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Transcript
Chapter 7
Chemical
Reactions
Day One and Two
Tro's "Introductory Chemistry", Chapter 7
2
Day Three
Tro's "Introductory Chemistry", Chapter 7
3
Chemical Reactions

Reactions:
 chemical changes in matter resulting in new
substances

Reactions involve rearrangement and exchange
of atoms to produce new molecules
 Elements DO NOT CHANGE during a reaction.
 Atoms of different elements can combine to make new
compounds
 Molecules can decompose into smaller molecules or
atoms
 Atoms can gain or lose electrons, turning them into
ions.
○ Or changing the charge on ions that are already there.
Tro's "Introductory Chemistry", Chapter 7
4
Combustion Reactions

Reactions in which O2 is consumed by combining
with another substance are called combustion
reactions.
Reactants

Products
5
Precipitation Reactions

Combining of ions resulting in formation of a
material that is insoluble in water:
precipitation reactions
Tro's "Introductory Chemistry", Chapter 7
6
Evidence of Chemical Reactions


Look for evidence of a new substance
Visual clues (permanent)
 Color change
 Precipitate formation
○ Solid that forms when liquid solutions are mixed
 Gas bubbles
 Large energy changes
○ Container becomes very hot or cold
○ Emission of light

Other clues.
 New odor
 Whooshing sound from a tube
 Permanent new state
Tro's "Introductory Chemistry", Chapter 7
7
Evidence of Chemical Change
Release or Absorption of Heat
Color Change
Formation of a Gas
Emission of Light
Formation of Solid Precipitate
Tro's "Introductory Chemistry", Chapter 7
8
Practice—Decide Whether Each of the
Following Involve a Chemical Reaction.
•
•
•
•
•
Photosynthesis Yes, CO2 and H2O combine into carbohydrates
Heating sugar until it turns black Yes, sugar decomposing
Heating ice until it turns liquid No, molecules still same
Yes, food decomposing and combining
Digestion of food with stomach acid
Dissolving sugar in water No, molecules still same
Tro's "Introductory Chemistry", Chapter 7
9
Chemical Equations
Short-hand way of describing a reaction.
 Provides information about the reaction.

 Formulas of reactants and products.
 States of reactants and products.
 Relative numbers of reactant and product
molecules that are required.
 Can be used to determine masses of
reactants used and products that can be
made.
Tro's "Introductory Chemistry", Chapter 7
10
Conservation of Mass

Matter cannot be created or destroyed.
 Therefore, the total mass cannot change.
 mass of reactants = total mass of products.

In a chemical reaction, all the atoms
present at the beginning are still present
at the end.
 If all the atoms are still there, then the mass
will not change.
Tro's "Introductory Chemistry", Chapter 7
11
The Combustion of Methane

Methane gas burns to produce carbon
dioxide gas and gaseous water.
 Whenever something burns it combines with
O2(g).
Tro's "Introductory Chemistry", Chapter 7
12
Combustion of Methane

Methane gas burns to produce carbon dioxide gas
and gaseous water.
 Whenever something burns it combines with O2(g).
CH4(g) + O2(g)  CO2(g) + H2O(g)
Assignment

Problem Set #1
 Page 232
○ #29- 33
Tro's "Introductory Chemistry", Chapter 7
14
Day Four
Tro's "Introductory Chemistry", Chapter 7
15
Chemical Equations
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g)
CH4 and O2 are the reactants, and CO2
and H2O are the products.
 The (g) after the formulas tells us the state
of the chemical.
 The number in front of each substance tells
us the numbers of those molecules in the
reaction.

 Called the coefficients.
Tro's "Introductory Chemistry", Chapter 7
16
Symbols Used in Equations

Symbols used to indicate state after
chemical.
 (g) = gas; (l) = liquid; (s) = solid.
 (aq) = aqueous = dissolved in water.

