Ch7_Section7.9 to 7.10_PrintableHandout.pdf

1
Chemical Reactions
Chapter 7: Section 7.9 to 7.10
Section 7.11 is NOT included for this
course. And In Section 7.9, the concept
of Oxidation Numbers calculations are
not included.
CHEM 1020 Instructor: Dr. Rekha, MCTC 2
Balancing of chemical equation; where you need to
do balance a last standing element or compound
by using fractions as coefficient, and later multiply
the whole equation to convert to whole numbers.
C2H6+O2⟶H2O+CO2(unbalanced)
• Following the usual inspection approach, one might
first balance C and H atoms by changing the
coefficients for the two product species, as shown:
• C2H6+O2⟶3H2O+2CO2(unbalanced)
3
• This results in seven O atoms on the product side of
the equation, an odd number—no integer coefficient
can be used with the O2 reactant to yield an odd
number, so a fractional coefficient, 7/2, is used
instead, to yield a provisional balanced equation:
• C2H6+7O2⟶3H2O+2CO2
2
• Since fractions of molecules don’t exist, multiply
each coefficient by 2 in the entire equation. You get
• 2C2H6+7O2⟶6H2O+4CO2
4
Predicting Whether a Reaction Will Occur
• So far, we have discussed three “forces”
that drive a reaction.
 Precipitate (solid) formation from two solutions
reacting with each other
 Water formation through Acid base reactions
 Gas formation
• Now let us look at the fourth force, that is
transfer of electrons.
5
Review of Transfer of electrons
• In chapter-5 you learned about how metals can lose
electrons and become cations.
• Nonmetals can gain those electrons and become anions.
• Therefore, Reaction between a metal and a
nonmetal involves transfer of one or more
electrons.
• metal + nonmetal → ionic compound
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Oxidation-Reductin
The nonmetal may or may not be oxygen. However,
 Loss of electron is considered as oxidation.
 Metal is said to be oxidized.
 Gain of electron is considered as reduction.
 Nonmetal is said to be reduced
Therefore, the reactions between a metal and a
nonmetal are called as Oxidation-Reduction or
simply Redox reactions.
When one reactant gets oxidized, the other gets reduced/
1 2
3 4
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Example-1: When Zinc reacts with Sulfur, the ionic compound
Zinc Sulfide is formed.
 From all the previous chapters you have learned Zinc is a metal
and Sulfur is a nonmetal from Group VI A; 2 steps away from
noble gas group, and hence can gain 2 electrons given up byZinc.
Therefore, Zinc is oxidized and Sulfur is reduced.
The half-reactions for Zn oxidation, and the half-reaction for S
reduction can be written as:
Zn Zn2+ + 2e-
S + 2e- S2-
The two half reactions occur simultaneously. And overall the
reaction is written as: Zn(s) + S(s) ZnS(s)
.
8
▪ To write balanced chemical equation for oxidation-
reduction reactions, Identify which metal and which
nonmetal are reactants.
▪ Which ionic compound is formed as the product.
▪ Write the chemical equation using correct symbols for
reactants and correct formula for product.
▪ Make sure you use the subscript 2 for the elements that
exist as diatomic molecules.
▪ Balance the equation
▪ Make sure you show the physical states
chemical equations for oxidation-reduction
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Example-1: When sodium reacts with gaseous chlorine, the
ionic compound sodium chloride is formed.
 From all the previous chapters you have learned sodium is a metal
and all metals are solids, except a few. And the ionic compounds
are all solids, unless you dissolve in water. Therefore, show
proper physical states. But the question is not saying anything
about water. Hence,
Na(s) + Cl2(g) → NaCl(s)
• Cl there are 2 atoms on reactant side, but only 1 on product side.
To balance it, you need to add coefficient 2 before NaCl.
Na(s) + Cl2(g) → 2NaCl(s)
 Now, Na becomes 2 on the product side. But there is only 1 on
reactant side. Therefore, add coefficient 2 in front of Na also.
2Na(s) + Cl2(g) → 2NaCl(s)
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OtherExamples:
2Al(s) + N2(g) → 2AlN(s)
Fe (s) + Br2(l) → FeBr2(s)
2Fe(s) + 3 Br2(l) → 2FeBr3(s)
Do you notice that all the above equations are
balanced, formulas are correct?
Refer to your book for more examples and pictures.
