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Chapter 11
Chemical Reactions
Hingham High School
Mr. Dan Clune
All chemical reactions
• Two parts:
• Reactants – what you start with
• Products- what you end with
• Reactants turn into the products.
• Reactants  Products
In a chemical reaction
• The way atoms are joined is
changed.
• Atoms aren’t created
or destroyed.
In a chemical reaction
• Can be described several ways:
1. In a sentence
Copper reacts with chlorine to
form copper (II) chloride.
2. In a word equation
Copper + chlorine  copper (II) chloride
Symbols in equations-p.323
Cu + Cl2  CuCl2
• Arrow () separates reactants from
products
• Read “reacts to form”
• Plus (+) sign read “and”
Symbols used in equations
Cu(s) + Cl2(g)  CuCl2(s)
•
•
•
•
(s) = solid
(g) = gas
(l) = liquid
(aq) - dissolved in water,
an aqueous solution.
Symbols used in equations
• ↑ after product, indicates
gas produced
•same as (g) - H2↑
•after product, indicates
solid produced
•same as (s) - PbI2↓
Symbols used in equations
indicates reversible
reaction

heat
shows that
heat is supplied to the reaction
Pt
  is - indicates a catalyst
is supplied, in this case, platinum.
  ,   
What is a catalyst?
• Speeds up a reaction
• Is NOT changed or
used up by the reaction.
• Enzymes are biological or protein
catalysts.
Skeleton Equation
• Uses formulas to describe a reaction
• doesn’t indicate how many.
• NOT balanced
Convert this to an equation
• Solid iron (III) sulfide reacts with
gaseous hydrogen chloride to form
iron (III) chloride and
hydrogen sulfide gas.
Fe2S3(s) + HCl(g) 
FeCl3 + H2S(g)
Nitric acid dissolved in water reacts with
solid sodium carbonate to form liquid
water, carbon dioxide gas,
sodium nitrate dissolved in water.
HNO3(aq)
H2O(l)
+
Na2CO3(aq)
+ CO2(g) +

NaNO3(aq)
Now, read these:
• Fe(s) + O2(g)  Fe2O3(s)
• Cu(s) + AgNO3(aq) 
Ag(s) + Cu(NO3)2(aq)
• NO2 (g)
Pt
 
N2(g) + O2(g)
Balancing Chemical
Equations
Balanced Equation
• Atoms can’t be created or destroyed
• All the atoms we start with
we must end up with
• A balanced equation has the same
number of each element on
both sides of the equation.
C
+
O
O

O C
O
•
C + O2  CO2
• This equation is already balanced
• What if it isn’t?
C
+
O
O

C
O
•
C + O2 
CO
• Need one more O in the products.
• Can’t change the formula,
because it describes what it is (carbon
monoxide in this example)
C
+
O
O

C
O
C
O
• Must be used to make another CO
• But where did the other C come from?
C
+
C
O
O