Energy symbols used above the arrow for
decomposition reactions.
 D = heat.
 hn = light.
 shock = mechanical.
 elec = electrical.
Tro's "Introductory Chemistry", Chapter 7
17
Balancing Equations
1.
Write the CORRECT formulas for each compound
(silicon dioxide + carbon  silicon carbide + carbon
monoxide)
2. If element occurs in only one compound
balance first. Balance METALS first.
Tro's "Introductory Chemistry", Chapter 7
18
Balancing Equations, cont.
SiO2 + C  SiC + CO
3. If element occurs as a FREE element,
balance LAST.
4. Check to make sure equation is balanced.
Tro's "Introductory Chemistry", Chapter 7
19
Example

When magnesium metal burns in
air, it produces a white, powdery
compound magnesium oxide.
1.
Write a skeletal equation
2. Count the number of atoms on each
side.
 Mg 
O
Tro's "Introductory Chemistry", Chapter 7
20
Another Example

Under appropriate conditions at 1000°C, ammonia gas
reacts with oxygen gas to produce gaseous nitrogen
monoxide and steam
Tro's "Introductory Chemistry", Chapter 7
21
Practice #1
When aluminum metal reacts with air, it
produces a white, powdery compound called
aluminum oxide.
Tro's "Introductory Chemistry", Chapter 7
22
Practice #2
Acetic acid reacts with the metal aluminum to make
aqueous aluminum acetate and gaseous hydrogen.
 Acids are always aqueous.
 Metals are solid except for mercury.
Tro's "Introductory Chemistry", Chapter 7
23
Practice #3
Combustion of ethyl alcohol (C2H5OH) in
flambé (a brandied flaming dessert).
Tro's "Introductory Chemistry", Chapter 7
24
Practice #4
Combustion of liquid butane (C4H10) in a lighter.
C4H10(l) + O2(g)  CO2(g) + H2O(g)
Tro's "Introductory Chemistry", Chapter 7
25
Assignment

Problem Set #2
 Page 233
○ #37, 38, 39
Tro's "Introductory Chemistry", Chapter 7
26
Day Five
Tro's "Introductory Chemistry", Chapter 7
27
Aqueous Solutions

Chemicals we are react together are most of
the time dissolved in water
 Mixtures of a chemical dissolved in water are
called aqueous solutions

Dissolving the chemicals in water helps them
to react together faster
 The water separates the chemicals into individual
molecules or ions
 The separate, free-floating particles come in
contact more frequently so the reaction speeds
up
Tro's "Introductory Chemistry", Chapter 7
28
Predicting Whether a
Reaction Will Occur in
Aqueous Solution

“Forces” that drive a reaction:





Formation of a solid.
Formation of water.
Formation of a gas.
Transfer of electrons.
When chemicals (dissolved in water) are
mixed and one of the above-noted forces
occur, the reaction will generally happen.
Tro's "Introductory Chemistry", Chapter 7
29
Dissociation

When ionic compounds
dissolve in water, the anions
and cations are separated
from each other. This is called
dissociation.
 However, not all ionic
compounds are soluble in water!

When compounds containing
polyatomic ions dissociate,
the polyatomic group stays
together as one ion.
Tro's "Introductory Chemistry", Chapter 7
30
Dissociation, Continued

Potassium iodide dissociates in water into
potassium cations and iodide anions.
KI(aq) → K+1(aq) + I-1(aq)
K

I
K+1
I-1
Copper(II) sulfate dissociates in water into
copper(II) cations and sulfate anions.
CuSO4(aq) → Cu+2(aq) + SO4-2(aq)
Cu
SO4
Cu+2
Tro's "Introductory Chemistry", Chapter 7
SO4-2
31
Dissociation, Continued

Potassium sulfate dissociates in water
into potassium cations and sulfate
anions.
K2SO4(aq) → 2 K+1(aq) + SO4-2(aq)
K
SO4 K
K+1
SO4-2
K+1
Tro's "Introductory Chemistry", Chapter 7
32
Electrolytes
Electrolytes are
substances whose
water solution is a
conductor of electricity
 All electrolytes have
ions dissolved in water

Tro's "Introductory Chemistry", Chapter 7
33
Electrolytes, Continued
In strong electrolytes, all
the electrolyte molecules or
formula units are separated
into ions
 In nonelectrolytes, none of
the molecules are separated
into ions
 In weak electrolytes, a small
percentage of the
molecules are separated into
ions

Tro's "Introductory Chemistry", Chapter 7
34
Types of Electrolytes

Salts = Water soluble ionic compounds.
 All strong electrolytes.

Acids = Form H+1 ions and anions in water
solution.
 In binary acids, the anion is monoatomic. In oxyacids, the
anion is polyatomic.
 Sour taste.
 React and dissolve many metals.
 Strong acid = strong electrolyte, weak acid = weak electrolyte.

