Download Oxidation-Reduction Reactions Oxidation-Reduction

Continuation of Chapter 4
Oxidation-Reduction Reactions
OxidationOxidation-reduction reactions involve the
transfer of electrons from one species to
another.
Oxidation is defined as the loss of electrons.
Reduction is defined as the gain of electrons.
Oxidation and reduction always occur
simultaneously.
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Oxidation-Reduction Reactions
The reaction of an iron nail with a solution of
copper(II) sulfate, CuSO4, is an oxidationoxidation- reduction
reaction.
The molecular equation:
Fe(s ) CuSO 4 (aq )
FeSO 4 (aq ) Cu(s )
Total net ionic:
Fe (s) + Cu2+(aq) + SO42- (aq)
Fe2+(aq) + SO42 -(aq) + Cu (s)
Net ionic:
Fe(s ) Cu 2 (aq )
Fe 2 (aq ) Cu(s )
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The net ionic equation shows the reaction
of iron metal with Cu2+(aq) to produce
iron(II) ion and copper metal.
Loss of 2 e- oxidation
Fe(s ) Cu 2 (aq )
Fe 2 (aq ) Cu(s )
Gain of 2 e- reduction
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Oxidation Numbers
The concept of oxidation numbers is a simple
way of keeping track of electrons in a reaction.
The oxidation number (or oxidation state) of an
atom in a substance is the actual charge of the
atom if it exists as a monatomic ion.
Alternatively, it is hypothetical charge
assigned to the atom in the substance by simple
rules.
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Oxidation Number Rules
Rule Applies to
Statement
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Elements
The oxidation number of an atom in an
element is zero.
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Monatomic
ions
The oxidation number of an atom in a
monatomic ion equals the charge of the
ion.
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Oxygen
The oxidation number of oxygen is 2 in
most of its compounds. (An exception is O
in H2O2 and other peroxides, where the
oxidation number is 1.)
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Oxidation Number Rules
Rule Applies to
Statement
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Hydrogen
The oxidation number of hydrogen is
+1 in most of its compounds.
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Halogens
Fluorine is 1 in all of its compounds.
The other halogens are 1 unless the
other element is another halogen or
oxygen.
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Compound The sum of the oxidation numbers of
s and ions the atoms in a compound is zero. The
sum in a polyatomic ion equals the
charge on the ion.
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Sample Question
The oxidation state of nitrogen given for all the
following species is correct EXCEPT
1) N2H4 ( 2).
2) NH2OH ( 1).
3) N2O (+1).
4) HN3 ( 1).
5) HNO2 (+3).
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Sample Problem
Assign oxidation numbers to all atoms in the
following:
CO2
K2Cr2O7
PCl5
HNO2
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Describing Oxidation-Reduction Reactions
A halfhalf-reaction is one of the two parts of an oxidationoxidationreduction reaction. One involves the loss of electrons
(oxidation) and the other involves the gain of electrons
(reduction).
Fe(s ) Cu 2 (aq )
Fe 2 (aq ) Cu(s )
We can write this reaction in terms of two half-reactions.
Fe(s )
Fe 2 (aq )
Cu 2 (aq )
2e
2e
Cu(s )
oxidation half-reaction
reduction half-reaction
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Describing Oxidation-Reduction Reactions
An oxidizing agent is a species that oxidizes
another species; it is itself reduced.
reduced.
A reducing agent is a species that reduces
another species; it is itself oxidized.
oxidized.
Loss of 2 e- oxidation
reducing agent
Fe(s ) Cu 2 (aq )
oxidizing agent
Fe 2 (aq ) Cu(s )
Gain of 2 e- reduction
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Sample Question
In the following reactions, label the
oxidizing agent and the reducing agent.
a) CO2 (g) + C (s)
2CO (g)
b) 8 Fe (s) + S8 (s)
8FeS (s)
Sample Question
Which of the following conversions requires an
oxidizing agent?
1) Mn3+
Mn2+
2) C2H4
C2H6
3) SiF4
SiF62
4) 2CrO42
Cr2O72
5) SO2
SO3
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Sample Question
Which of the following reactions is an oxidation
reduction reaction?
1) CaCO3 + 2HCl
CaCl2 + H2O + CO2
2) NH4NO3
N2O + 2H2O
3) AgNO3 + KI
AgI + KNO3
4) H2SO4 + 2NaOH
Na2SO4 + 2H2O
5) CaO + SO3
CaSO4
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Some Common Oxidation-Reduction
Reactions
Most fall into one of the following simple
categories:
Combination Reactions
Decomposition Reactions
Displacement Reactions
Combustion Reactions
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A combination reaction is a reaction in which two
substances combine to form a third substance.