C
O
C
O
• Must have started with two C
• 2 C + O2  2 CO
Finding the number of atoms
• The subscript in front of an element is the
number of atoms of that element / polyatomic.
• Ex) CO2
B2(SO4)3
• C= 1
O= 2
B= 2
SO4= 3
• A coefficient in front of the formula multiplies
the amount of elements by the coefficient.
• Ex) 3CO2
2B2(SO4)3
• C= 3
O= 6
B= 4
SO4= 6
Finding the number of atoms
H2 O
H= 2
O= 1
2H2O
H= 4
O= 2
B(NO3)2
B= 1
NO3= 2
3B(NO3)2
B= 3
NO3= 6
Pb3(PO4)4
Pb= 3
PO4= 4
2Pb3(PO4)4
Pb= 6
PO4= 8
Rules for balancing:
. Determine the
correct formulas
for reactants and products.
. Write a skeleton equation.
Rules for balancing:
3. Count the number of atoms
of each element appearing
on both sides of the equation.
4. Balance the elements one at a
time by adding coefficients
(the numbers in front)
-Save H and O until LAST!
Rules for balancing:
5. Check to make sure it is balanced.
6. Make sure the coefficients are
in the lowest possible ratio.
Don’t you ever…
• Never change a subscript
to balance an equation.
• H2O is a different compound than H2O2
• Never put a coefficient in the
middle of a formula
X
• 2 NaCl is okay, Na2Cl is not.
Example
2 H2 + O2  2 H2O
R
P
4 2
H
2 4
2
O
1 2
2 H2 + O2  2 H2O
2 AgNO3 + Cu  Cu(NO3)2 + 2Ag
R
P
21
1
2 1
Ag
Cu
NO3
12
1
2
3Mg + N2  Mg3N2
R
P
3 1 Mg
2
N
3
2
4 P + 5 O2  P4O10
R
41
10 2
P
P
4
O
10
2 Na + 2H2O  H2 + 2 NaOH
R
21
4 2
2 1
P
Na
H
O
12
3 4
1 2
CH4 + 2 O2  CO2 + 2 H2O
R
P
1
4
42
C
H
O
1
2 4
3 4
Section 11.2
Types of Chemical Reactions
• OBJECTIVES:
• Identify a reaction as combination,
decomposition, single-replacement, doublereplacement, or combustion
Types of Reactions
• 5 major types.
• predict the products
• predict whether or not they will happen at all
• How? We recognize them by their reactants
#1 - Combination Reactions
• Combine - put together
• 2 or more substances combine to
make one compound.
A + B 
• Ca +O2 CaO
• SO3 + H2O  H2SO4
#1 - Combination Reactions
• We can predict the products if
they are two elements.
3 Mg + N2 Mg3N 2
Write and balance
• Ca + Cl2  CaCl2
• 2 Fe + O2  iron (II) oxide 2 FeO
• 4 Al + 3O2  2 Al2O3
• Remember that the first step is to write
the correct formulas
• Then balance by using coefficients only
#1 – Combination Reactions
• Additional Notes:
a) Some nonmetal oxides react with
H2O - produces acid:
SO2 + H2O H2SO3
(how “acid rain” forms)
b) Some metallic oxides react with
H2O - produces base:
CaO + H2O Ca(OH)2
#2 - Decomposition Reactions
• Decompose = fall apart
• One reactant falls apart into
two or more elements or
compounds.
AB  A + B
#2 - Decomposition Reactions
•NaCl   
Na + Cl2

•CaCO3   CaO + CO2
•Note that energy is
usually required to
decompose
electricity
#2 - Decomposition Reactions
• Binary compounds (made of 2 elements)
falls apart into its elements
• 2H2O
• 2 HgO
electricity



 
2 H2
O
+ 2
2 Hg + O2
#2 - Decomposition Reactions
• If the compound has more than two
elements, one product must be given.
• The other product will be from the
missing pieces