Bases = Water-soluble metal hydroxides.
 Bitter taste, slippery (soapy) feeling solutions.
 Increases the OH-1 concentration.
Tro's "Introductory Chemistry", Chapter 7
35
When Will a Salt Dissolve?

A compound is soluble in a
liquid if it dissolves in that
liquid.
 NaCl is soluble in water, but
AgCl is not.

A compound is insoluble if
a significant amount does
not dissolve in that liquid.
 AgCl is insoluble in water.
○ Though there is a very small
amount dissolved, but not enough
to be significant.
Tro's "Introductory Chemistry", Chapter 7
36
Solubility Rules:
Compounds that Are Generally Soluble in
Water
Compounds containing the
following ions are generally
soluble
Exceptions
(when combined with ions on the
left the compound is insoluble)
Li+, Na+, K+, NH4+
none
NO3–, C2H3O2–
none
Cl–, Br–, I–
Ag+, Hg22+, Pb2+
SO42–
Ca2+, Sr2+, Ba2+, Pb2+
Tro's "Introductory Chemistry", Chapter 7
37
Solubility Rules:
Compounds that Are Generally Insoluble
Compounds containing the
following ions are generally
insoluble
OH–
S2–
CO32–, PO43–
Exceptions
(when combined with ions on the
left the compound is soluble or
slightly soluble)
Li+, Na+, K+, NH4+,
Ca2+, Sr2+, Ba2+
Li+, Na+, K+, NH4+,
Ca2+, Sr2+, Ba2+
Li+, Na+, K+, NH4+
Tro's "Introductory Chemistry", Chapter 7
38
Determine if Each of the
Following Is Soluble in Water
KOH
 AgBr
 CaCl2
 Pb(NO3)2
 PbSO4

Tro's "Introductory Chemistry", Chapter 7
39
Determine if Each of the Following Is
Soluble in Water, Continued





KOH
Soluble, because the cation is K+.
AgBr
Insoluble, even though most
compounds
with Br− are soluble,
this is an exception.
CaCl2 Soluble, most compounds with Cl− are
soluble.
Pb(NO3)2 Soluble, because the anion is NO3−.
PbSO4 Insoluble, even though most
compounds
with SO42− are soluble,
this is an exception.
Tro's "Introductory Chemistry", Chapter 7
40
Precipitation Reactions

If the ion exchange results
in forming a compound that
is insoluble in water, it will
come out of solution as a
precipitate.
Tro's "Introductory Chemistry", Chapter 7
41
Precipitation Reactions, Continued
2 KI(aq) + Pb(NO3)2(aq)  2 KNO3(aq) + PbI2(s)
Tro's "Introductory Chemistry", Chapter 7
42
No Precipitate Formation =
No Reaction
KI(aq) + NaCl(aq)  KCl(aq) + NaI(aq)
All ions still present,  no reaction.
Tro's "Introductory Chemistry", Chapter 7
43
Process for Predicting the Products of
a Precipitation Reaction
Write the formula for the reactants and Determine
what ions each aqueous reactant has.
2. Exchange ions.
1.

(+) ion from one reactant with (-) ion from the other.
Balance charges of combined ions to get formula
of each product.
4. Balance the equation.
3.

5.
Count atoms.
Determine solubility of each product in water.



Use the solubility rules.
If product is insoluble or slightly soluble, it will
precipitate.
If neither product will precipitate, no reaction.
Tro's "Introductory Chemistry", Chapter 7
44
Example 7.7—When an Aqueous Solution of Sodium
Carbonate Is Added to an Aqueous Solution of
Copper(II) Chloride, a White Solid Forms.
Write the formulas of the reactants and
Determine the ions present when each
reactant dissociates.
Na2CO3(aq) + CuCl2(aq) 
1.
2.
Exchange the ions.
Tro's "Introductory Chemistry", Chapter 7
45
Practice–Predict the Products and Balance
the Equation

KCl(aq) + AgNO3(aq) 

