Chapter 11
“Chemical
Reactions”
Mater Lakes Academy
Chemistry
Mrs.Nunez
1
Section 11.1
Describing Chemical Reactions
 OBJECTIVES:
–Describe how to write a word
equation.
–Describe how to write a skeleton
equation.
–Describe the steps for writing a
balanced chemical equation.
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All chemical reactions…

3
have two parts:
1. Reactants = the substances you
start with
2. Products = the substances you
end up with
A+B
C+D
REACTANTS
PRODUCTS
- Page 321
Products
Reactants
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Signs of a Chemical Reaction
Evolution of heat and light
 Formation of a gas
 Formation of a precipitate
 Color change

5
Law of Conservation of Mass
Mass is neither created nor
destroyed in a chemical reaction.
 Total mass stays the same.
 Atoms can only rearrange.

6
In a chemical reaction
A reaction can be described several ways:
#1. In a sentence (every item is a word)
Copper reacts with chlorine to form copper (II)
chloride.
#2. In a word equation (some symbols used)
Copper + chlorine  copper (II) chloride
#3.In a Skeleton Equation (Uses formulas and

symbols)
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Fe(s) + O2(g)  Fe2O3(s
Chemical Equations
8
C. Johannesson
What is a catalyst?
A substance that speeds up a
reaction, without being
changed or used up by the
reaction.
 Enzymes are biological or
protein catalysts in your body.

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Writing Equations
2H2(g) + O2(g)  2H2O(g)

Identify the substances involved.

Use symbols to show:
 How many? - coefficient
 Of what? - chemical formula
 In what state? - physical state

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Remember the diatomic elements.
Writing Equations
Two atoms of aluminum react with three
units of aqueous copper(II) chloride to
produce three atoms of copper and two
units of aqueous aluminum chloride.
• How many?
• Of what?
• In what state?
2 Al(s) + 3 CuCl2 (aq)  3 Cu (s) + 2 AlCl3 (aq)
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Describing Equations

Describing Coefficients:
– individual atom = “atom”
– covalent substance = “molecule”
– ionic substance = “unit”
3CO2  3 molecules of carbon dioxide
2Mg
 2 atoms of magnesium
4MgO  4 units of magnesium oxide
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Describing Equations
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
• How many?
• Of what?
• In what state?
One atom of solid zinc reacts with
two molecules of aqueous hydrochloric acid
to produce one unit of aqueous zinc chloride
and one molecule of hydrogen gas.
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Write a skeleton equation for:
Solid iron (III) sulfide reacts with
gaseous hydrogen chloride to form
iron (III) chloride and hydrogen
sulfide gas.
Read the following equation:
Cu(s) + AgNO3(aq)  Ag(s) + Cu(NO3)2(aq)
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Balanced Chemical Equations
A
balanced equation has the same
number of each element on both
sides of the equation.
 They
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are balanced!!!
A. Balancing Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient  subscript = # of atoms
4. Reduce coefficients to lowest
possible ratio, if necessary.
5. Double check atom balance!!!
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Helpful Tips
Balance one element at a time.
 Update ALL atom counts after adding
a coefficient.
 If an element appears more than once
per side, balance it last. (H and O)
 Balance polyatomic ions as single
units.
“1 SO4” instead of “1 S” and “4 O”

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Never change a subscript to balance an
equation (You can only change coefficients)
– If you change the subscript (formula) you
are describing a different chemical.
– H2O is a different compound than H2O2
 Never put a coefficient in the middle of a
formula; they must go only in the front

2NaCl is okay, but Na2Cl is not.
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Balancing Example
Aluminum and copper(II) chloride react
to form copper and aluminum chloride.
2 Al + 3 CuCl2  3 Cu + 2 AlCl3
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2 1
Al
1 2
3 1
Cu
1 3
6 2
Cl
3 6
C. Johannesson
Section 11.2
Types of Chemical Reactions
 OBJECTIVES:
–Describe the five general types of
reactions.
–Predict the products of the five
general types of reactions.
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Synthesis (Combination)

the combination of 2 or more
substances to form a compound

only one product
A + B  AB
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Combination Reactions
 Combine
= put together
 Ca + O2 CaO
 SO3 + H2O  H2SO4
 We can predict the products, especially
if the reactants are two elements.
Mg3N2 (symbols, charges, cross)
 Mg + N2 _______
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Combination Reactions
Important Notes:
 Some nonmetal oxides react with
water to produce an acid:
SO2 + H2O
H2SO3
 Some metallic oxides react with
water to produce a base:
CaO + H2O
Ca(OH)2
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Decomposition

a compound breaks down into 2 or
more simpler substances

only one reactant
AB  A + B
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C. Johannesson
Decomposition Reactions
 decompose
= fall apart
electricity
 NaCl   
Na + Cl2

