Chapter 3
WAJA F5 Chemistry 2009Chapter 3 : Oxidation And Reduction
CHAPTER 3: OXIDATION AND REDUCTION
A.REDOX REACTIONS
Learning Outcomes:
You should be able to,
state what oxidation is
state what reduction is
explain what redox reaction is
state what oxidising agent is
state what reducing agent is
calculate the oxidation number of an element in a compound.
relate the oxidation number of an element to the name of its
compound using the IUPAC
nomenclature.
explain with examples oxidation and reduction processes in terms
of electron transfer
explain with examples oxidising and reducing agents in redox
reactions.
write oxidation and reduction half-equations and ionic
equations.
Activity 1
1)The meaning of oxidation and reduction in terms of:
(a)
Gain or Loss of Oxygen
Oxidation
Reduction
The process of gaining oxygen
The process of losing oxygen
Example :
Mg + PbO ( MgO + Pb
Mg gains oxygen to form MgO : The process is
PbO loses oxygen to form Pb : The process is ..
Mg is a reducing agent because
PbO is an oxidizing agent because ...
(b)
The loss and gain of hydrogen
Oxidation
Reduction
The process of losing hydrogen
The process of gaining hydrogen
Example :
H2S + Cl2 ( 2HCl + S
H2S loses hydrogen to form S : The process is ...
Cl2 gains hydrogen to form HCl : The process is .
H2S is a reducing agent because ..
Cl2 is an oxidizing agent because ..
(c)
the transfer of electrons
Oxidation
Reduction
The process of losing electrons
The process of gaining oxygen
Example :
2Na + Cl2 ( 2NaCl
Na ( Na+ + e // Cl2 + 2e ( 2Cl-
Na loses electron to form Na+ : The process is
Cl2 gains electrons to form 2Cl- : The process is ..
Na is a reducing agent because .
Cl2 is an oxidizing agent because ..
(d)
The change in oxidation number
Oxidation
Reduction
The increase in oxidation number
The decrease in oxidation number
Example :
2Na + Cl2 ( 2NaCl
Na ( Na+ // Cl2 ( 2Cl-
Oxidation number Na : 0 ( +1 // Cl : 0 ( -1
Na is oxidized to Na+ : Na is
Cl2 is reduced to Cl- : Cl2 is .
2)What are redox reactions?
Activity 2
Mark with arrows the oxidation and reduction processes as shown
in the example
below:
CuO + C Cu + CO2
ZnO + Mg ( MgO + Zn
Fe2O3 + 3CO ( 2Fe + 3CO2
2HI + Cl2 ( I2 + 2HCl
2NH3 + 3Br2 ( N2 + 6HBr
Activity 3
(a)Fill in the blanks with suitable words.
(i)The substance that causes oxidation is called the .
agent.
(ii)The substance that causes reduction is called is the
agent.
(b)(i)2Zn + O2 ( 2ZnO.
Oxidising agent:..
Reducing agent:..
(ii)2Mg + CO2 ( 2MgO + C
Oxidising agent:..
Reducing agent:..
Activity 4
Example :
a)Determine the oxidation number for the underlined elements
(i) H3PO4
(13) + P + (-24) = 0
3 + P + (-8) = 0
P = 8 3
P = +5
( the oxidation number for phosporus is +5
(ii)N2H4
2N + (14) = 0
2N + 4 = 0
2N = -4
N = -2
( the oxidation number for nitrogen is -2
1.State the changes in oxidation nmber for the bolded element in
the boxes provided and state whether the element undergoes
oxidation or reduction process .
a)Cr2O7 2- ( Cr3+
Process
b)4HCl + MnO2 ( MnCl2 + Cl2 + 2H2O
Process
c)2Fe + 3Cl2 ( 2FeCl3
Process
d)Cu + 2AgNO3 ( Cu(NO3)2 + 2Ag
Process
2. Calculate the oxidation number for chlorine and nitrogen in
their compounds below.
Chlorine compound
HCl
HClO
HClO2
ClO2
HClO3
HClO4
Nitrogen Compound
NH3
N2O
NO
NO2-
NO2
NO3-
3.Determine the oxidation number for the underlined elements in
the table below and name the compound using the IUPAC
nomenclature.
