B. The meaning of oxidation and reduction in terms of :
The loss and gain of oxygenOxidation / Reduction
· The process of gaining oxygen / · The process of losing oxygen
Example :
· Mg + PbO à 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 ……………………………………………..
The loss and gain of hydrogen
Oxidation / Reduction
· The process of losing hydrogen / · The process of gaining hydrogen
Example :
· H2S + Cl2 à 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 ……………………………………………..
the transfer of electrons
Oxidation / Reduction
· The process of losing electrons / · The process of gaining oxygen
Example :
· 2Na + Cl2 à 2NaCl
· Na à Na+ + e // Cl2 + 2e à 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 ……………………………………………..
The change in oxidation number
Oxidation / Reduction
· The increase in oxidation number / · The decrease in oxidation number
Example :
· 2Na + Cl2 à 2NaCl
· Na à Na+ // Cl2 à 2Cl-
· Oxidation number Na : 0 à +1 // Cl : 0 à -1
· Na is oxidized to Na+ : Na is ……………………………………
· Cl2 is reduced to Cl- : Cl2 is ………………………………….
C. OXIDISING AGENTS AND REDUCING AGENTS
AN OXIDISING AGENT· Bromine water / Br2 à 2Br- / The brown coloured bromine water is decolourised
· Chlorine water / Cl2 à 2Cl- / The pale yellow solution is decolourised
· Acidified potassium manganate (VII), KMnO4 solution / MnO4- + 8H+ +2e à Mn2+ + 4H2O / The purple coloured solution is decolourised
· Acidified potassium dichromate (VI), K2Cr2O7 solution / Cr2O72- + 14H+ + 6e à 2Cr3+ + 7H2O / The orange coloured solution turns to green
· Iron(III) sulphate solution / Fe3+ + e à Fe2+ / The brown iron(III) sulphate solution changes to green
A REDUCING AGENT
· Metals that are more electropositive than iron / X à Xn+ + ne / The metal dissolves/ corrodes
· Sulpur dioxide gas / SO2 + 2H2O à SO42- + 4H+ + 2e / -
· Hydrogen sulphide gas / H2S à S + 2H+ + 2e / A yellow precipitate forms
· Iron(II) sulphate solution / Fe2+ à Fe3+ + e / The green iron(II) sulphate solution changes to brown
· Potassium iodide solution / 2I- à I2 + 2e / A colourless changes to brown
D. WHERE DOES THE OXIDATION AND REDUCTION OCCURS?
1. In chemical cellDiagram 1 : (Using porous pot) Diagram 2 :( Using salt bridge)
Diagram 3 : ( Using U tube)
· The oxidation reaction occurs at the negative terminal
· The reduction reaction occurs at the positive terminal
· The porous pot or salt bridge functions to separate the two solutions but allows the ions to flow through it to complete the circuit.
· The transfer of electrons occur from the reducing agent to the oxidizing agent.
· The energy conversion that occurs in a chemical cell :
Chemical energy à electrical energy
Oxidation / Reduction
Zn à Zn2+ + 2e / Cu2+ + 2e à Cu
Zn acts as ……………………………….. / Cu2+ acts as ……………………………
Observation : Zn dissolves / corrodes / Observation : The blue solution turns to colourless.
2. In electrolytic cell
Diagram 1 : (molten) Diagram 2 : (aqueous solution)
· The oxidation reaction occurs at the anode
· The reduction reaction occurs at the cathode
· The transfer of electrons occur from the the anode to the cathode
· The energy conversion that occurs in an electrolytic cell :
electrical energy à chemical energy
Oxidation / Reduction
4OH- à 2H2O + O2 + 4e
2Cl- à Cl2 + 2e
2I- à I2 + 2e
2Br- à Br2 + 2e / Pb2+ + 2e à Pb
Cu2+ + 2e à Cu
2H+ + 2e à H2
The anions act as ……………………………… / The cations act as …………………………..
The oxidation numbers of the anions are ……………….. / The oxidation numbers of the cations are …………………..
Observation : Colourless gas / coloured gas are released
Cl2 :………………………….
I2 : ………………………………
Br2 : brown coloured gas / Observation : Grey solid is formed
H2 : colourless gas
3. Displacement Reaction
a) The displacement of metal from its salt solution
Electrochemical series
K Na Ca Mg Al Zn Fe Sn Pb (H) Cu Hg Ag
Electropositivity decreases
……………………………………. Least electropositive
· The element in the higher position in the electrochemical series or the more
electropositive element, is the element that tends to release electrons to form positive
ions.