2 Na (s)
Cl 2 (g)
2 NaCl 2 (s)
A decomposition reaction is a reaction in which a
single compound reacts to give two or more
substances.
2 HgO (s)
2 Hg (l) O 2 (g)
A combustion reaction is a reaction in which a substance
reacts with oxygen, usually with the rapid release of heat
to produce a flame.
4 Fe (s) + 3 O 2 (g)
2 Fe 2 O 3 (s)
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Figure 4.16:
Combustion reaction
Photo courtesy of James
Scherer.
4 Fe (s) + 3 O 2 (g)
2 Fe 2 O 3 (s)
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Figure 4. 14:
Decomposition
Reaction
Photo courtesy of
James Scherer.
2 HgO (s)
2 Hg (l) O 2 (g)
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A displacement reaction (also called a singlesinglereplacement reaction) is a reaction in which an element
reacts with a compound, displacing an element from it.
Zn(s ) 2HCl(aq )
ZnCl 2 (aq ) H 2 (g )
here, Zn has displaced H+ from solution.
Net ionic: Zn (s) + 2 H+ (aq)
Zn2+ (aq) + H2 (aq)
NOTE: any element higher in the activity series (Table
4.6) will reduce (displace) the ion of any element lower
in the activity series
Activity series looks at the relative reactivity of a neutral
metal (M ) with an aqueous cation (Yn+ )
Net ionic:
M (s) + Yn+ (aq)
Mnn+ (aq) + Y (s)
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Table
4.6
Sample Problem
Using the activity series (Table 4.6),
write balanced chemical and net ionic
equations for the following reactions . If
no reaction occurs write down NR.
(a) Al(s) + ZnCl2(aq)
(b) Ni (s) + MnBr2 (aq)
(c ) Zn (s) + HCl (aq)
(d)
Pt(s) + HBr (aq)
(e) Ca (s) + H2O (l)
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Types of Chemical Reactions
Oxidation-Reduction Reactions
Balancing Simple Oxidation-Reduction
Reactions
At first glance, the equation representing
the reaction of zinc metal with silver(I)
ions might appear to be balanced.
Zn(s ) Ag (aq )
Zn 2 (aq ) Ag(s )
However, a balanced equation must have
a charge balance as well as a mass
balance.
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Balancing Simple Oxidation-Reduction
Reactions
Since the number of electrons lost in the
oxidation halfhalf-reaction must equal the number
gained in the reduction halfhalf-reaction,
Zn(s )
2 Ag
Zn 2 (aq ) 2e
(aq )
2e
2Ag(s )
oxidation half-reaction
reduction half-reaction
we must double the reaction involving the
reduction of the silver.
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Balancing Simple Oxidation-Reduction
Reactions
Adding the two halfhalf-reactions together, the
electrons cancel,
Zn(s ) Zn 2 (aq ) 2e
2 Ag (aq ) 2e
2 Ag(s )
Zn(s) 2Ag (aq)
oxidation half-reaction
reduction half-reaction
Zn 2 (aq) 2Ag(s)
which yields the balanced oxidation-reduction reaction.
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Sample Problem
Balance the following oxidation-reduction
reactions by the half-reaction method:
Cr3+ (aq) + Zn (s)
Cr (s) + Zn2+ (aq)
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Working with Solutions
The majority of chemical reactions discussed here
occur in aqueous solution.
When you run reactions in liquid solutions, it is
convenient to dispense the amounts of
reactants by measuring out volumes of reactant
solutions
When we dissolve a substance in a liquid, we
call the substance the solute and the liquid the
solvent.
solvent.
The general term concentration refers to the
quantity of solute in a standard quantity of
solution..
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Molar concentration, or molarity (M), is defined
as the moles of solute dissolved in one liter
(cubic decimeter) of solution.
Molarity (M)
moles of solute
liters of solution
Let s try an example.
A sample of 0.0341 mol iron(III) chloride, FeCl3,
was dissolved in water to give 25.0 mL of
solution. What is the molarity of the solution?
Since molarity
then M
moles of FeCl 3
liters of solution
0.0341 mole of FeCl 3
0.0250 liter of solution
1.36 M FeCl 3
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Sample Problem
What is the molarity of 100 mL
solution containing 15.6g of NaOH?
SAMPLE PROBLEM
What is the molarity of Br - ions in a 0.225 M
aqueous solution of FeBr3 assuming complete
dissociation?
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Working with Solutions
Molar Concentration
The molarity of a solution and its
volume are inversely proportional.
Therefore, adding water makes the
solution less concentrated.
This inverse relationship takes the form
of:
M i Vi
Mf
Vf
So, as water is added, increasing the
final volume, Vf, the final molarity, Mf,
decreases.
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Sample Problem
How many moles of HCl are contained
in 250 mL of a 0.600 molar solution?
