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Chemical Reactions
I. Chemical Reactions
A. Definition
Chemical reaction- process where one or
more substances are rearranged to form
new substances
B. Indications/Signs of a Chemical Reaction
Sign of a Chemical Reaction:
Precipitate Formation
Precipitate is a solid
that forms from two
liquids - suspension
Sign of a Chemical Reaction:
Gas Formation
Sometimes when
two substances
come in contact a
reaction occurs
producing a gas.
Sign of a Chemical Reaction:
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
Sign of a Chemical Reaction:
Temperature Change
• Exothermic reactions feel warm
• Endothermic reactions feel cold
Sign of a Chemical Reaction:
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
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
C. Representing Chemical
Reactions
Reactant 1 + Reactant 2 
Product 1 + Product 2
(the number of reactants and products will vary)
C. Representing Chemical
Reactions
The large number in front of elements or
compounds are called coefficients
Cu + 2AgNO3  Cu(NO3)2 + 2Ag
Determining States of Matter
• Most states of matter will be provided
• Acids are aqueous, unless stated
otherwise
• Metals are solid at room temperature,
except mercury (liquid)
Determining States of Matter
• Most diatomics are gases at room
temperature (bromine is liquid and
iodine is solid)
• For products that are ionic compounds
in water: use the solubility rules on the
back of your periodic table to
determine the state of matter. Insoluble
substances will exist as solids.
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
“brinclhof” elements can not exist alone
(Ex. Cl2 & NaCl not NaCl2)
A. Writing Equations
 If they are not combined with a
different element, they will bond with
themselves, forming a diatomic
molecule.
Br I N Cl HO F
Bromine
Iodine
Nitrogen
Chlorine
Hydrogen
Oxygen
Fluorine
Diatomics
Br I N Cl HO F
Symbols used in reactions
These symbols must
be memorized!
NOTE: All acids will
be aqueous unless
stated otherwise
Common Compounds to Know
NH3 = ammonia
CH4 = methane
Word Equation
Not in
your
notes!
equation expressed in words
Iron (s) + chlorine (g)  iron (III) chloride (s)
Equations that only show formulas of
substances involved, but are not balanced
are called skeletal equations
Ex 1:
Solid sodium hydrogen carbonate
(bicarbonate) reacts with hydrochloric acid
to produce aqueous sodium chloride, water,
and carbon dioxide gas.
(not balanced)
NaHCO3(s) + HCl (aq) NaCl(aq) + H2O(l) + CO2 (g)
All acids will be aqueous
Ex: In your notes
BaCl2 (aq) + Na2SO4 (aq) 
BaSO4 (s) + 2NaCl (aq)
“A solution containing 1 mole of aqueous
barium chloride reacts with an aqueous
solution containing 1 mole of sodium sulfate to
yield 1 mole of solid (or precipitated) barium
sulfate and 2 moles of aqueous (dissolved)
sodium chloride.”
Practice
Practice
1. Write equations for each chemical reaction.
a. Sulfur burns in oxygen to form sulfur dioxide.
S + O2 (g)  SO2
No Δ !!
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.
Practice (Not in your notes)
Hydrofluoric acid (HF) is used to etch grooves in
glass because it reacts with the silica (SiO2(s)) in
glass. The products of the reactions are aqueous
silicon tetrafluoride and water. What is the
coefficient of silica in the balanced equation for the
reaction?
HF(aq) + SiO2(s)  SiF4 (aq) + H2O(l)
III. Balancing Chemical
Equations
A. Law of Conservation of Mass
•
•
•
Mass can not be created nor 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 reactants
must equal the sum of the mass of
products
Rules for balancing equations
•
When balancing an equation you can
only add coefficients, never add or
change the subscripts
(Ex. For 3H2O,
3 is the coefficient and 2 is the subscript)
Coefficient vs. Subscript
Example: 3H2O
H2O = formula for water molecule
1 molecule of water = 2 Hydrogens & 1 Oxygen
Coefficient of 3 means there are 3 molecules of water
3 molecules of water = 6 Hydrogens & 3 oxygens
Rules for balancing equations
•
The coefficient tells you the number of
particles of the element or compound
used/produced in the reaction.
(Ex. In 3H2O, there are 3 molecules of water)
Particles can be:
atoms, molecules, formula units, or moles
4Fe(s) + 3O2(g)  2Fe2O3(s)
4 atoms of iron used
3 molecules of oxygen used
2 formula units of iron (III) oxide produced
The coefficient also tells you the number of moles of
the element or compound used/produced in the
reaction.
4Fe(s) + 3O2(g)  2Fe2O3(s)
4 moles of iron used
3 moles of oxygen used
2 moles of iron (III) oxide produced
• For the sake of space, we sometimes
leave off the state of matter after each
of the substances
• 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)
63.5g
148.3g
123.4g
?
88.4g
A balanced equation
CH4(s) + 2 O2(g)  CO2(g) + 2 H2O(g)
Reactants
Products
Carbon
1
1
4
Hydrogen 4
4
4
oxygen
Ex 1:
Write a balanced equation for the
reaction of copper metal and an
aqueous solution of silver nitrate to
form copper (II) nitrate and silver.
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.
Na +
Cl2 
NaCl
Ex 2:
Balance the following reaction:
Na(s) + Cl2(g)  NaCl(s)
2
2
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)
Extra Example
Balance the following reaction:
Al(s) + O2(g)  Al2O3(s)
42
3
4 2 1
6 2
2
Al 2 4
O 3 6
Balanced!