Formula of compound
Oxidation number
Name of compound
CuSO4
Cu2O
CrCl3
Cr2 O72-
NO3-
NH4+
MnO2
Activity 5
For each of the reactions below,
- write the half-equations,
- identify the following:
(i) oxidised substance(iii) oxidising agent
(ii) reduced substance(iv) reducing agent
Example :
Zn + 2Ag+ ( Zn2+ + 2Ag
Oxidation Half-equation : Zn ( Zn2+ + 2e-
Reduction Half-equation : 2Ag+ + 2e- ( 2Ag
Oxidised substance : Zinc (Zn)
Reduced substance : Silver ion (Ag+)
Oxidising agent : Silver ion (Ag+)
Reducing agent : Zinc (Zn)
a)
Cl2 + 2I( ( 2Cl( + I2
b)
c)
d)
Activity 6
Changing Iron(II) ions, Fe2+ to Iron(III) ions, Fe3+
Procedure :
1.The test tube contains 2.0 cm3 of
............................................. solution.
2. Add .................................. drop by drop into the
test tube and heat the mixture.
3.Record the
.............................................................
4. Observation :
The colour of iron(II) sulphate solution change from
....................... to ................. The bromine water
change from ....................... to ........................ In
order to detect iron(III) ion in the solution, add
...................................................................................in
excess. .................................... precipitate of
iron(III) hydroxide is formed.
5. Concept :
a) Iron(II) ion is ..................... to iron(III) ion by
................................... At the same time,
.............................................. is reduced to
............................... ion.
b) Half equation
Fe2+ ( ............. + ............
Br2 + ............ ( .....................
c) Iron(II) ions .............. electrons to become iron(III)
ions. Iron(II) ions
are .........................................
d) Bromine molecules ............. electrons to form bromide
ions. Bromine
molecules are ....................................
e)Reducing agent : ...................................
Oxidising agent : ...................................
Changing Iron(III) ions, Fe3+ to Iron(II) ions, Fe2+
Procedure :
1. The test tube contains 2.0 cm3 of
............................................. solution.
2. Add ......................................... into the test
tube and heat the mixture.
3. Record the
.............................................................
4. Observation :
a. The colour of iron(III) chloride solution change from (i)
.......................
to (ii) .................
b. In order to detect iron(II) ion in the solution, add
(i)........................................
in excess. A (ii)....................................
precipitate of iron(II) hydroxide is
formed.
5. Concept :
a. Iron(III) ion is (i)..................... to iron(II) ion by
(ii)................................... At the
same time, zinc is (iii)......................... to
(iv)............................... ion.
b. Half equations :
Fe3+ + ....................( ..............
Zn ( .................. + ..................
c. Iron(III) ions (i) .............. electrons to become
iron(II) ions. Iron(III) ions
are (ii) .......................
d. Zinc atom (i).................. electrons to form
(ii)................ ions. Zinc atoms
are (iii)........................
e. Reducing agent : (i) .................................
Oxidising agent : (ii) ...................................
Activity 7
Displacement of metal from its salt solution
M ( Mn+ + ne
1.The (i) . (more/ less) electropositive element, is oxidized
more
(ii) . (easily / harder )and acts as a (iii) ..
( stronger / weaker) reducing agent.
2. The element that is located higher in the electrochemical
series can displace other elements that are (i) .. in the
electrochemical series from its salt solution.
Example :
Zn + CuSO4 ( ZnSO4 + Cu
Zn ( Zn2+ + 2e [ Oxidation ]
Cu2+ + 2e ( Cu [ Reduction ]
Zinc, Zn is more electropositive than copper,Cu.
Thus Zn atom releases two electrons to form ion Zn2+.
The electrons are transferred from atom Zn to the copper(II)
ion, Cu2+.
The copper(II) ion, Cu2+ receives the two electrons to form atom
copper, Cu.
Zn atom acts as (ii)
Cu2+ ion acts as (iii)
Activity 8
Displacement of halogens from their halide solutions by other
halogens.
lessreceive
halide
seventeendecreases
oxidising agentmore
1)Halogens are located in Group .. of the Periodic Table
2) Halogen elements tend to electrons to achieve a stable octet
electron arrangement.
3) Halogens are reduced to ions
4) The electronegativity of halogens or their tendency to accept
electrons .. when going down Group 17.
5) Thus, the reactivity of halogens acting as .. decreases when
going down the group 17.