· M à Mn+ + ne
· The ………………. (more/ less) electropositive element is oxidized more
…………………. (easier / harder )and acts as a ……………………..
( stronger / weaker) reducing agent.
· The element that is located higher in the electrochemical series can displace other
elements that are …………………….. in the electrochemical series from its salt solution.
· Example :
Zn + CuSO4 à ZnSO4 + Cu
Zn à Zn2+ + 2e [ Oxidation ]
Cu2+ + 2e à Cu [ Reduction ]
Zinc, Zn is more electropositive than copper,Cu.
Thus atom Zn 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.
Atom Zn acts as ………………………………………
Ion Cu2+ acts as …………………………………………
b) Displacement reaction of halogens from their halide solutions by other halogens.
· Halogens are located in Group ………….. of the Periodic Table
· Halogen elements tend to receive electrons to achieve a stable electron arrangement.
· Halogens are reduced to halide ions
· Therefore, halogen acts as ………………………………..( a reducing agent/ an oxidizing agent ).
· The tendency of halogen s to receive electrons decreases when going down Group 17.
· Thus, the reactivity of halogens acting as ………………………….. decreases when going down the group 17.
Flourine, F2 Chlorine,Cl2 Bromine, Br2 Iodine, I2
Reactivity decreases
· Example :
Cl2 + NaBr à NaCl + Br2
Cl2 + 2e à 2Cl-
2Br- à Br2 + 2e
· Chlorine water, Cl2 oxidises bromide ion, Br- to form bromine, Br2
· Bromide ion, Br-, releases electrons to form bromine, Br2
· Chlorine, Cl2 receives electrons to form chloride ion, Cl-
Halogen / Colour of halogens in tetrachloromethane.Chlorine water, Cl2 / Yellow or pale yellow
Bromine water, Br2 / Brown
Iodine solution, I2 / Purple
4. Rusting as a redox reaction
4.1 Corrosion of metals
The corrosion of metals occur when a metal loses electrons and is oxidized to form the
metal ion. The metal is corroded.
M à Mn+ + ne
Examples :
Atom sodium, Na, loses 1 electron to form sodium ion, Na+. Sodium is oxidized.
Na à ……………. + e.
Atom Aluminium, Al loses 3 electrons to form aluminium ion, Al3+ . Aluminium is oxidized.
Equation : ………………………………………………………………………….
4.2 Rusting of iron
· Iron, Fe gets rusted or corroded when it loses electrons to form iron(II)ion, Fe2+.
Fe à Fe2+ + 2e
· The rusting of iron requires à water
à oxygen / air
· The rusting of iron occurs in five stages.
Stage / Explanation1 / At the centre of the water droplet (anode), the iron loses two electrons and is oxidized to iron(II) ion, Fe2+ . The potassium hexacyanoferrate(III) solution is used to detect the presence of iron(II) ion, Fe2+. ( dark blue spot )
Fe à Fe2+ + 2e
2 / The electrons that are released by the iron, flow to the edge of the water droplet through the iron
3 / At the edge of the water droplet (cathode) ,oxygen and water received electron and is reduced to hydroxide ions, OH- . The OH- ion can be detected by the phenolphthalein indicator, in which a pink colour is formed.
O2 + 2H2O + 4e à 4OH-
4 / Iron(II) ions, Fe2+, and hydroxide ions, OH-, combine to form a green solid, namely iron(II) hydroxide, Fe(OH)2
Fe2+ + 2OH- à Fe(OH)2
5 / iron(II) hydroxide, Fe(OH)2 is then oxidized to a brown solid, namely hydrated iron(III) oxide, Fe2O3.3H2O (rust) by the oxygen in the air.
Fe(OH)2 à Fe2O3.3H2O
Oxygen in the air
Draw a labeled diagram (rusting of iron) according to the above explanation.
4.3 Preventing the rusting of iron
Using protective layer ( surface protection ) / · Coating by a layer of metal such as tin.· Applying paint, oil or grease on the surface of iron.
· Wrapping the iron ‘s surface with a layer of plastic.
Electrochemical protection / The surface of the iron is protected by the more electropositive metal
· The coating of iron with a protective layer of zinc is called galvanization
Using sacrificial metal (cathode protection) / Items made of iron are tied to a big filled with magnesium or zinc powder. Magnesium or zinc will be corroded.