Na2S(aq) + CaCl2(aq) 
Tro's "Introductory Chemistry", Chapter 7
46
Assignment

Problem Set #3
 #61, 67, 70
Tro's "Introductory Chemistry", Chapter 7
47
Day Six
Tro's "Introductory Chemistry", Chapter 7
48
Day Seven
Tro's "Introductory Chemistry", Chapter 7
49
Practice—Write an Equation for the Reaction
that Takes Place when an Aqueous Solution
of ammonium sulfate is Mixed with an
Aqueous Solution of lead (II) acetate.
Tro's "Introductory Chemistry", Chapter 7
50
Ionic Equations
Equations that describe the chemicals put into the
water and the product molecules are called
molecular equations.
2 KOH(aq) + Mg(NO3)2(aq)  2 KNO3(aq) + Mg(OH)2(s)
 Equations that describe the actual dissolved species
are called complete ionic equations.

 Aqueous electrolytes are written as ions.
○ Soluble salts, strong acids, strong bases.
 Insoluble substances and nonelectrolytes written in molecule
form.
○ Solids, liquids, and gases are not dissolved, therefore, molecule form.
2K+1(aq) + 2OH-1(aq) + Mg+2(aq) + 2NO3-1(aq) K+1(aq) + 2NO3-1(aq) +
Tro's "Introductory Chemistry", Chapter 7
51
Writing Complete Ionic Equations
Rewrite the molecular equation, but
dissociate strong electrolytes into
individual ions.


Strong electrolytes must be aqueous.
○


All soluble ionic compounds are strong
electrolytes.
Strong acids are strong electrolytes.
○
○

Solids, liquids, or gases cannot be electrolytes.
HCl, HNO3, H2SO4..
Weak acids are not written in the dissociated ion
form.
Molecular compounds do not have ions, leave
in the molecular form.
Tro's "Introductory Chemistry", Chapter 7
52
Ionic Equations

Ions that are both reactants and products are called
spectator ions.
2K+1(aq) + 2OH-1(aq) + Mg+2(aq) + 2NO3-1(aq) K+1(aq) + 2NO3-1(aq) +
Mg(OH)2(s)
• An ionic equation in which the spectator ions are
removed is called a net ionic equation.
2OH-1(aq) + Mg+2(aq) Mg(OH)2(s)
Tro's "Introductory Chemistry", Chapter 7
53
Writing Net Ionic Equations

First, identify the spectator ions in the
complete ionic equation.
 Identical ions on both sides of the equation.

Cancel out the spectator ions—the
result is the net ionic equation.
Tro's "Introductory Chemistry", Chapter 7
54
Summary
A molecular equation is a chemical
equation showing the complete, neutral
formulas for every compound in a
reaction.
 A complete ionic equation is a chemical
equation showing all of the species as
they are actually present in solution.
 A net ionic equation is an equation
showing only the species that actually
participate in the reaction.

Tro's "Introductory Chemistry", Chapter 7
55
Practice–Write the Ionic and Net Ionic
Equation.
K2SO4(aq) + Ba(NO3)2(aq)  2 KNO3(aq) + BaSO4(s)
Na2CO3(aq) + 2 HCl(aq)  2 NaCl(aq) + CO2(g) +
H2O(l)
Tro's "Introductory Chemistry", Chapter 7
56
Assignment

Problem Set #4
 # 73,75, 76
Tro's "Introductory Chemistry", Chapter 7
57
Day Eight
Tro's "Introductory Chemistry", Chapter 7
58
Properties of Acids
Sour taste
 React with “active” metals, not
noble metals

 I.e., Al, Zn, Fe, but not Cu, Ag or Au.
Zn + 2 HCl ZnCl2 + H2
 Corrosive

React with carbonates, producing
CO2.
 Marble, baking soda, chalk, limestone.
CaCO3 + 2 HCl CaCl2 + CO2 + H2O

React with bases to form ionic
salts
 And often water
Tro's "Introductory Chemistry", Chapter 7
59
Common Acids
Chemical name
Formula
Old name
Strength
Nitric acid
HNO3
Aqua fortis
Strong
Sulfuric acid
H2SO4
Vitriolic acid
Strong
Hydrochloric acid
HCl
Muriatic acid
Strong
Phosphoric acid
H3PO4
Moderate
Chloric acid
HClO3
Moderate
Acetic acid
HC2H3O2
Hydrofluoric acid
HF
Carbonic acid
H2CO3
Boric acid
H3BO3
Vinegar
Weak
Weak
Soda water
Tro's "Introductory Chemistry", Chapter 7
Weak
Weak
60
Properties of Bases

Taste bitter.