 CaCO3   CaO + CO2
that energy (heat, sunlight,
electricity, etc.) is usually required
 Note
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Decomposition Reactions
 We
can predict the products if it is
a binary compound
–It breaks apart into the elements:
electricity

 H2O   

 HgO  
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Decomposition Reactions
 If
the compound has more than
two elements you must be given
one of the products
–The other product will be from
the missing pieces

 NiCO3   CO2 + ___
heat
 H2CO3(aq) CO2 + ___
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Single Replacement

one element replaces another in a
compound
– metal replaces metal (+)
– nonmetal replaces nonmetal (-)
A + BC  B + AC
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C. Johannesson
C. Single Replacement

Products:
– metal  metal (+), or H
– nonmetal  nonmetal (-)
– free element must be more active
(check activity series)
Fe(s)+ CuSO4(aq)  Cu(s)+ FeSO4(aq)
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Br2(l)+ NaCl(aq)  N.R.
C. Johannesson
Single Replacement Reactions
 We
can even tell whether or not a single
replacement reaction will happen:
–Because some chemicals are more
“active” than others
–More active replaces less active
 There is a list on page 333 - called the
Activity Series of Metals
 Higher
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on the list replaces those lower.
The “Activity Series” of Metals
Higher
activity
Lower
activity
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Lithium
Potassium
Calcium
Sodium
Magnesium
Aluminum
Zinc
Chromium
Iron
Nickel
Lead
Hydrogen
Bismuth
Copper
Mercury
Silver
Platinum
Gold
1) Metals can replace other
metals, provided they are
above the metal they are
trying to replace
(for example, zinc will replace lead)
2) Metals above hydrogen can
replace hydrogen in acids.
3) Metals from sodium upward
can replace hydrogen in
water.
The “Activity Series” of Halogens
Higher Activity
Fluorine
Chlorine
Bromine
Iodine
Lower Activity
Halogens can replace other
halogens in compounds,
provided they are above the
halogen they are trying to
replace.
2NaCl(s) + F2(g) 
MgCl2(s) + Br2(g) 
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2NaF
??? (s) + Cl2(g)
???Reaction!
No
Double Replacement
ions in two compounds “change
partners”
 Must be ionic compounds in aqueous
solution
 cation of one compound combines
with anion of the other

AB + CD  AD + CB
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C. Johannesson
E. Double Replacement

Products:
– switch negative ions
– one product must be insoluble
(check solubility table)
Pb(NO3)2(aq)+ 2KI(aq)  PbI2(s)+2KNO3(aq)
NaNO3(aq)+ KI(aq)  N.R.
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Double Replacement Reactions
 Have
certain “driving forces”, or reasons
–Will only happen if one of the
products:
a) doesn’t dissolve in water and forms
a solid (a “precipitate”), or
b) is a gas that bubbles out, or
c) is a molecular compound (which will
usually be water).
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Combustion
Combustion means “add oxygen”
 the burning of any substance in O2 to
produce heat

A + O2  CO2 + H2O
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
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C. Johannesson
Combustion Reactions
 If
the combustion is complete, the
products will be CO2 and H2O.
 If the combustion is incomplete, the
products will be CO (or possibly just
C) and H2O.
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Section 11.3
Reactions in Aqueous Solution
 OBJECTIVES:
–Describe the information found
in a net ionic equation.
–Predict the formation of a
precipitate in a double
replacement reaction
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Net Ionic Equations
 Many
reactions occur in water- that
is, in aqueous solution
 When dissolved in water, many
ionic compounds “dissociate”, or
separate, into cations and anions
 Now we are ready to write an ionic
equation
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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 an ionic equation by
splitting the compounds into their ions:
Ag1+ + NO31- + Na1+ + Cl1- 
AgCl + Na1+ + NO31Note that the AgCl did not ionize, because it is a “precipitate”
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Net Ionic Equations
3. simplify by crossing out ions not
directly involved (called spectator ions)
Ag1+ + Cl1-  AgCl
This is called the net ionic equation
Let’s talk about precipitates before we
do some other examples
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Predicting the Precipitate


Insoluble salt = a precipitate
[note Figure 11.11, p.342 (AgCl)]
General solubility rules are found:
a) Table 11.3, p. 372 in textbook
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Let’s do some examples together of
net ionic equations, starting with
these reactants:
BaCl2 + AgNO3 →
NaCl + Ba(NO3)2 →
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