Heep Woh College

Form 5 / Combined Science (Chemistry) / Reading Assignment 1 / 2

Heep Woh College

F.5 Combined Science (Chemistry Part) Reading Assignment 1

Group Name: ______Class: 5 ( )

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

3. ______( ) 4. ______( )

Part I: 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.

Questions

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

______

______

______

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

______

______

______

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?

______

______

______

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

______

______

______

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

______

______

______

End of Part I

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

Using fuel cells to power cars

What sort of car do you expect to be driving 20 years from now? Will it be just a newer and lighter version of the cars you see on the road today? How much will it cost to fill the petrol tank of a car? Will you have trouble breathing the polluted air?

There are severe concerns over diminishing fuel reserves and pollution problems caused by petrol-driven cars. Using efficient energy conversion devices such as fuel cells to power cars is a possible solution.

Fuel cells that convert the chemical energy released by the oxidation of hydrogen into electrical energy are ideal for vehicles aiming at zero emissions.

How to generate hydrogen for fuel cells

One major issue of using fuel cells is how hydrogen can be generated. Electrolysis of water is the most likely method for large-scale hydrogen production. The energy needed for the electrolysis may be obtained from some renewable sources, such as solar, wind or hydroelectric power. However, in the short-term, the most likely solution to generate hydrogen is by extracting it from hydrocarbon fuels, such as natural gas, methanol or gasoline, by steam reforming.

For example, methane (CH4) is the major component of natural gas. In steam reforming, methane is mixed with steam in the presence of a catalyst to produce hydrogen and carbon monoxide.

CH4(g) + H2O(g) D CO(g) + 3H2(g)

This method is a highly developed and cost effective method for generating hydrogen.

Questions:

1. What design features would you expect to appear in future cars?

______

______

______

2. How do you think chemistry can help in the demand for better cars?

______

______

3. Suppose the Hong Kong Government wishes to launch a plan for switching petrol-driven cars to fuel cell-driven cars. Suggest what actions the Government may take to attract users to participate.

______

______

______

______

The End

S.Mo & C.K.Lau