Download OXIDATION AND REDUCTION REACTION

OXIDATION AND REDUCTION
REACTION
REDOX
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Types of Reactions
REDOX
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Redox Reaction
 Is the reaction that involve the
transfer of electrons from a reducing
agent to an oxidizing agent
REDOX
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TYPE OF REDOX REACTION
 OUT OF BODY
 IN THE BODY
REDOX
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Electrochemistry
 Is the branch of chemistry that
deals with the interconversion of
electrical energy and chemical
energy
 Electrochemical processes are
redox reactions in which the energy
released by a spontaneous reaction
is converted to electricity or in
which electrical energy is used to
cause a nonspontaneous reaction
to occur
REDOX
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Definition of Oxidation and
Reduction
Oxidation:
Is defined as the
removal of electrons
Reduction:
is defined as the gain
of electrons
REDOX
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Common uses of the terms
oxidation and reduction
Term
Oxidation
Meaning
To Combine with oxygen
To lose hydrogen
To lose electron
To increase in oxidation
Reduction number
To lose oxygen
To combine with hydrogen
To gain electron
To decrease in oxidation
number REDOX
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Oxidation of Ferrous to
ferric ion
REDOX
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OXIDATION NUMBER
 Oxidation numbers are the charges
atoms in a compound would have if
the electrons of each bond belonged
to the more electronegative atoms.
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Properties of oxidizing and
reducing agents
oxidizing agents
- Gains electrons
- Oxidation number
decrease
- Becomes reduced
reducing agents
- Loses electron
- Oxidation number
increase
- Becomes oxidized
REDOX
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THE RULES FOR ASSIGNING
OXIDATION NUMBER
1. Atoms of any element not
combined with atoms from another
element have oxidation numbers of
zero.
Examples: The oxidation number of
the atom in N2, Br2, Cl2, P4 and S2
are zero
REDOX
continued
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2. The oxidation numbers of
monatomicions equal their ionic
charge.
Examples:
The oxidation numbers of Na+ and
K+ are +1, of Ca2+, Cu2+, and Mg2+
are +2, and Cl¯ and Br¯ are –1.
REDOX
continued
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3. In their compounds, the
oxidation number of any
atoms of the
- Group I A elements is +1
(e.g., Na+, K+)
- Group II A elements is +2
(e.g., Cu2+, Mg2+)
- Group III A is +3
REDOX
continued
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4. The oxidation number of any
nonmetal in its binary compounds
with metals equals the charge of the
monatomic anion
e.g., The oxidation number of Br in
CrBr3 is –1 because the monatomic
ion of Br is the bromide ion, which
has
a charge of 1-.
REDOX
continued
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5. In compounds, the oxidation number
of
- O is almost always –2.
(Exceptions occur only when the
rules for H or F would be violated)
- H is almost always +1.
(the exceptions are binary
compounds with metals, like NaH, in
which the H has the oxidation
number –1.)
- F is always –1. (No exceptions.
Fluorine is the most electronegative
of all elements).
continued
REDOX
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6. The sum of the oxidation numbers
of all the atoms in the formula of
the atom, ion, or molecule must
equal the overall charge given for
the formula – the sum rule
continued
REDOX
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Problem No. 1
Calomel, long use in medicine, has the
formula Hg2Cl2. What are the oxidation
numbers on the atoms in this compound?
The rule for assigning oxidation number: No. 4 & 6
4. The oxidation number of any nonmetal in its
binary compounds with metals equals the
charge of the monatomic anion
6. The sum of the oxidation numbers of all the
atoms in the formula of the atom, ion, or
molecule must equal the overall charge given
for the formula – the sum rule
REDOX
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The oxidation number of Hg (x)
Hg2Cl2
Rule 4 : Cl 2 atoms X (-1) = -2
Hg 2 atoms X (x) = 2x _
Rule 6 :
sum
= 0
The value of x comes from the sum,
2x + (-2) = 0
2x = +2
x = +1
The oxidation number of Hg in the Hg2Cl2 is +1
REDOX
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BALANCING REDOX REACTIONS
Redox reaction
Oxidation reaction
Reduction reaction
REDOX
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BALANCING REDOX REACTIONS
BY THE ION-ELECTRON METHOD
Simply list the steps to balance a redox
reaction in an acidic (or neutral) medium.