6) In a displacement of halogen, a .. electronegative halogen
displaces a electronegative halogen from its halide solution.
Activity 9
Transfer of electrons at a distance
1.Redox reaction involving bromine water and potassium iodide
solution
2.The half equation for the reaction that occurs around the
carbon rod on the left is as follow.
MnO4- + 8H + + 5e ( Mn 2+ + 4H2O
a)State the change in oxidation number of manganese.
..
b) A brown vapour was observed around the carbon rod on the
right. Name the brown vapour formed.
..
c) Write a half equation for the formation of the violet
vapour.
..
d)On the diagram above, show the direction of the flow of
electron by
using arrows.
B.RUSTING AS A REDOX REACTION
Learning Outcomes :
You should be able to,
state the conditions for the rusting of iron
.state what corrosion of metal is.
describe the process of rusting in terms of oxidation and
reduction.
Generate ideas on the use of other metals to control
rusting,
Explain with examples on the use of a more electropositive metal
to control metal corrosion,
Activity 10
1. Fill in the blanks with suitable terms
a) (i).. is the oxidation of a metal while (ii) is the oxidation
of iron when the metal or iron interact with the environment by
losing electron.
b) The presence of (i). and (ii).will cause iron to rust, the
rusting can be accelerated by adding (iii). or (iv) .
c) Copper forms a green coating as a result of corrosion, it
contains .. ions.
d) Metals like (i). and (ii) forms a very tough oxide which
adhere tightly to the surface of the metal preventing further
oxidation, thus protecting the metals from further corrosion.
e) To prevent corrosion, metal A can be coated with a layer of
metal B which is more (i). than A. Metal B will corrode first, thus
preventing metal A from corrosion. In this situation, metal B is
also called the (ii). metal.
f) If iron is in contact with another (i)which is less
electropositive than iron, the rate of rusting for iron will be
(ii).
g) Zinc is used to protect iron or steel by coating a thin layer
of zinc onto it, this process is called (i).. In industry, zinc is
chosen to serve the purpose rather than other metals because zinc
is (ii) in cost.
h) Some household and bathroom equipments are coated with a
layer of shiny finishes, the metals usually used for these coatings
are (i). and (ii). .
2.Write the equations for the processes below
a) The formation of iron(II) ions from the metal:
When iron contacts with water, the iron surface oxidizes to form
iron(II) ions.
Equation: .
b) The formation of hydroxide ions:
Electrons travel to the edges of the water droplets, where there
is high concentration of dissolved oxygen. Water and oxygen
molecules receive the electrons, they are reduced to form hydroxide
ions.
Equation: .
c) Fe2+ readily combines with OH- to form Fe(OH)2
Equation:
d) With excess oxygen, the rust is formed:
The Fe2+ ions are further oxidized to form Fe3+ ions, which
reacts with OH- ions to form the hydrated iron (III) oxide, Fe2O3.
xH2O, known as rust.
Equation: .
Activity 11
Answer the questions below.
1.Name three main ways to prevent rusting.
(i)
(ii)
(iii)
2.Galvanising involves coating an iron or steel sheet with a
thin layer of..
3.Name three metals that can be used in sacrificial protection
for an underground pipe.
4.Name a reagent that is usually used to detect the presence of
iron (II) ion in an experiment to investigate the rusting of
iron.
..
Activity 12
1
Diagram below shows the apparatus set-up for the experiment to
study the effect of metals P and Q on the rusting of iron nail. The
results are recorded after one day.
Experiment
After 1 day
Observation
A
Some dark blue precipitate.
B
Large amount of dark blue precipitate
C
No dark blue precipitate. Solution turns pink.
(a)
Write the half-equation for the formation of iron(II) ion from
iron.
..............................................................................................................................
(b)
State the function of potassium hexacyanoferrate(III) solution
in the experiment.
..............................................................................................................................
(c)
Which test tube shows the highest rate of rusting of iron?
Explain your answer.
..............................................................................................................................
..............................................................................................................................
(d)
Arrange the metals Fe, P and Q in order of decreasing
electropositivity.
..............................................................................................................................
(e)
(i)
What happens to metal Q in test tube C?
.................................................................................................................
(ii)
Suggest a metal that can be used as metal Q.
.................................................................................................................