When powdered aluminum and iodine (shown in the
foreground) are mixed (and a little water added),
they react vigorously.
Write a Balanced Chemical Equation for this.
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Exchange of Anion in an oxidation-
Reduction Reaction
Another Kind of Oxidation-Reduction Reaction involving a metal
and an ionic compound, wherein the exchange of the anion
takes place: (Example Thermite Reaction see next slide for
picture); The ionic compound may or may not involve oxygen.
2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s)
The aluminum is originally present as an elemental metal; loses 3
electrons (as it is in Group-3A). The oxygen now gets exchanged
from Fe to Al. So, Al ends up as Al2O3.
Al → Al3+ + 3e-
The opposite (gain of electrons by Fe) occurs with iron wherein it
forms elemental iron.
Fe3+ + 3e- → Fe
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7 8
9 10
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The thermite reaction:
[2Al(s) +Fe2O3(s) → 2Fe(s) + Al2O3]
gives off so much heat that the iron
formed, although is solid is molten
(melted).
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Oxidation-Reduction due to oxygen and
hydrogen
Another type of Oxidation-Reduction is involving oxygen or
hydrogen.
A substance that takes up (accepts) oxygen (and its new
compound now has oxygen as one of the elements)) or gets rid of
hydrogen (now its new compound does not have hydrogen) is said
to be oxidized
Likewise, a substance that takes up hydrogen or gets rid of
oxygen, is said to be reduced.
Addition of Oxygen = Oxidation
Or Removal of Hydrogen = Oxidation
Addition of Hydrogen = Reduction
Or Removal of Oxygen = Reduction
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Example-2:
Sr(s) + O2(g) → SrO (s)
This is both an electron-transfer involving, as well as oxygen
involving reaction. Sr is strontium metal in Group II; Oxygen is
the diatomic nonmetal. They have reacted to form the solid ionic
compound Strontium oxide.
But the equation needs balancing. There are 2 oxygen atoms in
reactant side, as it is diatomic molecule in natural existence.
But on Product side, only 1 oxygen, Therefore we will put
coefficient of 2 in front of SrO.
Sr(s) + O2(g) → 2SrO (s)
And now Sr is 1 on reactant side, but 2 on product side. So, put
coefficient of 2 in front of Sr on reactant side also.
2Sr(s) + O2(g) → 2SrO (s)
Now it is balanced.
Not Covering Oxidation Numbers
In your ebook, in Section 7.9, there is a part of
description about Oxidation Numbers and how to
determine it.
For your course we are not covering it.
We now move on to Section 7.10
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Other Ways to Classify Reactions
• The chemical reactions described so far, starting
from section 7.2, are only a small number of
types.
• There are many other types of reactions.
• And sometimes, the same reaction can be
classified into more than one type.
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There are five fundamental types of reactions.
1) Oxidation-Reduction
2) Combination or Synthesis
3) Acid-Base Reactions
4) Exchange reactions (single exchange and double
exchange)
5) Decomposition (which is opposite of combination or
synthesis)
Even among the above, sometimes they can be
classified into more than one of the above categories
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15 16
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Description/Review of the five fundamental types of
reactions.
1) Oxidation-Reduction: You already learnt that these
are either due to transfer of electrons or can involve
oxygen and hydrogen.
Metal + nonmetal Ionic compound
Or A + B AB
These can also be classified as Combination or
Synthesis reactions, because a new compound is
formed (synthesized) from combination of simple
elements.
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2) Combination or Synthesis Reactions: a new
compound is formed (synthesized) from combination
of simple elements.
Well…. The oxidation-reduction reaction between metal
and nonmetal was indeed a Combination reaction also.
Metal + nonmetal Ionic Compound was
synthesized.
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Synthesis (or Combination) Reactions
The metal can be simple metal from Group IA, IIA
and IIIA, or a transition metal, as shown below in the
case of Iron where more than one type of compound is
possible for synthesis.
2Na(s) + Cl2(g) → 2NaCl(s)
2Al(s) + N2(g) → 2AlN(s)
Mg(s) + F2(g) → MgF2(s)
Fe (s) + Br2(l) → FeBr2(s)
2Fe(s) + 3 Br2(l) → 2FeBr3(s)
Remember these are also electron-transfer and oxidation-
reduction reactions. Therefore, these are classified under
three categories!