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
4) Check each atom or polyatomic ion
to be sure the equation is balanced
• 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.
2Na + 2H2O(l)  2NaOH(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) + 2H2O(aq)
Practice
Practice
2. Balance the following reactions.
a. 2SO2 +
b.
O2  2SO3
Fe2O3 + 3H2  2Fe + 3H2O
c. 4P + 5O2 
d. 21Al +
P4O10
6HCl  2 AlCl3 + 3H2
3
e. 2C2H6 + 7O2  4CO2 + 6H20
More Practice
2
2
2
4
4
2
42
2
Xe 1 2
F 1 2 4
H1 2 4
O2
___ XeF2 + ___ H2O  __ Xe + __ O2 + __ HF
1
2
2
1
Practice (Not in your notes)
Hydrofluoric acid (HF) is used to etch grooves in
glass because it reacts with the silica (SiO2(s)) in
glass. The products of the reactions are aqueous
silicon tetrafluoride and water. What is the
coefficient of silica in the balanced equation for the
reaction?
1
4 HF(aq) + SiO2(s) 
41 H
41 F
1 Si
2 O
SiF4 (aq) + 2 H2O(l)
24
4
1
12
IV. Types of Chemical Reactions
A. Classifying Chemical Reactions
There are 5 basic types of chemical reactions:
• 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 or more parts combine and
make ONE product
General Form:
A + B  AB
EX: 2Na + Cl2  2NaCl
Hey baby let’s get jiggy.
Synthesis
aka
Combination
Element + Element  Compound
Synthesis
reaction of two elements
Ex:
2 Al + ___
3 Cl2  ___
2 AlCl3
___
Al
3+
Cl
2. Decomposition
• A complex substance is broken
down into two ore more simpler
substances (decompose means “to
break down”)
 Look for (usually) one reactant and
more than one product to identify a
DECOMPOSITION REACTION.
General Form:
AB  A + B
Break yoself fool!
Decomposition
Ex:
2 NaCl  2Na + 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
Single Replacement
For Single Replacement Reactions
FYI: Not all single replacement reactions
will occur. You must consider the
activity series. Metals that are lower on
the activity series are “less active” and
will not displace more metals. Although
Hydrogen is not a metal, it can act as a
metal and is included in the series.
4. Double Replacement
Ions from two ionic compounds switch
places
Basically: an exchange of cations between
two ionic compounds
(H2O and HOH are the same thing)
4. Double Replacement: ions from
two ionic compounds switch places
Double Replacement
4. Double Replacement
General Form:
AX + BY  AY + BX
switch
4. Double Replacement
4. Double Replacement
Ex:
Li1+
I-
Ag1+ NO3-
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.
5. Combustion
Combustion of hydrocarbons
ALWAYS produces CO2 and H2O
General Form:
CxHyO + 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!
Practice (Not in your notes)
Tin was among the first metals used by humans.
Elemental tin is produced by heating tin(IV)
oxide, the principal ore of tin, with carbon. The
products of this reaction are tin and carbon
dioxide. When the equation is written and
balanced, ID the type of reaction.
SnO2 + C  Sn (s) + CO2 (g)
Sn4+ O2O2-
Single
Replacement
V. Predicting Products for
Replacement Reactions
To predict products of chemical
reactions, it is important that you
recognize ions
V.
Predicting Products
from chemical reactions
a. Single Replacement Reactions (SR)
To predict products of SR reactions, create a
new compound with the lone metal ion
and the anion from the reactant
compound. The cation from the reactant
compound becomes a lone element on
the product side.
•You must consider the
activity series
(on the back of your
periodic table)
•NR = no reaction
Ex 1
Mg(s) + Zn(NO3)2(aq) 
Mg
Zn2+ NO3-
2+
+
Mg(NO3)2 + Zn
Cation replaces cation
Ex 1
Mg(s) + Zn(NO3)2(aq) 
Magnesium + Zinc nitrate
Magnesium nitrate + Zinc
Mg2+
NO3Zn
Mg(NO3)2 + Zn
Mg(s) + AgNO3(aq) 
Magnesium + silvernitrate
Magnesium nitrate + silver
Mg2+
NO3Ag
Mg(NO3)2 + Ag
Ag(s) + Mg(NO3)2(aq) 
silver + Magnesium nitrate
NR
Practice
1.Write a balanced reaction for each SR
reaction.
a. Zn(s) + H2SO4(aq)  ZnSO + H
4
2
b. 2 Na(s) + 2H2O(l) 2 NaOH + H2
HOH
c. Sn(s) + NaNO3(aq)  NR
b. Double Replacement Reactions and
Precipitates
to predict products of a double replacement
reaction, simply switch ions, being very
careful to create new formulas from the
new combination (assume constant
oxidation #’s)
Ex 1
Pb(NO3)2(aq) + KI(aq) 
Lead (II) nitrate + Potassium iodide
Lead (II) iodide + potassium nitrate
PbI2 + KNO3
Ex 2
FeS(s) + HCl(aq) 
Iron (II) sulfide + hydrogen chloride
Iron (II) chloride + Hydrogen sulfide
FeCl2 +H2S
Ex 3
HCl(aq) + NaOH(aq) 
hydrogen chloride + sodium
hydroxide
hydrogen hydroxide + sodium chloride
HOH
+NaCl
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
c. Combustion Reactions
All combustion reactions will have products of
CO2 & H2O.
Ex 1:
C2H4 + O2 
^All produce CO2 + H2O^
C2H4 + O2  CO2 + H2O
Balanced:
C2H4 + 3O2  2CO2 + 2H2O