(f)
State the ion that causes the solution in test tube C to turn
pink.
..............................................................................................................................
C.THE REACTIVITY SERIES OF METALS WITH OXYGEN AND ITS
APPLICATION
Learning Outcomes :
You should be able to,
compare the differences in the vigour of the reactions of some
metals with oxygen.
deduce the reactivity series of metals.
determine the position of carbon and hydrogen in the reactivity
series of metals.
state what the reactivity series of metals are.
describe the extraction of iron and tin from their ores.
explain the use of carbon as the main reducing agent in metal
extraction.
use the reactivity series of metals to predict possible
reactions involving metals
Activity 13
1.
Figure below shows the set-up of apparatus for an experiment to
determine the order of metals in the reactivity series. Solid
potassium manganate (VII) is heated to release oxygen gas to react
with metal powder.
The experiment is carried out using metal powders of copper,
zinc, magnesium and lead . The observation of the experiments on
the metal powders of copper, zinc, magnesium and lead in the
experiments are shown in table below.
Type of metal
Observation
Copper
Faint glow
Zinc
A bright flame spreads slowly
Magnesium
A bright white shiny flame spreads quickly
Lead
Red hot and embers slowly
(a) Based on the observations in table above, arrange copper,
zinc ,
magnesium and lead in descending order of reactivity of metal
towards
oxygen.
Descending order of reactivity of metal towards oxygen.
(b) Name two other substances that can be used to release oxygen
gas.
(i)
(ii)
(c) Write a balanced chemical equations for the reactions
below.
(i) Copper + oxygen
(ii) Zinc + oxygen
(iii) Magnesium + oxygen
(iv) Lead + oxygen
.
2) According to the equation below, Carbon reacts with oxygen
to
form ..
C ( s ) + O2 (s ) CO2 ( g )
3) Carbon displaces a metal from its metallic oxide. Thus,
by
heating a mixture of metal oxide and carbon , the reactivity of
carbon can be
determined.
0 Oxidation +4
C ( s ) + 2PbO (s ) CO2 ( g ) + 2Pb(s)
+2 Reduction 0
4) Carbon is to carbon dioxide and lead (II) oxide is reduced
to
lead.
5) Carbon acts as a (i).. agent which displaces a metal from its
oxides while the metallic oxide is the (ii) .agent.
6) Carbon is not able to displace a . metal from its metallic
oxide. Hence, there is no displacement reaction when a mixture of
magnesium oxide and carbon is heated.
7) Carbon is positioned in between (i) and (ii) .. in the
reactivity series.
8)
Oxides of metals W,X, Y, Z are heated with equal amount of
carbon powder in an experiment to compare their reactivity with
carbon. The changes observed are recorded in table 2.2
Mixture
Observation
Carbon + oxide of W
No change
Carbon + oxide of X
Dim glow.Grey residue is formed
Carbon + oxide of Y
No change
Carbon + oxide of Z
Bright glow. Brown residue is formed
(a) Based on the observation , classify the metals in groups
that are
(i) more reactive than carbon
(ii) less reactive than carbon
(b) Suggest the possible elements for metals X and Z
X : .
Z : .
(c) The reactivity of W and Y can be compared by heating an
equal amount
of W powder with oxide of Y in a crucible lid using the same
apparatus
set-up.
(i) What is the expected observation if W is more reactive than
Y?
Explain.
(ii) Suggest the possible elements for W and Y
W : .
Y : ..
(iii) Write a balanced chemical equation for the reaction
between W
(charge of +2) and oxide of Y (charge of +3).
..
(iv) Identify the reducing agent and oxidising agent in the
reaction
between W and oxide of Y.
Reducing agent :.
Oxidising agent : .
9.
Complete the reactivity series with oxygen below by writing the
name of the elements.
Potassium
Calcium
Aluminium
Iron
Tin
Mercury
Gold
Reactivity
decreases
10.The following shows part of the reactivity series of metals
with oxygen.
K
Na
Ca
Mg
Al
Zn
Fe
Sn
Pb
Cu
Hg
Ag
Au
Insert the positions of carbon and hydrogen in the above series
by using arrows.
11.Predict what will be observed when,
(a)hydrogen gas is heated with copper (II) oxide.
............
.
(b)a piece of burning magnesium ribbon is dropped into a gas jar
filled with carbon dioxide.