Balancing redox reactions by the
ion-electron method has 8 steps.
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Those are:
1. Write a skeletal equation that shows only
the ions or molecules involved in the
reaction.
2. Divide the skeletal equation into two halfreactions.
3. Balance all atoms that are not H or O.
4. Balance O by adding H2O.
5. Balance H by adding H+ (not H or H2 or H¯,
but H+).
6. Balance the net charge by adding e¯.
(remember its minus sign.)
REDOX
continued
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7. Multiply Entire Half-reactions by
simple whole numbers, as needed,
to get the gain of e¯ in one halfreaction to match the loss of e¯ in
the other. Then add the half-reactions.
8. Cancel whatever is the same on
both sides of the arrow.
REDOX
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The oxidation of methyl alcohol, CH3OH, to
formic acid, HCHO2, using the dichromate ion,
Cr2O72¯ in an acidic medium.
As this reaction proceeds, the chromium in
Cr2O72¯ change to Cr3+.
Step 1. Write a skeletal equation showing
reactants and products as given.
CH3OH + Cr2O72¯
HCHO2 + Cr3+
Step 2. Divide the skeletal equation into two
half-reactions. Except for H and O, the
same elements must appear on both sides
of each half-reaction.
CH3OH
HCHO2
3+
Cr2O72¯
Cr
continued 23
REDOX
Step 3. Balance all atoms that are not H or O
CH3OH
HCHO2 (No change, yet.)
Cr2O72¯
2Cr3+ (Balances Cr atoms.)
Step 4. Balance O by adding H2O
CH3OH + H2O
HCHO2 (Cs and Os balance.)
Cr2O72¯
2Cr3++ 7H2O (Crs and Os balance)
Step 5. Balance H by adding H+
CH3OH + H2O
HCHO2 + 4H+ (All atoms
now balance.)
Cr2O72¯ + 14H+
2Cr3+ + 7H2O (All atoms
now balance.)
REDOX
continued
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LOOK AT THE STEP 5, AND THEN
MAKE THE NET CHARGE ON THE LEFT
SIDE OF THE ARROW EQUAL TO THE
NET CHARGE ON THE RIGHT SIDE.
Step 5. Balance H by
adding H+
(a) CH3OH + H2O
HCHO2 + 4H+
Step 6. Balance the net
charge by adding e¯.
(a) CH3OH + H2O
HCHO2 + 4H++ 4e¯
2¯ +
(b) Cr2O72¯ + 14H++ 6e¯
2Cr3+ + 7H2O
14H+
(b) Cr2O7
2Cr3+ + 7H2O
REDOX
continued
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Step 7. Multiply half-reaction by whole
numbers so that the electrons will
cancel when the half-reactions are
added.
3 X [CH3OH + H2O
HCHO2 + 4H++ 4e¯]
2 X [Cr2O72¯ + 14H++ 6e¯
2Cr3+ + 7H2O]
+
Sum:
3CH3OH + 2Cr2O72 ¯+ 3H2O + 28H++ 12e¯
3HCHO2 + 4Cr3++ 12H+ + 14H2O +12e¯
REDOX
continued
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Step 8. Cancel everything that can be canceled
a). The 12 electrons on each side obviously
cancel.
b). Water molecule: there are 3 on the left and
14 on the right, so we can strike those on
the left and change those on the right to 11.
…. + 3H2O +….
…. + 14H2O +….
becomes: ….
.… + 11H2O + ….
c). And then also cancel some H+
…. + 28 H+ + ….
…. + 12 H+ + ….
becomes: …. + 16 H+ + ….
…..
