Rossita Radzak SASER

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REDOX Redox reactions are chemical reactions involving oxidation and reduction occurring simultaneously.

1. Transfer of electron, Mg Mg2+ + 2e // Cu2+ + 2e Cu 2. Loss or gain oxygen, C + 2CuO 2Cu + CO2 3. Loss or gain hydrogen, H2S + Cl2 2HCl + S 4. changes in oxidation number

Rusting of iron 1. When iron exposed to water and oxygen 2. Iron atom releases 2 electrons to form iron (II) ion, Fe2+ / is oxidized to form iron (II) ion, Fe2+ 3. Fe Fe2+ + 2e // (anode) 4. iron acts as reducing agent 5. Oxygen and water receives /gain electrons to form hydroxide ions. 6. O2 + 2H2O + 4e 4OH- ( cathode ) 7. Oxygen acts as oxidizing agent. 8. Iron (II) ion , Fe2+ combine with hydroxide ion ,OH- to form iron (II) hydroxide, Fe(OH)2. 9. Iron (II) hydroxide, Fe(OH)2 oxidized by oxygen to form iron (III) oxide, brown solid/precipitate, Fe2O3.x H2O. Fe2+ Fe3+ + e Effect of the contact of other metals on the rusting of iron. Aim : To investigate the effect of the contact of other metals on the rusting of iron. Problem statement : How does the effect on rusting when iron is in contact with another metal? Hypothesis : When a more electropositive metal is in contact with iron, the metal inhibits rusting. When a less electropositive metal is in contact with iron, the metal speeds up the rusting. Variable: Manipulated : Type of metal that in contact with iron. Responding : Rusting of iron Fixed : Iron nails, temperature, medium in which iron nail are kept.

Rossita Radzak SASER

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Apparatus : Test tube, test tube rack Materials : iron nails, magnesium ribbon, copper strip, zinc strip, tin strip, hot jelly solution, potassium hexacyanoferat (III) , K3Fe(CN)6 solution, phenolphthalein indicator, sand paper. Procedure:

1. Five iron nails, magnesium ribbon, copper strip, zinc strip and tin strip were cleaned with sand paper.

2. four iron nails were coiled tightly with the magnesium ribbon, copper strip, zinc strip and tin strip respectively.

3. All five iron nails were placed in separate test tube. 4. The volume of hot jelly solution that was mixed with a little K 3Fe(CN)6 solution and

phenolphthalein indicator was poured into the each test tube to completely cover all the nails.

5. The test tubes were kept in a test tube rack and were aside for a day. 6. All observations were recorded.

Observation

Observation Metal Intensity of dark blue colouration

Intensity of pink colouration

Condition of nail

Fe Low The surface of the nail was partially covered with reddish brown solid

Fe-Mg

High No reddish brown solid was found on the surface of the nail.

Fe-Zn High No reddish brown solid was found on the surface of the nail.

Fe-Sn Moderate Low The whole surface of the nail was covered with reddish brown solid

Fe-Cu High Low The whole surface of the nail was heavily covered with reddish brown solid

The nail in test tube A rusted a little. No rusting occurred to the nails in test tubes B and C .The nail in test tube D rusted but the nail in test tube E rusted even more. Discussion

1. Based on the observations magnesium and zinc metals inhibit rusting of iron, while copper and tin metals speed up rusting of iron.

2. This is because magnesium and zinc are more electropositive than iron. Magnesium atom or zinc atom releases its electron more easily than iron.

3. Copper and tin are less electropositive than iron. Iron atom releases its electrons more easily than copper atom or tin atom.

4. The less electropositive metals that in contact with iron, the faster the rusting of iron occurs.

5. The more electropositive metals that in contact with iron, prevent iron from rusting.

Rossita Radzak SASER

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Conclusion Rusting can be prevented when iron is in contact with a more electropositive metal. Rusting occurs faster when iron is in contact with a less electropositive metal. Displacement reaction 1. Displacement of metal : Example : Zn + CuSO4 ZnSO4 + Cu // Zn + Cu2+ Cu + Zn2+

- Zn atom oxidized to Zn2+ - oxidation number of Zn changes / increase from 0 to +2, - Zn acts as reducing agent.

- Copper (II) ion reduced to Cu - oxidation number of copper changes / decrease from +2 to 0 - Cu2+ ion acts as oxidizing agent 2. Displacement of halogen : Example : Cl2 + 2KI 2KCl + I2 // Cl2 + 2I- I2 + 2Cl- Cl2 + 2e 2Cl- ( reduction) 2I- I2 + 2e (oxidation) Transfer of electron at a distance – U-tube Procedure:

1. clamp a U-tube to a retort stand 2. pour dilute sulphuric acid 3. add solution into one end of the arm of the U-tube 4. add solution into the other one end 5. place / dip carbon electrodes into each arm of the U-tube 6. connect the electrodes to a voltmeter/ galvanometer using connecting wire 7. leave the apparatus for 30minutes 8. record the observation

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 answer. Sample answer: 1. Fe+2 Fe+3 + e 2. Br2 + 2e 2Br – 3. Iron(II) ions lose electrons to become iron(III) ions. Iron(II) ions are oxidised 4. Bromine molecules gain electrons to form bromide ions. Bromine molecules are reduced (a: any suitable reducing agent) 1. Fe+3 + e Fe+2 2. Zn Zn+2 + 2e 3. Iron(III) ions gain electrons to become iron(II) ions. Iron(III) ions are reduced 4. Zinc atoms lose electrons to form zinc ions. Zinc atoms are oxidized (a: any suitable reducing

agent)

Carbon electrodes Chlorine water

Potassium iodide solution

Dilute sulphuric acid

Rossita Radzak SASER

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Reactivity series

1. reactive metal with oxygen 2Mg + O2 2MgO

2. hydrogen gas with oxide of less reactive metal H2 + PbO Pb + H2O

3. carbon with oxide metal C + 2CuO 2Cu + CO2

4. carbon dioxide with metal

CO2 + 2Mg 2MgO + C Application of reactivity series in the extraction of metals Extraction of iron from its ores, hematite, Fe2O3 Extraction of tin from its ores, cassiterite, SnO2 - in blast furnace , carbon / coke as a reducing agent. Example: C + O2 CO2 C + CO2 2CO C, CO2 , 2CO reduced the iron oxides to iron 2 Fe2O3 + 3C 4Fe + 3CO2 Fe2O3 + 3CO 2Fe + 2CO2

CaCO3 CaO + CO2 ( lime stone decomposed) CaO + SiO2 CaSiO3 ( impurities )

K Na Ca Mg Al C Zn H Fe Sn Pb Cu Hg Ag Au

Positions of carbon and hydrogen in the reacting series of metal

Oxidizing reducing agent agent

Produce oxygen