...
(c)carbon is heated with magnesium oxide.
....................
12.Complete the following table by giving the main mineral ore
and the metal extracted from ore.
Ore
Main mineral in ore
Metal extracted
Name
Formula
(a) Bauxite
(b) Hematite
(c) Magnetite
(d) Cassiterite
D.ELECTROLYTIC AND CHEMICAL CELLS
Learning Outcomes:
You should be able to,
explain with examples the oxidation and reduction reactions at
the electrodes of various chemical cells.
explain with examples the oxidation and reduction reactions at
the electrodes of various electrolytic cells.
state the differences between electrolytic and chemical cells in
terms of basic structure, energy conversion and the transfer of
electrons at the electrodes .
compare and contrast electrolytic and chemical cells with
reference to the oxidation and reduction process.
Activity 14
1)The differences between electrolytic and chemical cells
2)The figure above shows the electrolysis process of molten lead
(II) bromide. Answer the questions below.
(a)State the ions contain in the molten lead (II) bromide.
.
(b)Pb2+ ions move to the (i) . while Br( ions move to the (ii)
..
(c)Br( ions act as the (i) . agent, losing electrons to become
(ii) .. molecules. Thus, Br( ions undergo (iii) .. process.
(d)Oxidation half-equation : .
(e)Pb2+ ions act as the (i) . agent, accepting electrons to
become (ii) .. metal. Thus, Pb2+ ions undergo (iii) .. process.
(f)Reduction half-equation : ...
3) Figure below shows a chemical cell.
a) Label the negative terminal and positive terminal and show
the direction of the flow of electrons by using arrows the above
figure.
b) Write the half-equation for the reaction at the positive
terminal.
...
c) Write the half-equation for the reaction at the negative
terminal.
...............
d)State the substance that undergoes oxidation process.
...
e)State the substance that undergoes reduction process.
...
4)Similarities and differences of the redox reactions in
electrolytic cell an chemical cell
Similarities
In both cells,
electrons are transferred from the agent to the agent.
oxidation occurs at the anode.
occurs at the cathode
Differences
Activity 15
1.Draw and label one example of primary cell. Discuss the
oxidation and reduction processes that occur in the cell.
2. Draw and label one example of secondary cell . Discuss the
oxidation and reduction processes that occur in the cell.
Reduction
Oxidation
Oxidised Substance:
Reduced Substance:
Oxidising Agent:
Reducing Agent:
Oxidation Half-equation:
Reduction Half-equation:
Reduction Half-equation:
Oxidation Half-equation:
Reducing Agent:
Oxidising Agent:
Reduced Substance:
Oxidised Substance:
Mg + 2HCl ( MgCl2 + H2
Reduction Half-equation:
Oxidation Half-equation:
Reducing Agent:
Oxidising Agent:
Reduced Substance:
Oxidised Substance:
Pb + Br2 ( PbBr2
Reduction Half-equation:
Oxidation Half-equation:
Reducing Agent:
Oxidising Agent:
Reduced Substance:
Oxidised Substance:
4Na + O2 ( 2Na2O
(1) 2.0 cm3 of Iron(II) sulphate solution (light green)
(1) 2.0 cm3 of iron(III) chloride solution (yellow)
1. Electrons flow from ..
to
2. The colour of potassium iodide solution
changes from to
.
3. The colour of bromine water changes
from to
4. Oxidation half-equation :
.
Reduction half-equation :
.
Overall Ionic Equation :
.
6. Oxidising
agent :
7. Reducing
agent :
Carbon
Electrode
Bromine potassium
Water iodide
Solution
Dilute
Sulphuric acid
The electrodes may be of
the . material such as
as
It requires a source of
.
It does not require a source of
The electrodes must be of
two ...... metals.
Potassium Iodide Solution, 1.0 mol dm-3
Acidified Potassium Manganate (VII) solution, 1.0 mol dm-3
Dilute Sulphuric acid,
1.0 mol dm-3
Carbon rod
The chemical reactions that occur at
the electrodes produce
an
The electrical energy
causes reactions
to occur at electrodes.
Electrons flow from the .
electrode (anode) to the .
electrode (cathode).
Electrons flow from the more . metal ( terminal) to the less
metal (. terminal)
Chemical cell
Electrolytic cell
Y
X