1972-11-16 — Page 26

真二第張七第日一十月十年子壬歷夏

WAH KIU YAT PO

報日僑茶

Example.

Mathane, CH

reacts with

南僑榮

1973英文中學會考試題預習專

化學科

眼科

(四)

Chemistry (4)

Solution for last week

Part I.

C 2. B. 3.B

Part II.

The volumes diffusing are in the inverse ratio of the square

roots of the densities;

volume of hydrogen

volume of air

volume of air

100 x

0.09

0.09

1.293

26.4 ml.

Formula and Equationr

The formula of a compound re- presents the composition of one molecule of that compound, which indicates three points:

1. The kind of element, thei

chemically combine together to form that molecule.

The number of atoms of differ ent elements that combine to form that molecule.

3. A definite proportion by weight as represented by the sum of the weights of the different

Į atoms in the molecule.

Example:

A substance on analysis gave the following composition:- carbon 64.86%, hydrogen 13.51%. oxygen 21.63%. Find the simplest formula.

Element

by wt.

atomic wt"

relative no. of aton

simplest ratio of atoms

carbon

64.86

5.046

13.51

Oxygen

21.63

16.

подохран

Therefore the simplest 19)

CH

A chemical equation respresents a chemical change

The requirement for writing the Oorrect equation for a reaction may now be summarized

a) The reactants and the products

of the reaction must be known, b) The formule for all reactantg“

and products must be known

c) The law of conservation of mass

must be obeyed.

d) The equation should be written

vith whole-number coefficients The smallest possible

oxygen to form water and carbon dioxide only. Write the equation for the reaction,

CH, +20,

CO2+ 2H20

A plain equation does not gives 1) The conditions under which the reaction occurs, e.g. heat, cat- alyst etc

ii) The speed of reaction iii) The heat is given out (exathermic) or absorbed (endoth- ermic).

iv) The state of substances, eg, solid, liquid or in concentration or dilute solution.

Caleulation:

1. Magnesium chloride and hydr were formed by the reaction of magnesium and hydrochlonic acid. What weight of magnesium must have reacted if the weight of the chloride form was 1.64 gm? (Atomic wt. Mg:24.3, C1:35.5)

Mg+ 2HC1

→ MgCl2 + H2 mol. wt. 95.3

95.3 gm. of magnesium chloride is produced from 24.3 gm. of magnesium

1.64 gm.of magnesium chloride - is produced from 1.64

350 em 243

0,42 gm, of magnesium

2. What volume of hydrogen au N.T.P. could be prepared by treat- ing 4.86 gm, of magnesium with an excess of hydrochloric acid?

Mg + 2HC1 24.3

Mg01

22.41

24.3 gm. of magnesium yields 2.41 of hydrogen at N.T.)

4.86

4.86 gm, of magnesium yields

x 22.41 = 4.481 of hydrogen

at N.T.P.

Type of chemical Reactions

1.) Direct combination This is a type of chemical change in which two or more substances directly combine to form one compound. Cxamples"

H2

2) DecomDUS1TION

ZH C

2H01

A substance

oreaking down into two or more simpler substances.

Examples

¿Cu (NO3)2

KC10

20uo + 4NO

2KC1 30,

3) Replacement An element acting on a compound which replaces an element in the compound,

xamples:

Zn + Cuso

Mg 2H01

Zuso

MeC1, +

This is

4) Double decomposition

a type of chemical change in which two compounds react together and undergo a mutual exchange of their constituent part.

Examples:

CaC0, + 2HC1

Na, SO + Ba(NO

CaC

BaSo

四期星

日六十月一十年二七九一歷公乍一十六國民華中育教儒華

8) Thermal Dissociation This is defined as the splitting up of a molecule of a compound on heating

into simpler molecules which recombine on cooling. Examples:

NH CI

CECO

HO

NH 3

Cao +60

9) Reversible reaction This is a réaction which can be made to pro- ceed in either direction by the alteration of the conditions of the experiment,

Examples:

3Fe+ 4H0

Ca (HCO

Exercise Part I

80304

412

Ca

1. One mole of hydrocarbon CXнy is burnt in excess oxygen to form 00.

and H.0. The number of molecules of oxygen used up is

A1 BX + C. x + 21 D. 2x + y

The information of which is not available from a balanced chemical equation is:

A. What substances react together B. What products are formed.

C. What conditions are necessary D. How many atoms or molecules

are involved.