Feel slippery.

React with acids to form ionic salts.
 And often water.
 Neutralization.
Tro's "Introductory Chemistry", Chapter 7
61
Common Bases
Chemical
name
Sodium
hydroxide
Potassium
hydroxide
Calcium
hydroxide
Magnesium
hydroxide
Ammonium
hydroxide
Formula
Strength
KOH
Common
name
Lye,
caustic soda
Caustic potash
Ca(OH)2
Slaked lime
Strong
Mg(OH)2
Milk of magnesia
Weak
NH4OH, Ammonia water,
{NH3(aq)} aqueous ammonia
Weak
NaOH
Tro's "Introductory Chemistry", Chapter 7
Strong
Strong
62
Acid–Base Reactions

Also called neutralization reactions because the
acid and base neutralize each other’s properties.
acid + base salt + water
2 HNO3(aq) + Ca(OH)2(aq)  Ca(NO3)2(aq) + 2
H2O(l)

The net ionic equation for an acid-base reaction
often is:
H+1(aq) + OH-1(aq)  H2O(l)
 As long as the salt that forms is soluble in water.
Tro's "Introductory Chemistry", Chapter 7
63
Process for Predicting the Products of
an Acid–Base Reaction
Determine what ions each aqueous reactant has.
2. Exchange ions.
1.

(+) ion from one reactant with (-) ion from the other.
 H+ combines with OH− to make water.
Balance charges of combined ions to get formula
of the salt.
4. Balance the equation.
3.

5.
Count atoms.
Determine solubility of the salt.


Use the solubility rules.
If the salt is insoluble or slightly soluble, it will
precipitate.
Tro's "Introductory Chemistry", Chapter 7
64
Example 7.11—Write the Molecular, Ionic, and NetIonic Equation for the Reaction of Aqueous Nitric
Acid with Aqueous Calcium Hydroxide.
Tro's "Introductory Chemistry", Chapter 7
65
Practice—Complete and Balance These
Acid–Base Reactions.
NH4OH(aq) + H2SO4(aq) 
Al(OH)3(aq) + H2SO3(aq) 
Ba(OH)2(aq) + H2SO4(aq) 
Tro's "Introductory Chemistry", Chapter 7
66
Gas Evolution Reactions

Reactions in which the driving force is the
production of a material that escapes as a gas are
called gas evolution reactions.

Gases that you need to memorize:
 H2S, NH3, SO2, CO2, NO2, NO, H2, N2, O2, F2, Cl2
Tro's "Introductory Chemistry", Chapter 7
67
Practice—Complete the Following
Reactions.
PbS(s) + H2SO4(aq) 
Tro's "Introductory Chemistry", Chapter 7
68
Assignment

Problem Set #5
 #81, 84, 85
Tro's "Introductory Chemistry", Chapter 7
69
Day Nine
Tro's "Introductory Chemistry", Chapter 7
70
Other Patterns in Reactions

The precipitation, acid–base, and gas evolving
reactions all involved exchanging the ions in the
solution.

Other kinds of reactions involve transferring
electrons from one atom to another. These are
called oxidation–reduction reactions.
 Also known as redox reactions.
Tro's "Introductory Chemistry", Chapter 7
71
Oxidation–Reduction
Reactions

We say that the element that loses
electrons in the reaction is oxidized.
And the substance that gains electrons in
the reaction is reduced.
 You cannot have one without the other

Tro's "Introductory Chemistry", Chapter 7
72
Summary

Redox reactions occur when:
 A substance reacts with O2.
 A metal combines with a nonmetal.
 In general, whenever electrons are
transferred.
Tro's "Introductory Chemistry", Chapter 7
73
Reactions of Metals with Nonmetals
(Oxidation–Reduction)

Metals react with nonmetals to form ionic
compounds.
 Ionic compounds are solids at room temperature.

The metal loses electrons and becomes a
cation.
 The metal undergoes oxidation.

The nonmetal gains electrons and becomes an
anion.
 The nonmetal undergoes reduction.