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Also, nonmetal can be Oxygen.
2Ba(s) + O2(g) → 2BaO(S)
4Al (s) + 3O2(g) → 2Al2O3(s)
4Fe(s) + 3O2(g) → 2Fe2O3(s) (formation
of rust)
Remember these are also electron-transfer and oxidation-
reduction reactions. Therefore, these are classified
under three categories!
23
Does it always have to be between a metal and a
nonmetal in all the Synthesis/Combination reactions?
No, sometimes two nonmetals can interact, for
synthesis forming “molecular compounds”. But Most
of the times in such reactions, oxygen is one of the
reactants. Hence, these are also classified as Oxidation-
reduction
2S(s) + O2(g) → SO2(g)
2S(s) + 3(g) → 2SO3(g)
Formation of the
colorful plastics
used in these
zippers is an
example of a
synthetic reaction.
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21 22
23 24

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Does it always have to be two simple elements,
irrespective of being metal and nonmetal or both
being nonmetals?
No! We can have one complex compound and one simple
nonmetal combine to form another single new
compound altogether!
Therefore, this is still a synthesis reaction.
26
Example-1: Carbon monoxide combines with another oxygen
molecule to form Carbon dioxide
2CO(g) + O2(g) → 2CO2(g)
Example-2: Calcium oxide combines with water molecule to
form Calcium hydroxide (lime)
CaO(s) + H2O(l) → Ca(OH)2(S)
Example-3: Sulfur trioxide reacts with water molecule to form
sulfuric acid. (too much pollution from man-made activities,
result in two much SO3 in the air, and when it rains, combines
with the water and falls as acid rain).
SO3(g) + H2O(l) → H2SO4(aq)
27
The second type among the five fundamental types of
reactions was
3) Acid-Base Reactions: You already learnt about these
reactions, when acid and a base react to form salt and
water.
It was also called Neutralization Reaction because the
acid neutralized the base and vice-versa.
28
4) The fourth type is Exchange reactions (single
exchange and double exchange):
Single exchange or single replacement: One element
replaces a similar element. You saw this already in the
gas-forming reactions.
Example: when metals react with acids, they release
hydrogen gas, by replacing the hydrogen from the acid.
Zn(s) + 2HCl(aq) → H2(g) + ZnCl2(aq)
So, this is both a gas-forming reaction and a single
replacement (hydrogen replacement reaction)
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Single exchange can also be between a metal and water
(not acid). In this case, metal replaces the hydrogen
from water.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Another type of single exchange is halogen
replacement, when a halogen reacts with a compound
containing another halogen but dissolved in water.
Cl2(g) + 2NaBr(aq) → 2NaCl (aq)) + Br2(l )
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Exchange reactions of double exchange or double
displacement type.
You learned many examples of these in the aqueous
ionic precipitation reactions, between two ionic
compounds dissolved in water, forming a precipitate
as one of the products.
Such, reactions are not only ionic precipitation reactions,
but also double-displacement or double-exchange
reactions.
2KI(aq)+Pb(NO3)2(aq)→2KNO3(aq)+PbI2(s)
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27 28
29 30

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Another example of double displacement is, gas-
forming reaction, when an ionic compound and an acid
react.
Na2S(aq)+2HCl(aq)→2NaCl(aq)+H2S(g)
A third type of double displacement is the acid-base or
neutralization reaction, when the H+ of the acid was
replaced by the cation of the base. No release of gas, but
a salt (compound was formed) along with water.
HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
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5) Decomposition (which is opposite of combination or
synthesis)
Never classify a decomposition reaction as a
combination (or synthesis) reaction. As they are
opposite to each other.
Reactions that involve breakup of a compound into
simpler substances are called Decomposition
Reactions. One of the products formed could be
oxygen (If the decomposing compound had oxygen in
it).
Once again, these involve electron transfer or oxygen
involvement (and therefore, Oxidation/Reduction)
Decomposition Reactions
Examples of Decomposition Reactions.
PbBr4(s) → Pb(s) + 2Br2(g)
H2O2(l) (hydrogen peroxide) → H2(g) + O2(g)
2HgO(s) → 2Hg(s) + O2(g)
2MgO(s) → 2Mg(s) + O2(g)
2H2O(l) → 2H2(g) + O2(g)
2NaCl(s) → 2Na(l) + Cl2(g)
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Combustion Reaction
In all combustion reactions, there is burning and
release of energy in the form of flame/heat/light.
