1970-02-05 — Page 25

GERARY

育教僑華頁一第張七第日九十月二十年酉己曆墓

10英文中學會考試題預習專欄

文書院主編

化學科(十四)

SOLUTIONS FOR LAST WEEK.

CHEMISTRY (14) ▼

Chloride ion can be detected in solution waen excomm dilute nitric acid and silver nitrate

solution are added. A white, ourdy precipitate of silver chloride... readily soluble in ammonia, is produced.

+01

Ag01/

In the case of the bromide ion, precipitate of silver bromide soluble in ammonia.

12 + Br

For the iodide ion in solut

a pale yellow, ourdy

formed, sparingly

a curdy yellow

precipitate of silver iodide in obtained, which is insoluble in ammonia.

A confirmatory test on the different one may be performed by warming the respective Bamples with concentrated sulphuric acid and manganese dioride z

201 ̄ + Kn©2 + 4H* - Un

+ 28.0+,

2BT

21+

**** 2 + 48"

MnO2 + 4H*

+ 280 +352 1

+ 28 0 + 1 ↑

The halogene evolved may be distinguished by their) different colours. Chlorine is greenish yellow, bromine reddish brown, iodine purple in coloration.

Atomic weight of bromine - 80 ::

Atomic weight of silver 108

Thus molecular weight of Agr

188

Number of gm. moles in 3.2375 g. AgBr.

This is also the number of moles in 2.0517 gm. KEX Therefore molecular weight of Br

2.0517 x

119.1

3.2375

Therefore atomic weight of potassium 119.1-80

2*(a) Manganese dioxide is an oxidizing agent whilst

carbon acts as a reducing agant, thus the two may be distinguished utilizing these criteria,

When concentrated hydrochloric acid is added to manganese dioxide, and the mixture gently heated, chlorine gas is evolved, detacte able through its colour and odour. No reuotion ooours if the sample is.

· X20, +4нCI - HACI, bon.

2

+ 2H20

When black copper oxide is mixed wita carbon in a test-tube fitted with a delivery. tube, heat, and any evolved gas passed througo line-water, the line-water turns cloudy indicating the generation of carbon dioxide. NO rasation ocours in oere of nanganese dioxide,

0 € 200 + COM

Hydrogen is a reducing gas taking part in many reactiona, but nitrogen is very inert.

If • burning splinter is introduced to the open", south of a container containing hydrogen, the gas burne with a blue flame. If the flame is allowed to be in contact with a cold surface, drops of water condenses. In case the gas is nitrogen, no combustion will occur,

28+ 228.0

(e) Sulphuric and hydrochloric acid contain different anions, so that the two: may - bɩ distinguished by anion tests,

When barium ohloride solution is added, tho sulphuric acid will yield a white précipitate of barium chloride, no change will occur in case. it is hydrochloric acid. When silver nitrate solution is added to the soida, silver chloride is formed an a white precipitate for hydrochlori acid, but no reaction can be observed in the #alphuric acid solution,

BaCha +

H290 - BASO /

+2H01

+ H02 – 4801 | + INO

(d) Red phosphorus is the stable allotropic form of

the non-metallic element phosphorus, mercuric oxide is a basic oxide of the metallic element mercury. The two may be distinguished by observation of the action of an acid, e.g. dilate nitric acid. Mercuric oxide readily ✨ dissolves to give the nitrate, while no reacti scours in case of phosphorus.

Potassium nitrate and potassium sulphate are distinguishable through identification of the anions. Application of the brown ring test, wing ferrous sulphate solution and then cono. #ulphuric acid gives a brown ring in case of the nitrate, but no action for the sulphate. Conversely, addition of barium chloride solution gives a white precipitate only when the sulphate is present.

SO

+2N03 2HNO 20+30 + 280

690

2HNO

VETKIU YATI PO

報日橋

3. Le Chatelier's Principle states that when the

conditions under which a system operating in

equilibrium are changed, the system will shift its equilibrium position in a way that will reduce the effect of the change.

(a) In the vapour state, phosphorus pentachloride

dissociates to an equilibrium mixture containing) phosphorus trichloride and chlorine, further- more the process is endothermic.

+ 012

It cán be seen that the forward reaction

(reaction towards the right) has the effect of doubling the volume with no change in mass (all three behaves as gases and conform with Avogadrole Hypothesis at the experimental conditions). The application of Le Chatelier's theorem shows that a diminution in pressure

must bring about a greater degree of diasociatfor

Increasing the temperature of the system will.

also increase the dissociation as this process, is endothermic.

((b) For the equilibrium C0 + H2O

GO.

as all four are gases at experimental conditions,

there is no change in volume during the

reactions,

hence changes in pressure will have

no effect upon the equilibrium position.