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Sample Problem
How many mL of 15.0 M nitric acid
solution should be dilute to make 250. mL
of a 0.50 M solution?
Quantitative Analysis
Analytical chemistry deals with the determination of
composition of materials that is, the analysis of
materials.
Quantitative analysis involves the
determination of the amount of a substance or
species present in a material.
Gravimetric analysis is a type of quantitative
analysis in which the amount of a species in
a material is determined by converting the
species into a solid product that can be
isolated and weighed.
Precipitation reactions are often used in
gravimetric analysis.
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Gravimetric Analysis
Consider the problem of determining the amount of
lead in a sample of drinking water.
Adding sodium sulfate (Na2SO4) to the sample
will precipitate lead(II) sulfate.
Na 2SO 4 (aq ) Pb 2 (aq )
2Na (aq ) PbSO 4 (s )
The PbSO4 can then be filtered, dried, and weighed.
Suppose a 1.00 L sample of polluted water was analyzed
for lead(II) ion, Pb2+, by adding an excess of sodium
sulfate to it. The mass of lead(II) sulfate that precipitated
was 229.8 mg. What is the mass of lead in a liter of the
water? Express the answer as mg of lead per liter of
solution.
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First we must obtain the mass percentage
of lead in lead(II) sulfate, by dividing the
molar mass of lead by the molar mass of
PbSO4, then multiplying by 100.
%Pb
207.2 g/mol
100
303.3 g/mol
68.32%
Then, calculate the amount of lead in the
PbSO4 precipitated.
Amount Pb in sample = 229.8 mg PbS04 x 0.6832
= 157.0 mg Pb
Concentration (mass/V) of Pb in the water = 157.0 mg/L
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Sample Problem
How much AgCl will be formed from
mixing 1.50 L of 0.500 M AgNO3 with
1.75 L of 0.300 M NaCl?
Quantitative Analysis
Volumetric Analysis
An important method for determining the
amount of a particular substance is based
on measuring the volume of the reactant
solution.
Titration is a procedure for determining the
amount of substance A by adding a carefully
measured volume of a solution with known
concentration of B until the reaction of A and B
is just complete. (See Figure 4.20)
Volumetric analysis is a method of analysis
based on titration.
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Figure 4.20: Titration of an unknown
amount of HCl with NaOH
(reaction completion is indicated by an indictor
colour change (i.e., clear to pink)
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Quantitative Analysis
Volumetric Analysis
Consider the reaction of sulfuric acid,
H2SO4, with sodium hydroxide, NaOH:
H 2SO 4 (aq ) 2NaOH(aq )
2H 2O(l ) Na 2SO 4 (aq )
Suppose a beaker contains 35.0 mL of 0.175 M
H2SO4. How many milliliters of 0.250 M NaOH
must be added to completely react with the
sulfuric acid?
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Quantitative Analysis
Volumetric Analysis
First we must convert the 0.0350 L (35.0 mL) to
moles of H2SO4 (using the molarity of the H2SO4).
Then, convert to moles of NaOH (from the
balanced chemical equation).
Finally, convert to volume of NaOH solution (using
the molarity of NaOH).
0.175 mole H 2SO 4 2 mol NaOH 1 L NaOH soln.
( 0.0350L )
1 L H 2SO 4 solution 1 mol H 2SO 4 0.250 mol NaOH
0.0490 L NaOH solution
or
(49.0 mL of NaOH solution)
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Sample Problem
What volume of 0.200 M HCl solution is need to
neutralize 25.0 mL of a 0.450 M KOH solution?
The balanced molecular equation is:
HCl(aq
HCl(aq)) + KOH(aq
KOH(aq))
KCl(aq)
KCl(aq) + H2O(l
O(l)
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SAMPLE PROBLEM
The concentration of aqueous I3- solution can be determined by
titration with aqueous sodium thiosulfate, Na2S2O3. What is the
molarity of I3- if 24.55 mL of 0.102 M Na2S2O3 is needed for
complete reaction with 10.00 mL of the I3- solution? The net
ionic equation is:
2 S2O32- (aq) + I3- (aq)
S4O62- (aq) + 3 I - (aq)
Operational Skills
Using solubility rules.
Writing net ionic equations.
Deciding whether precipitation occurs.
Classifying acids and bases as weak or strong.
Writing an equation for a neutralization.
Writing an equation for a reaction with gas
formation.
Assigning oxidation numbers.
Balancing simple oxidationoxidation-reduction reactions.
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Operational Skills
Calculating molarity from mass and volume.
Using molarity as a conversion factor.
Diluting a solution.
Determining the amount of a substance by
gravimetric analysis.
Calculating the volume of reactant solution needed.
Calculating the quantity of a substance by titration.
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