Alloying / Iron is alloyed with other metals such as chromium or nickel to produce stainless steel.
1 / Diagram 1 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.
(Rajah 1 menunjukkan susunan radas bagi eksperimen untuk mengkaji kesan logam P dan Q ke atas pengaratan paku besi. Keputusan dicatatkan selepas satu hari)
Experiment / After 1 day / Observation
A / / / Some dark blue precipitate.
(Sedikit mendakan biru)
B / / / Large amount of dark blue precipitate
(Banyak mendakan biru)
C / / / No dark blue precipitate. Solution turns pink.
(Tiada mendakan biru. Larutan bertukar merah jambu)
DIAGRAM 1
(a) / Write the half-equation for the formation of iron(II) ion from iron.(Tuliskan setengah persamaan bagi pembentukan ion ferum(II) daripada ferum.)
......
[2 marks]
(b) / State the function of potassium hexacyanoferrate(III) solution in the experiment.
(Nyatakan fungsi larutan kalium heksasianoferat(III) dalam eksperimen ini.)
......
[1 mark]
(c) / Which test tube shows the fastest rusting of iron? Explain your answer.
(Tabung uji yang manakah menunjukkan pengaratan ferum yang paling cepat. Terangkan jawapan anda.)
......
......
[2 marks]
(d) / Arrange the metals Fe, P and Q in order of decreasing electropositivity.
(Susun logam Fe, P dan Q mengikut urutan keelektropositifan menurun.)
......
[1 mark]
(e) / (i) / What happens to metal Q in test tube C?
(Apakah yang berlaku kepada logam Q dalam tabung uji C?)
…......
[1 mark]
(ii) / Suggest a metal that can be used as metal Q.
(Cadangkan satu logam yang boleh diguna sebagai logam Q.)
…......
[1 mark]
(f) / State the ion that causes the solution in test tube C to turn pink.
(Nyatakan ion yang menyebabkan larutan dalam tabung uji C bertukar kepada merah jambu.)
......
[1 mark]
2 (a) Define oxidation and reduction in term of changes in oxidation number.
[2 marks]
(b) Based on electron transfer, explain the oxidation and reduction reaction in
(i) changing of Fe2+ ions to Fe3+ ions
(ii) changing of Fe3+ ions to Fe2+ ions
Use a suitable example for each of the reaction. Include half equations in
your anwers.
[8 marks]
(c)
Describe an experiment to prove the above statement by the transfer of
electrons at a distance. In your description, include
· labelled diagram which shows the set up of apparatus
· procedure of experiment
· observations at both electrodes
[10 marks]
3. / Diagram 3 shows the set up of the apparatus for an experiment to investigate electron transfer through a distance.Rajah 2 menunjukkan susunan radas suatu eksperimen untuk mengkaji pengaliran elektron melalui satu jarak.
a) Label the negative terminal and the positive terminal of the cell on in Diagram 2
Labelkan terminal negatif dan positif bagi sel dalam Rajah 2.
[1 marks]
b) State the function of the dilute sulphuric acid.
Nyatakan fungsi asid sulfurik cair.
……………………………………………………………………………
[1 mark]
c) Name the reducing agent in the experiment
Namakan agen penurunan dalam eksperimen
……………………………………………………………………………
[1 mark]
d) Write the half equation for the reaction that occurs at silver electrode
Tuliskan persamaan setengah bagi tindakbalas yang berlaku pada elektrod argentum.
……………………………………………………………………………
[1 mark]
e) State two observations for the experiment.
Nyatakan dua pemerhatian bagi eksperimen .
……………………………………………………………………………
……………………………………………………………………………
[2 marks]
f) What is the change in oxidation number of magnesium ?
Apakah perubahan nombor pengoksidaan magnesium?
……………………………………………………………………………
[1 mark]
g) (i) The experiment is repeated by replacing the sulphuric acid with glacial ethanoic acid.
State one observation of this experiment
Eksperimen diulangi dengan menggantikan asid sulfurik dengan asid etanoik glacial.
Nyatakan satu pemerhatian eksperimen ini..
……………………………………………………………………………
[1 mark]
(ii) Explain your answer in (g)(i)
Jelaskan jawapan anda di (g)(i)
………………………………………………………………………
……………………………………………………………………………
…………………………………………………………………………….
[2 marks]