However, the forward reaction is an exothermic

procass, implying that a higher temperature

will favour the equilibrium towards a greater concentration of CO + H2O

(0) N2 + 02

210

In this process, again the number or moles of gases at the right, and left hand sides of the

equation remain the same, implying that changes in pressure are w thout effect upon the squilibrium position. The oxidation of r nitrogen is an endothermic process, the proportion of nitric oxide at equilibrium therefore increases with rising temperature

(4) 2502 +

2503

Here two moles of sulphur dioxide combines with one mole of oxygen to give two moles of sulphur triozride. Therefore there is a reduotidm in volume during the forward reaction, and the Le Chatelier's Principle predicts that increases in pressure will drive the equilibrium position towards sulphur trioxide. Furthermore the

process, 18 exothermic for the forward reaction, so that the reverse change, leading to the...... decomposition of the trioxide, becomes more and more important as the temperature rises.

(•) ¤à¤Oz —0a0

20+ CO2

aguilibrium will be established. As 80oda

This is an example of a heterogenous reaction involving two solids and a gao, If the

and products are kept in a closed vessel,

ocoupy negligible volumes compared with gases, an increase in prezaura will inhibit the forward reaction to a great extent, The

decomposition of calcium carbonate is an endothermic process, and izsrsagen witä an increase in temparature.·

4. (a) Industrial preparation of nitric acid from

amronis.

In the Catwald process, ammonia 18 oxidised te nitric oxide by pacoing a mixture of ammonia and air over, red-hot platinum gauzes

43

+

- 480 + 50%

66,0 + 214.6 kcal.

The nitric oxide is converted into nitrogen dioxide by mixing with air, and then dissolved in water to nitric acid.

(b) Lead chamber arncess for manufacture or asi phuric

aoid

In this process sulphur dioxide 18 oxikisou by the action of air, oxides of nitrogen and, water, The changes, that take place may be representeds

0.

802

NO

2000+

▪2:0。

Thus the brides of nitrogen have a catalyti cetion, serving as an oxygen carrier.

(o) Industrial preparation of methanol

fsthanol is manufactured on a very large cale by passing a mixture of carbon monoxiãe and hydrogen under a pressure of 200-300 atmospheres, over a catalyst heated to about 350°

CO + 252 - CH2OE + 33.9 kcal

The catalyst consists of zinc oxide and gnather

ozide such as: chronic oxide.

(a) Anti-knock properties of gasoline

For efficient running in an internal sombustion engine, the explosion of gasoline must take place smoothly and it must not occu prematurely. The anti-knock properties of gasoline can be improved greatly by the addition of a negative catalyst such as lead tetraethyl Pb(C25/4*

Since practically all chemical changes can be oatalysed by some substances, it is probable that the manner in which a oatalyat acts will not always be the same in every case. T theories of catalysis are given belows

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ARE DRACH ENFERMANJE ZATEKS KUR

:轉訊》中文大學文化,一心中國文一十幷來在英國各大學主

***********(H) EKOSEHO

MAKAN BES

月11十日起,微星期五一代文化巡動之意義及其參加本做榻泡答,林無B、ㄖㄜ街,暲氏在一爲廣視陜氏綱及5】,銳東,流行李异文,黄梁淑,黄天賜,崔始林,藝、

*-*-

林熊易林

專電線無南東

單名業畢届本

料

【科學無港:

李寶

i. The "intermediate compound" theory

postulates.

that the catalyst combines with a reagent to yield a more reactive intermediate compound, that undergoes reaction spontaneously, thus Increasing the rate of reaction,

The adsorption theory was suggested to

xplain the action of heterogenous catalysts, *.g. catalytic power of certain surfaces. The surfaces are remarkable for their power of adsorbing substances, especially gases. The increase in the velocity of the reaction was attributed to the close contiguity of the reactants in the adsorbed films. However, a difference in orientation of the adsorbed molecules on the catalyst may affect profoundly the course of the reaction.

It should be noted that the explanations offered for the catalysis meopaniem, indicated the faster achievement of equilibrium and hot shifting the equilibrium position. It im because the presence of the catalysts increases the rates of both the forward and backward reactions.

QUESTIONS FOR NEXT WEEK

1. Give an account of the different types of fuels,

their preparatione and their uses,

Describe (a) the preparation of sulphur dioxide. from sulphuric acid, (b) the manufacture of sulphut dioxide, free from impurities other than oxygen and nitrogen, from zinc blende.

How, and under what conditions, does sulphur dioxide react with (i) ohlorine, (ii) potassium dichromate, (iii) nitric acid, (iv) sqdium hydroxide?

Describe the reactions which take place between water or steam and each of the following carbon, iron, sodium carbonate, plaster of Paris phosphorus pentachloride, calcium carbide aluminium sulphide.

The sulphate of a monovalent metal contained 55.96%-water of oxystallisation. When treated with barium chloride, 1:61 gm. of the crystals precipitated 1.16 gm. of barium sulphate. How many molecules of water of crystallization are contuined in the formula?,

Indicate the chemical reactions involved in following observationë t

the blue flame at the top of a clear fire." the change in colour when hydrochloric acid) and small pieces of zinc are added to a solution of ferrio ohloride.

the formation of stalactites. a

the hardening of mortar.

the disappearance of the colour when chlorine water is exposed to sunlight.

the disappearance of the odour when coal gan burned.

Page 25Page 26

真二弟張七第,日九廿月二十年己磨夏 WAH KIU YAT PO