1974-04-27 — Page 23

REFERENCE LIBRARY

育教偡灣T原三第張六第日六初月四年寅甲歷夏、WAH KIU YAT PO

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1974 中學會考試題預習專欄 堅道書院主鍋

化學科(乙) (廿四) ̇王兆女

CHEMISTRY.

week:

Auggested answers for last we The Haber Process is reversible. and therefore obeys Le ChateHerk Principle. When a constraint 'is applied to a reversible -

system in a state of equilibriug the system will be-adjust itsel1 so as to tend to minimise the effects of the constraint." Thus in the system:

+36

2013

Cals.

Heat 16 evolved in the forward reaction and hence a rise in temperature will cause the back reaction to predominate, thereby removing heat energy and tending: to minimize, the increase. In temperature, Hence amdderately Low-temperature (about 500°C.).

is best for optimum yields of ammonia, a lower one making the | reaction too slow to be economic

Moreover a high pressure (from . 200 atmospheres in practice) d1] increase the yield of ammonia," Bince there is a decrease in volume during the forward reactu Lon

32

vol. 3 vols.

2NE

and increased pressure will tend: to be alleviated by the formator of more ammonia.

A catalyst of ferric and alumin- ium oxides is used to increase. the speed at which equilibrium. is attained...

2. (a) On neating cogevner a

mixture of nitrogen and oxygen, nitric oxide is formed. The nitric oxide, however, constant ly decomposes to form nitrogen Cand oxygen, and thus if the reaction is allowed to take place in a closed vessel at constant temperature and press. ure, certain definite amounta of each three substances will co-exist in reversible equili brium.

(b) At constant temperature in this system, it will be found that the ratio of the square of the concentration of the nitric oxide to the product of

the concentrations of the oxy- gen and nitrogen is constant.

This constant is called the equilibrium constant and at any definite temperature it win not varyy.no matter what the: initial concentrations of oxy- | gen and nitrogen may be. The magnitude of K gives a quantit- ative indication, of the relate

dve:proportions of reactants 滋的 and products present in the equilibrium mixture, Thus a large value of K shows that. much nitric oxide will exist in the equilibrium mixture: that. particular temperature.

(o) The beat of formation is the heat absorbed or evolved when one gram-molecule of a substance is formed from its component elements under stated or inferred, conditions. Thus theheat of formation of nitric. oxide would be -21 Calories," since: the equation is wr

4·2·

(d)"This shows that the for- ward reaction is endothermic,, that is heat is absorbed, from the environment, when oxygen and nitrogen combine. The apunt of heat, expressed in Calories, is termed the heat of reaction: and in the case represents the. heat. change when one gram-mols. dule of oxygen combines with. one gram-molecule of nitrogen according to the balanced equat: ion.

ELECTROLYS IS

An electrolyste is an electro valent compound, which, when

·malten or dissolved in water conducts electric current with decomposition. Examples: are copper sulphate. Cu

sulphuric acid

When substances dissolved in aolution split up completely:or largely into fons, such substan- ces. aretermed strong electrolye- tesi

For weak electrolystes, ion- ization is only-alight and most of electrolyte exists. in solut- ̇ion in the form of uniontaed

molecules.

A non-electrolysts is à co~ valent compound which having:no ions, does not conduct electric current, Examples are chloroform CHC1, and

and alcohol?

......... Electrolysis is the decompói” sition of a compound, when molt- en or in solution, by the

of electric current

報日僑華

Discharge offons from mixture, In a solution containing two or more cations, in the same: con- centrations and conditions, the Loistend to discharge in the inverse order of their electro- positivity. I.e., the least electropositive first- For the catieps:

Cu

for anions:

OH

Na

In this case, relative concent- rations may change the order,

Example of electrolysis. Dilute sulphuric acid:

¥2504.

At cathode :

H. discharges by election.gain.

20 +

Hydrogen is. liberated, Concent ion of H 50 16 decreased, At anode:

OH” discharges by electron loss

20H 42e

Oxygen is liberated. Concentration of H SO, Lncreases..

Faraday Is Laws of electrolysis:

1. The mass of an element liber-

·ated during electrolyale is dir ectly proportional to the quant= ity of electricity passing,

-The strength of the current (measured in amperes)

The time for which it flows (measured in seconds)

The quantity of electricity used is the product of these two and is measured in coulombs:

1 coulomb = 1 ampere/second

2. The masses of different. elements liberated in electroly eis by the same quantity of

Ag(CN)2.

at the cathode: Ag at: the anode:::

·AB

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日七十月 四年四七九一厝公年三十六國民華中

be man

Ag

則等號左邊的式就变

此呼B的個壎都瘦

The conditions necessary for electroplating are usually very. epecific.

1. Before immersion in the

electroplating, the cathode should be cleaned.

2. The metal ahould be deposited

on the cathode slowly, so that a hard tanacious surface is

produced. This can be achieved:

in

1) Make sure that the current

used is amalla 11) Use an electrolyte in

which only a few of the required long are present.

Correct the potential differe ence of the cell.

Carefully to control the

temperature when the plating is carried out.

Questions for this week:

By naaging a curront of 0.65

Afor 35 minutes through water

copper, and silver coulometers

the following weights of ele- ménta were liberated: 0.0143 gm of hydrogen, 0.114 gm of oxygan, 0.449 gm ofcopper and 1.542 gm of Alver.

(1) Show that this results.

agree with Faradays

second law; docht

(11):Calculate the electroahem.

ical equivalent of silver

Describe a simple experiment. to illustrate, what is méant by electrolysis, giving a clearly. Tabelled diagram of tha apparatue,

Briefly describe how electro- lysia is used

(a) to manufacture sodium (b), to purify copper.

electridity are directly proport| 1874中學會考試題預習專欄

ional to the chemical equivalent of the respective elements.

Electroplating:

The Important use of electrolyeds is in electroplating, which is used to obtain both a protective as well as decorative covering. 'for articles

For example, silver plating d done by ueing the: metal to be: silver plated as the cathode, the anode used being made of pure silver, and the electrolyte an aqueous solution of sodiun argentocyanide, which gives the

passage | Lon, Ag(CN)2.

數學科(甲)

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|城門水塘举行预树·爲減少供境的染传。此很 日當空之際,洪赴新界先進洒家家被用的 | 年級草敎師,昨日在烈 淡,這些方法均含外地 一名葛量洪敎育學院的!] 重新成瑳市區垃圾的

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原的自往

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樂末

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特色出演 路戲正開 | |

|

*聯校邦采展覽

廿五日開幕

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女準敦子系清掘土,每 出境內第一百六十位男 北、在城門水塘]

·最後乃進行郊野

|完汪定四月廿五日正式踞素,並得吾中質都最

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此有廿五日月一

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