• NiCO3  
CO2 + Ni
• H2CO3(aq) CO2 + ?
H2
#3 - Single Replacement Reactions
• One element replaces
another (new dance partner)
• Reactants are an element & cpd
• Products will be a different
element and different cmpd
• Li + KCl  K + LiCl
(Cations switched)
• F2 +2 LiCl  2LiF + Cl2
(Anions switched)
#3 Single Replacement Reactions
• Metals replace other metals (and H)
• 3 K + AlN  K3N + Al
• Zn + 2 HCl 
ZnCl2 + H2
• Think of water as: HOH
• Metals replace first H, then combines w/
hydroxide (OH).
• 2 Na + 2 H2O  2 NaOH + H2
#3 Single Replacement Reactions
• Sometimes, the reaction will not happen:
Some chemicals are more “active”
than others
• More active replaces less active
The “Activity Series” of Metals
Higher • Lithium
activity • Potassium
• Calcium
• Sodium
• Magnesium
• Aluminum
• Zinc
• Chromium
• Iron
• Nickel
• Lead
• Hydrogen
• Bismuth
• Copper
• Mercury
• Silver
Lower
• Platinum
activity
• Gold
If the lone metal is above
the paired metal,
replacement will occur.
Ex) Li + NaCl  Na + LiCl
If the lone metal is below
the paired metal,
replacement will not occur.
Ex) Na + LiCl  Na + LiCl
The “Activity Series” of Halogens
Higher Activity
Fluorine
Chlorine
Bromine
Iodine
If the lone halogen is
above the paired
halogen, replacement
will occur.
Lower Activity
2NaCl(s) + F2(g)  2NaF(s) +
Cl2(g)
MgCl2(s) + Br2(g)  No Reaction!
#3 Single Replacement Reactions Practice:
Higher • Lithium
activity • Potassium
• Calcium
• Sodium
• Magnesium
• Aluminum
• Zinc
• Chromium
• Iron
• Nickel
• Lead
• Hydrogen
• Bismuth
• Copper
• Mercury
• Silver
Lower
• Platinum
activity
• Gold
• Fe + CuSO4 
FeSO4 + Cu
• Pb + KCl  No reaction
• 2 Al + 6 HCl  2 AlCl3 + 3 H2
#4 - Double Replacement Reactions
• Two things replace each other.
• Reactants must be two ionic
compounds.
• NaOH + FeCl3 
• positive ions change place
• NaOH + FeCl3 Fe+3 OH- + Na+1 Cl-1
=
NaOH + FeCl3 Fe(OH)3 + NaCl
#4 - Double Replacement Reactions
• Have certain “driving forces”, or reasons
• only happens if one product:
a) doesn’t dissolve in water & forms solid
(a “precipitate”), or
b) is gas that bubbles out, or
c) is molecular compound (usually water)
Complete and balance:
• assume all of the following reactions
actually take place:
CaCl2 + 2 NaOH  Ca(OH)2 + 2 NaCl
CuCl2 + K2S  CuS + 2 KCl
3 KOH + Fe(NO3)3  3 KNO3 +
Fe(OH)3
K2SO4 + BaF2  2 KF +
BaSO4
How to recognize which type?
• Look at the reactants:
El + El = Combination
Cpd
= Decomposition
El + Cpd
= Single replacement
Cpd + Cpd = Double replacement
Practice Examples:
•
•
•
•
•
•
•
2 H2 + O2 
2H2O
2 H2O 2 H2 + O2
Zn + H2SO4  H2 + ZnSO4
2 HgO  2 Hg + O2
2 KBr + Cl2  2 KCl + Br2
AgNO3 + NaCl  AgCl + NaNO3
Mg(OH)2 + H2SO3  MgSO3 + 2 H2O
#5 – Combustion Reactions
• Combustion means “add oxygen”
• Normally, a cpd composed of only C, H,
(sometimes O) is reacted with oxygen –
called “burning”
• Complete combustion, products are
CO2 and H2O
Combustion Reaction Examples:
• C4H12 + 7 O2 
4 CO2 + 6 H2O
•
C6H12O6 + 9 O2  6 CO + 6 H O
2
2
•
C8H8 + 10O2  8 CO + 4 H O
2
2
SUMMARY: An equation...
•
•
•
•
Describes a rxn
Must be balanced
only balance by changing coefficients
special symbols to indicate physical state,
catalyst or energy required, etc.
Reactions
• 5 major types
• We can tell what type they are by looking at
reactants
• Single Replacement happens based on the
Activity Series
Section 11.3
p. 342
Reactions in Aqueous Solution
NiCl2
Co(NO3)2
K2Cr2O7
K2CrO4
CuSO4
KMnO4
Net Ionic Equations
• Many reactions occur in water,
or aqueous solution
• When dissolved in water, many ionic
cpds “dissociate”, or separate,
into cations & anions
• Now write ionic equation
Net Ionic Equations
• Example (needs to be a double replacement reaction)
AgNO3 + NaCl  AgCl + NaNO3
1. this is the full balanced equation
2. next, write it as ionic equation by splitting
the cpds into their ions:
Ag1+ + NO31- + Na1+ + Cl1- 
AgCl(s) + Na1+ + NO31Solids do not split up.
Net Ionic Equations
3. Crossing out ions that did not change
(called spectator ions)
Ag1+ + Cl1-  AgCl (s)
This is the net ionic equation
Let’s talk about precipitates before we do
some other examples
Predicting the Precipitate
• Insoluble salt is a precipitate
• i.e. a solid
• General solubility rules are found:
a) Table 11.3, p. 344 in textbook
Solubility Rules
BaCl2 + 2 AgNO3 → Ba(NO3)2 + 2 AgCl (s)
1. Break up into ions
Ba+2 + 2Cl- + 2Ag+ + 2NO3- 
Ba+2 + 2NO3- + 2AgCl (s)
2. Now write the net ionic equation.
2Ag+ + 2NO3-  2AgCl (s)
2 NaCl + Ba(NO3)2 → 2 NaNO3 +
BaCl2
1. Break up into ions
2Na+ + 2Cl- + Ba+2 + 2NO3- 
2Na+ + 2NO3- + Ba+2 + 2Cl-
2. Now write the net ionic equation.
Nothing changed, so no net ionic equation.
PbCl2 (s) + Li2O  PbO + 2 LiCl
1. Break up into ions
PbCl2 (s) + 2Li+ + O-2 
Pb+2 + O-2 + 2Li+ + 2Cl2. Now write the net ionic equation.
PbCl2 (s)  Pb+2 + 2Cl-