E. How many liters of a gaseous

product t are produced at S. T. P. 3. What volume of chlorine is re- quired to convent 22.41 of methane. to chlor form at S.T.P. B. 22.41 A 31,21

0.44.81

D. 67.21

E. 8961

of the following are reversible? 1) 3Fe

reactions

4H2O →→ Fe 304

4H

3) 2KC103

NO2

2K01

A. 1 &

B. 1.&

O2 &

E A11

2) N2

none of above

5) In balancing the following equation, what is the coefficient of nitric oxide?-

3Cu 2NO

3Cu + 8H + 2NO

•NO+

4H2O

A. 1 B 2

6) In the equation 2Pb(NO),

2760 + 02 4N0, the coefficients

represent

P. the mass ratio of the substance reacting and being produced

Q. the volume ratio of the subs- tances reacting and being produced R. the molecular ratio of the substances reacting and being proo sed

5. the exact number of molecules involved in the reaction

Tthe exact mass of the reactants and products involved

7) Which of the following sets of numbere balances the equation?

+2NaNO

r_H20 +_Cl2

Neutralisation Reactions be- tween acids and bases to form water and salt

Examplest

HỘI + NAOH

NaCl +

H2SO4 + 2NH OH - (NH),

SO4+2H2©

6) Oxidation Oxidation is the increase of oxygen or the decrease of hydrogen in a compound. Examples:

200

200

Pb0, + 4HCl → Pb01, + 2H 0 + 01.

7) Reduction Reduction is the increase of hydrogen or the de. crease of oxygen in a compound

Examples

CO

Duo

Cu + CO2 12 + H2O + SO - 2HC1

2,

HC1KC1

Mn01.

8) In a compound, 28 gm.of murogen 1a compined with 48 gm, of oxygen.! Ita empinical formula is P. NO

QNO

3. NO.

T. NO

Part II

1. The composition of sodium car bonate le 43.4% sodium; 113% carbon and 45.3% oxygen. Calculate its simplest formula

2 If nydrogen sulphiae te prepare by using ferrous sulphide, and ffd 2.2 m. of its substance are used up. Calculate the weight of hữ-

rogen du paide producéd

1978

中文中學會考試題預習專欄

英文科

English

Letter writing

(四)

A candidate is required to write a letter, the content of which is not less than 50 words. There are, broadly speaking two kinds of letters, formal ones and informal ones.

In actual life, a person can use any suitable type of language ine

an informal letter. He can use. notes, slang, colloquialisa etc., if he should find those appropri- ate. In an examination like the Hong Kong Certificate of Education (Chinese) Examination, however, it 19 better to use complete sentence vand avoid slang. The following are

a few reminders of points, which candidates may know already: Address--- This is put in the upper right-hand corner. Date--- This is put immediately underneath the Address (4 November. 1972 or 4th November 1972). Salutation--- This can be done in several ways. depending on the re- lationship or degree of affection between the writer of the Letter and the receiver of the letter, Sometimes,

3, this depends on custom, etiquette, social or business position, age,etc. For example, Phillip Wong could write to Maj Chan in

of these ways:

Dear Miss Chan, ¡Dear Mav.

My dear May, My darling May

The following way is a wrong way and should never be used:

Dear Miss May or Dear Mr. Phillip,

The ending of a letter depen 4 On the same factors as those affet- ting the choice of an opening salutation. In an informal letter, Phillip could end the letter in one of the following ways:

very sincerely, Yours affection" ately, Yours sincerely, Yours faithfully・・ Yours, Loure, as

ever

in an informal letter, a person can use any ending, as luu 89 it is courteous and suits the occasion. Thus, for example, he can end the letter like this:

With best wishes, Yours With much love,

Your,

Yours,

With kindest regards,

With kind personal regards. Yours Content and Style.

Get to the point as soon as possible. In an examination, it i helpful sometimes to start the body of the letter by thanking the receiver for a letter, which cand-

idate pretends to have received from him. Candidate should know what he wants to say. He should say it clearly and directly. He should think about the things in Which the receiver is interested. If he knows that receiver is main- ly interested in himself, then he must try to write about receiver or things which affect receiver. If the receiver (perhaps the father or mother of the writer) is more interested in the writer, then the Writer must try to write more about himself.

Candidates should write legibly unless they wish to insult the receivers, or in the case of an examination, take the risk of getting low marks. They should use short sentences and short paragra- phs rather than long ones, Accu- Bations should be avoided, unless they are directed to make them in an examiriation.

As for formal letters, ROVEITI <ment departments and many business films have their own way of wmte Ing letters, Generally, formal letters have the following salut- ations and endings: Salutations

(to a firm)

Dear Sir, Dear Sir, Dear Madam, Dear Mr. Soo, Dear Mrs. 500. Dear Mis9 500.

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