion



A substance combines with oxygen, releasing a
large amount of energy in the form of light or
heat
On the reactant side, look for a hydrocarbon (a
compound made of C and H) or an oxygenated
hydrocarbon (a compound made of C, H and O),
plus a diatomic oxygen.
Basically, a compound reacts with O2 to produce
energy, CO2 gas and a hydrocarbon
5. Combustion

Combustion of hydrocarbons
ALWAYS produces CO2 and H2O
General Form:
CxHy + O2
CO2 + H2O
5. Combustion

Ex:
Show combustion of propane (C3H8) gas
1 C3H8 + 5 O2
3 CO2 + 4 H2O
I sell propane
and propane
accessories!
V.
Predicting Products
from chemical reactions
Synthesis:
1.
Solid sodium reacts with chlorine gas.
Diatomic Element
Use oxidization #’s to
make a formula unit
___
2 Na(s) + ___
1 Cl2(g)  ___
2 NaCl
V.
Predicting Products
from chemical reactions
Synthesis:
1.
Solid zinc reacts with chlorine gas.
Diatomic Element
Use oxidization #’s to
make a formula unit
___
2 Zn(s) + ___
1 Cl2(g)  ___
2 ZnCl
V.
Predicting Products
from chemical reactions
Decomposition:
1.
Sodium Azide (NaN3) decomposes
Separate Elements
Diatomic atoms
2 NaN3(s)  ___
2 Na (s) + ___
3 N2(g)
___
This is used in car air bags.
V.
Predicting Products
from chemical reactions
Decomposition:
2.
Aluminum oxide decomposes.
Al3+
Separate Elements
O2Diatomic atoms
2 Al2O3(s)  ___
4 Al(s) + ___
3 O2(g)
___
V.
1.
Predicting Products
from chemical reactions
Single Replacement Reactions
To predict products of SR reactions, create a new
compound with the lone metal ion and the anion from the
reactanct compound. The cation from the reacctant
compound becomes a lone element on the product side.
Identify the lone element as a cation or anion (by its
oxidation number), and the anion and cation of the
compound on the reactant side of the equation
Ex 1:
2+
1-
Mg(s) + Zn(NO3)2(aq)  Mg(NO3)2 + Zn
Cation
Anion
V.
1.
Predicting Products
from chemical reactions
Single Replacement Reactions
The two cations will trade places.
Use oxidation numbers to make the compound.
Ex 1:
2+
1-
Mg(s) + Zn(NO3)2(aq)  Mg(NO3)2 + Zn
Cation
Anion
Not in notes!
Problem

Single Replacement Reactions
Identify the lone element as a cation or
anion (oxidation number), and the anion and
cation of the compound on the reactant side of
the equation
F2 + NaBr 
Not in notes!
Problem

Single Replacement Reactions
Identify the lone element as a cation or
anion (oxidation number), and the anion and
cation of the compound on the reactant side of
the equation
F2 + NaBr 
-1 +1 -1
Not in notes!
Problem

Single Replacement Reactions
In this reaction the two anions switch place
(remember + goes with a -)
F2 + NaBr  NaF + Br2
-1 +1 -1
Not in notes!
Problem

Single Replacement Reactions
Balance
F2 + 2NaBr  2 NaF + Br2
Ex 2:
Mg(s) + Ag(NO3)(aq) 
+2
+1 -1
Switch:
Mg(s) + Ag(NO3)(aq)  Mg(NO3)2 + Ag
Balance:
Mg(s) + 2Ag(NO3)(aq)  Mg(NO3)2 + 2Ag
Practice
1. Write a balanced reaction for each SR reaction.
a. Zn(s) + H2SO4(aq)  ZnSO4 + H2
b. 2Na(s) + 2H2O(l)  2 NaOH + H2
HOH
c. Sn(s) + 2NaNO3(aq)  Sn(NO3)2 +2Na
(use tin(II))
d. Cl2(g) + 2NaBr  2 NaCl + Br2
2. Double replacement reactions
To predict products of a double replacement
reaction, simply switch ions, being very careful to
create new formulas from the new combinations
(assume the constant oxidation #’s)


Identify the anions and cations of both compounds
(do not separate polyatomic ions!)
Switch the cation of one compound with the cation of
the other compounds.
2. Double replacement reactions
Ex 1:
Pb (NO3)2(aq) + KI(aq) 
+2 -1
+1 -1
USE OXIDATION
NUMBERS TO FORM
Switch:
PRODUCTS
Pb(NO3)2(aq) + KI(aq)  PbI2 + K(NO3)
Balance:
1Pb(NO3)2(aq) + 2KI(aq)  1PbI2 + 2K(NO3)
2. Double Replacement Reactions
Remember, iron needs
a roman numeral!
Ex 2:
FeS(s) + HCl (aq) 
+2 -2
+1 -1
Iron (II)
Switch:
FeS(s) + HCl (aq)  FeCl2 + H2S
Balance:
FeS(s) + 2HCl (aq)  FeCl2 + H2S
2. Double replacement reactions
Ex 3:
HCl(aq) + Na(OH) 
+1 -1
+1 -1
Switch:
HCl(aq) + Na(OH)  NaCl + HOH (or H2O)
Balance:
Practice
1. Write balanced equations for these DR reactions.
a. NaOH + Fe(NO3)3  Na(NO3) + Fe(OH)3
b. 3KOH (aq) + H3(PO4) (aq)  K3(PO4) + 3HOH
c. NaCl + KBr  NaBr + KCl
3. Combustion Reactions
All combustion reactions will have products of CO2
& H2O.
Ex 1:
C2H4 + O2 
^All produce CO2 + H2O^
C2H4 + O2  CO2 + H2O
Balance:
C2H4 + 3O2  2CO2 + 2H2O