Unit 8
Chemical Reactions
What is a Chemical Reaction?
 When
a substance is changed into
another substance by chemical
means
 Reactants = what you start with
 Products = what you end up with
During a Chemical Reaction…
 Bonds
are formed, broken, or both
 Atoms are rearranged, NOT created
or destroyed
 Conservation of Atoms
 Law of Conservation of Mass
Evidence of Chemical Reactions
 Evolution/production
of a gas
 Bubbling or odor change
 Forming a precipitate
 Two clear solutions  cloudy/particles
 Release or absorb energy
 Change temperature or give off light
 Color change in reaction system
Writing Chemical Equations

Reactants  Products

HC2H3O2 + NaHCO3  NaC2H3O2 + H2O + CO2

Can also include info about state of matter
s
= solid
 l= liquid
 g = gas
 aq = aqueous solution (water solution)
 HC2H3O2 (aq) + NaHCO3 (s)  NaC2H3O2 (aq)+
H2O (l) + CO2 (g)
Writing Chemical Reactions
Pt
  is used to indicate a catalyst
used in the reaction (in this case, it’s
platinum)
 What is a catalyst?

A
substance that speeds up a reaction
without being changed by the reaction.
 Enzymes are biological catalysts.
Writing Chemical Reactions
Word Equations – replaces symbols and
formulas with names
 Cu + Cl  CuCl2
 Copper + chlorine  copper (II) chloride
 Fe + O2  Fe2O3
 Iron + Oxygen → Iron (III) oxide

Balancing Chemical
Equations
Balancing Equations
Law of Conservation of Mass
 Mass of products = Mass of reactants
 Atoms can’t be created or destroyed
 A balanced equation has the same
number of atoms of each element on
both sides of the equation.

C
+
O
O

O C
C + O2  CO2
 Start with 1 carbon, 2 oxygens
 End with 1 carbon, 2 oxygens
 This equation is already balanced

O
C
+
O
O

C
O
C + O2  CO
 We need one more oxygen in the products.
 We can’t change the formula, because it
describes what actually happens

C
+
O
O

C
O
The O must be used to make another CO
 But where did the other C come from?

Must have started with two C
 Balanced equation = 2 C + O2  2 CO

Rules for balancing
 Write the correct formulas for all the
reactants and products
 Count the number of atoms of each type
appearing on both sides
 Balance the elements one at a time by
adding coefficients (the numbers in front)
 Check to make sure it is balanced.
Never…

Change a subscript to balance an
equation.
 If
you change the formula you are describing
a different reaction.
 H2O

is a different compound than H2O2
Put a coefficient in the middle of a formula
2
NaCl is okay, Na2Cl is not.
How do I balance an equation?
Start with elements that only appear once
on each side.
 CH4 + O2  CO2 + H2O
 If an atom appears more than once on a
side, balance it last.
 CH4 + O2  CO2 + H2O

Example
2 H2 + O2  H22H
O2O
R
P
4 2 H 2 4
2 O 1 2
The equation
Make
Changes
Also
Need
Recount
changes
twice
a table
theasO
to
is
the
much
keep
balanced,
HH
O
track
in the
has
of where
product
reactant
the same
you
number
are at of each kind of atom on both sides
Example
2 H2 + O2 
2 H2O
R
P
4 2 H 2 4
2 O 1 2
This is the answer
Not this
Example
N2 + H2  NH3
Examples
P+
O2 
P4O10
Examples
Al + N2 
Al2N3
Examples
CH4 + O2 
CO2 +
H2O
Examples
AgNO3 +
Cu 
Cu(NO3)2 +
Ag
Examples
Na +
H2O  H2 + NaOH
Examples
AgNO3 + H2S → Ag2S + HNO3
Word Equations
Iron (III) chloride + calcium hydroxide →
Iron (III) hydroxide + calcium chloride
FeCl3 + Ca(OH)2 → Fe(OH)3 +
CaCl2
Types of Chemical Rxns
 Synthesis:
2 things combine to form 1
 R + S → RS
 Decomposition: 1 thing breaks down to
form 2 or more products
 RS → R + S
Types of Chemical Rxns
Single-Replacement: 1 element replaces a
single element in a compound
 T + RS → TS + R
 Double-Replacement: 2 elements (ions)
switch
 RS + TU → RU + TS
 Combustion Rxn: must have O2 as a
reactant.
 Ex. 2C8H18 + 25O2 → 16CO2 + 18H2O

Synthesis Reactions
2 elements or compounds combine to
make 1 compound
 2Ca +O2 CaO

SO3 + H2O  H2SO4
 We can predict the products if there are
two elements as reactants
 Mg +
N2  MgN
 Mg +
N2  Mg3N2

Write and balance

Al +
O2 

First step - write the formula

Next – balance the equation
Write and balance

Ca +
Cl2 
Write and balance

Fe + O2 
Decomposition Reactions

One reactant breaks apart into two or
more elements or compounds
electricity
NaCl 

 Na + Cl2

 CaCO3   CaO + CO2

Decomposition Reactions
We can predict the products if the
reactant is a binary compound
 Binary compound = Made up of only two
elements


electricity
H2O   
 H2 + O2
Decomposition Reactions

HgO

 
Decomposition Reactions
If the reactant compound has more than
two elements, you must be given one of
the products
 The other product will be from the missing
pieces

 NiCO3   NiO + C

Single Replacement
One element replaces another
 Reactants must be an element and a
compound.
 Products will be a different element and a
different compound.
 Na + KCl  K + NaCl
 F2 + LiCl  LiF + Cl2

Single Replacement
Metals replace metals (and hydrogen)
 Al + CuSO4 
 Zn + H2SO4 
 Water - Think of water as HOH
 Metals replace one of the H to combine
with hydroxide
 Na + H2O 
 Na + HOH 

Single Replacement
When it comes to metals, we can
actually predict whether a
reaction is possible
 Some metals are more active
than others
 More active replaces less active
but not vice versa
 Can replace an element below it,
but not above it on this list.

Single Replacement

Al + HCl 
Single Replacement

Fe + CuSO4 
Single Replacement

Pb + KCl 
Single Replacement
 Nonmetals
can replace other nonmetals
 F2 + HCl 
 Br2 + KCl 
F2 + 2 LiCl  2 LiF + Cl2
F
Li
Cl
Li
Cl
F
Na + KCl  K + NaCl
Na
K
Cl
Double Replacement
 Two
things replace each other
 Reactants must be two ionic
compounds or acids
 NaOH + FeCl3 
 The positive ions change place.
 NaOH + FeCl3 Fe3+OH- + Na+Cl NaOH + FeCl3 Fe(OH)3 + NaCl
3NaOH + FeCl3 Fe(OH)3 + 3NaCl
Na+
O-
H+
Cl-
Na+
O-
H+
Cl-
Fe3+
Cl-
Na+
O-
H+
Complete and balance
 Assume
all of the reactions are possible
 CaCl2 + NaOH 
 CuCl2 + K2S 
Complete and balance

KOH + Fe(NO3)3 

H3PO4 + Ca(OH)2 
Combustion Reactions
A compound composed of only C, H, and
maybe O is reacted with oxygen
 Composed of C & H = hydrocarbon
 O2 will always be the second reactant
 If the combustion is complete, the
products will be CO2 and H2O
 If the combustion is incomplete, the
products will be C, CO, & H2O

Examples

Complete combustion of C4H10

Incomplete combustion of C4H10
Examples

Complete combustion of C6H12O6

Incomplete combustion of C2H6O