REDOX
continued
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3CH3OH(aq) + 2Cr2O72 ¯(aq) + 16H+(aq)
3HCHO2(aq) + 4Cr3+(aq) +11H2O
Check to see that both material and electrical
balance exist.
REDOX
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Home work
Problem No. 1
What is the oxidation number of carbon in
ethane, C2H6?
Problem No. 2
What are the oxidation numbers of the
atoms in the nitrate ion, NO3¯?
Problem No.3
Balance the following equation, which
occurs in an acidic medium.
Cu(s) + NO3¯(aq)
Cu2+(aq) + NO2(g)
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To Balance a Redox Equation When
the Medium is Basic.
MnO4¯(aq) + SO32¯(aq)
MnO2(s) + SO42¯(aq)
1. First Balance it for an Acid Medium and
2. Then Neutralize the Acid
Step 1. Through 8 for acidic solutions
2MnO4¯ + 3SO32¯ + 2H+
2MnO2 + 3SO42¯ + H2O
Step 9. Add as many OH as there are H+ to both
sides of equation. There are two H+ on the left.
So we add 2OH¯ to both sides.
2OH¯ + 2MnO4¯ + 3SO32¯ + 2H+
2¯ + H O + 2OH¯
2MnOREDOX
+
3SO
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2
4
2
Step 10. When they occur on the same
side of the arrow, combine H+
and OH¯ into H2O.
2OH¯ + 2MnO4¯ + 3SO32¯ + 2H+
2MnO2 + 3SO42¯ + H2O + 2OH¯
The left side has 2OH¯ and 2H+, so we
combine them into 2H2O.
The equation:
2H2O + 2MnO4¯ + 3SO32¯
2MnO2 + 3SO42¯ + H2O + 2OH¯
REDOX
continued
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Step 11. Cancel H2O molecules as possible
Step 10.
2H2O + 2MnO4¯ +
3SO32¯
2MnO2 + 3SO42¯
+ H2O + 2OH¯
Step 11.
Cancel H2O molecule
as possible.
The final equation:
H2O + 2MnO4¯(aq) +
3SO32¯(aq)
2MnO2(s)+ 3SO42¯(aq)
……… + 2OH¯(aq)
REDOX
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Reduction Potentials or Redox Potentials
Half Reaction
Fe3+ (aq) + e¯ ⇌ Fe2+(aq)
Fe2+(aq) + 2e¯ ⇌ Fe(s)
½ O2 + 2H+ + 2 e¯ ⇌ H2O
NAD+ + H+ + 2 e¯ ⇌ NADH
REDOX
E°
(volts)
+ 0.77
- 0,44
+ 0.815
- 0.315
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Rule for Combining Reduction
Half-Reactions
When two reduction halfreactions are combined into a
full redox reaction the one with
the more positive E° always runs
as written, as a reduction, and it
forces the other, with the less
positive E°, to run in reverse, as
an oxidation.
REDOX
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Na(s) and Cl2(g).
Will sodium react with chlorine?
Na+(aq) + e¯ ⇌ Na(s)
Cl2(g) + 2 e¯ ⇌ 2Cl¯
E°= -2,71 V
E°= +1.36V
When two reduction half-reactions are combined
into a full redox reaction the one with the more
positive E° always runs as written, as a
reduction, and it forces the other, with the less
positive E°, to run in reverse, as an oxidation.
Na(s) → Na+(aq) + e¯ (oxidation)
Cl2(g) + 2 e¯ → 2Cl¯ (aq)
(reduction)
Continued
next slide
REDOX
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to get the net reaction, multiply the coefficients
of the first half-reaction by 2.
2X [Na(s) → Na+(aq) + e¯ (oxidation)]
2 Na(s) → 2Na+(aq) + 2e¯ (oxidation)
Cl2(g) + 2e¯ → 2Cl¯ (aq)
(reduction)
Sum: 2Na(s) + Cl2(g)  2Na+(aq) + 2Cl¯ (aq)
Thus sodium and chlorine spontaneously react.