In the reaction, electrons are transferred from
the metal to the nonmetal.
Tro's "Introductory Chemistry", Chapter 7
74
Recognizing Redox Reactions
Any reaction where O2 is a reactant or a product is
a redox reaction.
 Any reaction between a metal and a nonmetal is
redox.
 Any reaction where electrons are transferred is
redox.

 When a free element gets combined into a compound, it
will be either oxidized or reduced.
N2(g) + H2(g)  NH3(g)
 When a metal cation changes its charge, it will be either
oxidized if its charge increases or reduced if its charge
decreases.
CuCl(aq) + FeCl3(aq)  FeCl2(aq) + CuCl2(aq)
Tro's "Introductory Chemistry", Chapter 7
75
Practice—Decide Whether Each of the
Following Reactions Is a Redox Reaction.
2 Al(s) + 3 Br2(l)  2 AlBr3(s)
CaSO3(s) + 2 HCl(aq)  CaCl2(aq) + SO2(g) +
H2O(l)
Fe2O3(s) + C(s)  2 Fe(s) + 3 CO(g)
SO2(g) + O2(g) + H2O(l)  H2SO4(aq)
Tro's "Introductory Chemistry", Chapter 7
76
Combustion Reactions
Reactions in which O2(g) is
a reactant are called
combustion reactions.
 Combustion reactions
release lots of energy. They
are exothermic.
 Combustion reactions are a
subclass of oxidation–
reduction reactions.

2 C8H18(g) + 25 O2(g)  16 CO2(g) + 18 H2O(g)
Tro's "Introductory Chemistry", Chapter 7
77
Products of Combustion

When a material burns that contains carbon
and hydrogen, the products are always
CO2(g) and H2O(g).
Tro's "Introductory Chemistry", Chapter 7
78
Practice—Write the Equation for Each Reaction.

Combustion of the anesthetic cyclopropane,
C 3H 6.

Combustion of the non-toxic antifreeze
propylene glycol, C3H8O2.
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79
Assignment

Problem Set #6
 #87-89
Tro's "Introductory Chemistry", Chapter 7
80
Day Ten
Tro's "Introductory Chemistry", Chapter 7
81
Classifying Reactions

One way is based on the process that
happens.
 Precipitation, neutralization, formation of a gas,
or transfer of electrons.
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Classifying Reactions,
Continued
 Another scheme classifies reactions by
what the atoms do.
Type of reaction
Synthesis
General equation
A + B  AB
Decomposition
AB  A + B
Displacement
A + BC  AC + B
Double displacement
AB + CD  AD + CB
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Synthesis Reactions
Also known as composition or combination
reactions.
 Two (or more) reactants combine together to
make one product.

 Simpler substances combining together.
2 CO + O2  2 CO2
2 Mg + O2  2 MgO
HgI2 + 2 KI  K2HgI4
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Decomposition Reactions

A large molecule is broken apart into
smaller molecules or its elements.
 Caused by addition of energy into the
molecule.

Have only one reactant, make 2 or more
products.
elec
2 FeCl ( s ) 
 2 FeCl (l )  Cl ( g )
3
2
2
D
2 HgO (s)  2 Hg(l)  O 2 ( g )
h
2 O3  3 O 2
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Single Displacement Reactions

Reactions that involve one atom
displacing another and replacing it in a
compound.
Other examples of displacement reactions
are:
Fe2O3(s) + Al(s)  Fe(s) + Al2O3(s)
2 Na(s) + 2 H2O(aq)  2 NaOH(aq) + H2(g)

Tro's "Introductory Chemistry", Chapter 7
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Double Displacement
Reactions

Two ionic compounds exchange ions.

X Yq (aq) + A Bq (aq)  XB + AY

Precipitation, acid–base, and gas evolving
reactions are also double displacement
reactions.
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Practice—Classify the Following Reactions as
Synthesis, Decomposition, Single Displacement, or
Double Displacement.
3 Mg(s) + 2 FeCl3(aq)  3 MgCl2(aq) + 2 Fe(s)
CO2(g) + H2O(l)  H2CO3(aq)
3 KOH(aq) + H3PO4(aq)  K3PO4(aq) + 3 H2O(l)
CaCO3 (s) 
 CaO(s)  CO2 ( g )
heat
Tro's "Introductory Chemistry", Chapter 7
88
Assignment

Review Sheet
Tro's "Introductory Chemistry", Chapter 7
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