Form 4 / Chemistry / Reading Assignment 2 / 1

Heep Woh College F.4 Chemistry Reading Assignment 2

Group Name: ______Class: 4 ( )

Group Member: 1. ______( ) 2. ______( )

3. ______( ) 4. ______( )

Part I: Read the following passage and answer the questions below.

Production of aluminium from aluminium ore

Aluminium is the most abundant metal in the Earth’s crust. Pure aluminium can be produced from the chief ore of aluminium – bauxite. It is a mixture of hydrated aluminium oxide and hydrated iron oxide.

Metallic aluminium has many properties that make it useful in a wide range of applications. It is lightweight, strong and non-toxic. The surface of aluminium quickly oxidizes to form an oxide layer that can resist corrosion. Furthermore, aluminium can be recycled easily.

History of aluminium extraction

In 1824, Hans Oersted, a Danish chemist, extracted aluminium using a three-step process.

Step 1 Prepare aluminium chloride by passing chlorine gas over a very hot mixture of aluminium oxide and carbon.

Step 2 Heat a mixture of aluminium chloride and potassium amalgam (an alloy of potassium and mercury), which produces potassium chloride and aluminium amalgam.

Step 3 The amalgam is distilled under vacuum. Aluminium metal is left as a residue and liquid mercury is collected.

In 1827, Friedrick Wohler repeated Oersted’s work and produced aluminium successfully when he replaced potassium amalgam with pure potassium metal.

In 1886, Charles Martin Hall from the United States and Paul Heroult from France independently discovered and patented an aluminium production process. In the process, aluminium oxide obtained from bauxite is dissolved in molten cryolite and decomposed by electricity. The Hall-Heroult process remains the only method by which aluminium is produced industrially.

S.Mo & C.K.Lau Form 4 / Chemistry / Reading Assignment 2 / 2

Industrial production of aluminium

The first step in the industrial production of aluminium is the separation of aluminium oxide from the iron oxide in bauxite. This is done by dissolving the mixture of aluminium oxide and iron(II, III) oxide in a concentrated sodium hydroxide solution. The aluminium ion forms a soluble complex ion with the hydroxide ion, while the iron ion does not.

Al 2O3(s) + 2NaOH(aq) + 3H 2O(l) → 2NaAl(OH) 4(aq)

After the insoluble iron(II, III) oxide is filtered from the solution, aluminium hydroxide is precipitated from the solution by adding acid to lower the pH to about 6. Then the precipitate is heated to produce dry aluminium oxide.

2Al(OH) 3(s) → Al 2O3(s) + 3H 2O(l)

The extraction of aluminium from the oxide is done by electrolysis, but first the aluminium oxide must be made molten so that electricity can pass through it.

Aluminium oxide has a very high melting point (over 2000 oC). It would be expensive to melt aluminium oxide. So, instead, aluminium oxide is dissolved in molten cryolite – an aluminium compound with a lower melting point. As the melting point of the mixture of aluminium oxide and cryolite is lower, less fuel is needed. Thus, the use of cryolite reduces the energy cost of the extracting process.

S.Mo & C.K.Lau Form 4 / Chemistry / Reading Assignment 2 / 3

Questions: 1. What type of reaction did Oersted carry out when he produced aluminium?

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2. Why do you think Oersted decided to use a potassium amalgam rather than a metal such as iron or copper?

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3. What environmental risks did Oersted’s experiment pose?

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4. Do you think Wohler’s method was cleaner than Oersted’s method? Give a reason for your answer.

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5. In 1886, using electricity to extract aluminium was welcomed because it was cheaper and produced purer aluminium than previous methods. Do you think this method was cleaner than previous ones? Give reasons for your answer.

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6. Write an ionic half-equation for the reaction that occurs at the cathode in the electrolytic cell.

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7. Why does the anode sometimes have to be replaced?

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______End of Part I

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Part II: Read the following passage and answer the questions below.

Why are Alkaline Cells Better than the Zinc-carbon Cells?

Alkaline cells are also called alkaline manganese cells. Compared with zinc-carbon cells (i.e. ordinary dry cells), alkaline cells have larger current and longer shelf life. They are durable and their case resists corrosive. All these advantages are due to the special structure and materials used in alkaline cells.

In a zinc-carbon cell, the conducting material of the positive electrode is a carbon-rod while that of the negative electrode is a zinc case. The zinc case is a cylinder which acts as the container of the cell. The space between the case and the carbon rod is filled with ammonium chloride, manganese(IV) oxide, etc. In an alkaline cell, the positive electrode is manganese(IV) oxide. Although steel acts as the container of the cell, it does not take part in chemical reactions. Therefore, the outer case of alkaline cells is very hard and resistant to corrosion. It prevents leakage and has a longer shelf life. The negative electrode in alkaline cells is also zinc. It is in powdered form, which is different from the zinc cylinder in ordinary dry cells. The zinc powder is wrapped by cellulose paper. It is soaked in the electrolyte, potassium hydroxide. A metal rod is located at the center of the zinc powder (also the center of the cell). Its end is connected to a steel disc.

Since the potassium hydroxide used in alkaline cells is in a liquid form, which is different from the paste in zinc-carbon cells, their resistance is smaller. Apart from that, zinc takes part in the reaction in powdered form, so the current produced is 3 to 5 times greater than that produced by zinc-carbon cells of the same size. Moreover, upon discharging, zinc-carbon cells produce gases inside the cell but alkaline cells do not. So the voltage of alkaline cells is more stable. In alkaline cells, most fillings are involved in chemical reactions, so it can be made smaller in size. In other words, when comparing alkaline cells with zinc-carbon cells of the same size, the former has greater charge capacity and longer life expectancy.

Alkaline cells are better than zinc-carbon cells in different aspects. Nevertheless, the requirement regarding techniques and materials used in manufacturing alkaline cells are stricter. Hence, they are more expensive. According to the properties of alkaline cells, we have to pay attention to the following when using them. First, since alkaline cells are not secondary cells, they are neither rechargeable after use nor heated. Second, their case can be neither broken down nor punched. This is because the alkaline solution inside is highly corrosive to skin and clothes.

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Questions

1. What are the conducting materials of the electrodes of zinc-carbon cells and alkaline cells?

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2. Why is the outer case of an alkaline cell harder than that of a zinc-carbon cell?

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3. Why is the current produced by alkaline cells 3 to 5 times greater than that produced by zinc-carbon cells of the same size?

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4. Why is the voltage produced by alkaline cells more stable than that produced by zinc-carbon cells?

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5. What is the advantage of zinc-carbon cells over alkaline cells?

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The End

S.Mo & C.K.Lau