REDOX
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TYPE OF REDOX REACTION
 OUT OF BODY
 IN THE BODY
REDOX
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Biological Oxidation
 The processes of oxidation are
essential for maintaining life
because oxidation and the
simultaneously occuring reduction
supply the free energy for the vital
work
REDOX
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Organic Reaction Mechanisms
 Organic reactions mechanisms has
classified biochemical reactions into four
categories:
 1. Group-transfer reactions
 2. Oxidations and reductions
 3. Eliminations, isomerizations, and
rearrangements
 4. Reactions that make or break carboncarbon bonds
REDOX
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Covalent bonds
 A covalent bond consists of an electron
pair shared between two atoms.
 In breaking such a bond, the electron pair
can either remain with one of the atoms
(heterolytic bond cleavage) or separate
such that one electron accompanies
each of the atoms (homolytic bond
cleavage).
REDOX
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Homolytic Bond Cleavage
 Homolytic bond cleavage, which
usually produces unstable radicals,
occurs mostly in oxidationreduction reactions
REDOX
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Heterolytic Bond Cleavage
 Heterolytic C-H bond cleavage
involves either carbanion and proton
(H+) formation or carbocation
(carboniun ion) and hydride ion (H-)
formation.
REDOX
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Biologically Importan
Nucleophillic Groups
Nucleophiles are the conjugate bases
of weak acids
REDOX
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1. Group-transfer reactions
 Types of metabolic group-transfer: Acyl
group transfer involves addition of a
nucleophile (Y) to the electrophilic carbon
atom of an acyl compound to form a
tetrahedral intermediate. The original acyl
carrier (X) is then expelled to form a new acyl
REDOX
compound.
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2. Oxidations and reductions
General
Base
Alcohol
General
acid
NAD+
Ketone
NADH
NAD+ IS AN ELECTRON ACEPTOR (OXIDATOR);
REDOX TWO ELECTRON FROM GENERAL45
BASE ARE TRANSFERRRED TO AN ELECTRON ACCEPTOR SUCH AS NAD+.
In Living Systems
The electron-transfer process
connecting these half-reactions occurs
through a multistep pathway that
harnesses the liberated free energy to
form ATP.
REDOX
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For Example: Biologic Oxidation
 The principal use of oxygen is in
respiration. Which may be defined as
the process by with cells derive
energy in the form of ATP from the
controlled reaction of hydrogen with
oxygen to form water.
REDOX
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For Example:
oxidation of primary alcohols in
the Body
REDOX
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ACCUT TOXICITY
Toxicity test
SUBCHRONIC TOXICITY
CHRONIC TOXICITY
REDOX
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FREE ENERGY CHANGES CAN BE
EXPRESSED IN TERMS OF REDOX POTENTIAL
 In reaction involving oxidation and
reduction, the free energy change is
proportionate to the tendency of reactants
to donate or accept electrons. Thus, in
addition to expressing free energy change
in terms of ΔG0.
 It is possible, in an analogous manner, to
express it numerically as an oxidationreduction or redox potential (E0).
REDOX
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Oxidation of a Metabolite catalyzed
by an oxidase forming H2O
REDOX
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Oxidation of a metabolite by Hydrogenases
and Finally by an Oxidase in a Respiratory
Chain.
REDOX
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The Citric Acid Cycle
REDOX
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The complete oxidation of glucose
by molecular oxygen
C6H12O6 + 6O2
6CO2 + 6H2O
Break this equation down into two half- reactions
1). C6H12O6 + 6H2O
6CO2 + 24H+ + 24e¯
The glucose carbon atoms are oxidized
2). 6O2 + 24H+ + 24e¯
12H2O
Molecular oxygen is reduced
REDOX
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The sites of electron transfer that form
NADH and FADH2 in glycolysis and
the citric acid cycle.
REDOX
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Cytochrome P450 hydroxylase cycle in
microsomes
REDOX
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A sugar that can be oxidized
by Cu2+ solutions
REDOX
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