Reaction of oxygen (nonmetal) with another substance
releasing flame/heat/ light/ energy.
 The substance that gains the oxygen is said to be
oxidized. The oxygen element is said to be reduced.
 The substance that is reacting with Oxygen, may or
may not contain carbon.
 If there is a reaction involving oxygen, but no
burning, heat or light released, then it is NOT
Combustions reaction; it just oxidation-reduction
or one of the other types.
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Examples of Combustion Reactions
CCombustion of carbon compounds of carbon compounds:
CH4(g)+ 2O2(g) → CO2(g) + 2H2O(g)
(combustion of methane; fire is involved: used in Bunsen
burners)
(combustion of methane in Bunsen burner)
C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
(combustion of propane : used for heating homes and gas
stoves/grills)
2C8H18(l) + 25O2(g) → 16CO2(g) + 18H2O(g)
(combustion of gasoline: used to power automobiles)
C(s) + O2(g) → CO2(g)
(combustion of coal to generate energy for production of electricity)
Combustion of Non-carbon compounds:
2Mg(s) + O2(g) → 2MgO(s) + light
(produces a flash of light; used in flash photography)
Each magnesium atom loses 2 electrons
(Mg → Mg2+ + 2e-).
Each oxygen gains 2 electrons (O + 2e- → O2-).
2H2(g) + O2(g) → 2H2O(l) + energy
N2H4(l) + O2(g) → N2(g) + 2H2O(g) + energy
(Combustion of hydrazine generating energy/flame;
used in rockets)
37
31 32
33 34
35 36

For launch, the space
shuttle orbiter is
attached to two solid-
fuel rockets (left and
right) and a fuel tank
(center) that supplies
hydrogen and oxygen
to the orbiter's
engines.
38
Summary of Oxidation-Reduction
In Oxidation-Reduction, so far you have studied:
1) Combustion Reactions: These reactions involve
oxygen and produce energy (heat) so rapidly that a
flame results.
2) Synthesis (Combination) reactions: Simple
elements combine to form a new compound. One
of the elements could be oxygen.
3) Decomposition Reactions: A compound breaks
down into simpler elements or smaller compounds.
One of the products could be oxygen.
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Summary of Ways to Classify Reactions
• Precipitation reactions: involve solid formation;
exchange of ions; double displacement reactions
(AB + CD → AD + BC)
• Acid-base reactions: formation of water due to the
reaction of an acid with a base
• Oxidation-Reduction reactions:
 Metals + Nonmetal forming compound; 2 nonmetals
combining (involves electron transfer; Synthesis or
Combination)
 Compounds breaking up into reactants (involves
electron transfer; Decomposition)
 O2 as a reactant or product, energy is released –
combustion
 Reactions that form gas (single replacement)
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• Now that you have studied various ways to
classify a reaction, is it possible to classify
the same reaction into more than one type?
• Let us look at some examples:
3KOH(aq) + Fe(NO3)3(aq) → 3KNO3(aq) + Fe(OH)3(s)
This is both a ionic-precipitation reaction and a double
displacement reaction.
HNO3(aq) + KOH(aq) + → KNO3(aq) + H2O(s)
This is both acid-base reaction and neutralization
reaction.
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2Mg(s) + O2(g) → 2MgO(s)
• This is electron-transfer, oxidation-reduction,
synthesis reaction.
2H2O(l) → 2H2(g) + O2(g)
• This is electron-transfer, oxidation-reduction,
decomposition reaction.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)+energy
This is oxidation-reduction and combustion
reaction.
Practice more examples from your book
Chemical Reactions
Precipitation
Reactions
(Double
Displacement)
Acid-Base
Reactions
(Neutralization)
Oxidation
Reactions
Electron Transfer
(Synthesis/Combination)-The
nonmetal may or may not be
oxygen
Gas Forming; Removal of
Hydrogen from one
substance (single
Replacement)
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39 40
41 42

Chemical Reaction
Precipitation Oxidation- Acid-Base
Reduction Reaction
(Double Displacement) (Neutralization)
Combustion Synthesis Decomposition
Reaction Reaction Reaction
(involves O2) (Reactants are elements; (Also involves
Transfer of electrons) transfer of electrons)
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