1973-01-17 — Page 28

頁四第張七第日四十月二十年子壬磨复

1913英文中學會考試題預習專欄

IN A SC YPA XR

化學科(十三)

Chemistry (13)

Solutions for last wee

Fart

1. A

2.E

8.

9. S

3. A

10. R

11. B

12. A

6. D

13 D

7.

14. A

Part II

* Dilute hydrochloric acid is added to zing pieces in a flask to produce hydrogen,

Zn + 2H01

ZnCl2 + H2

This hydrogen is dired by passing through several U-tubes containing fused calcium chloride before it is burnt at the tip of a metal jet. The water vapour formed is condensed by the cold surface of a retort containing cold running water, it runs down and is collected in a clean dry beaker,

2H2

+

02

2H 0

To show that this liquid is water, add a drop of it to some white dehydrated copper sulphate powder, and observe how the white powder turns quickly blue, showing that the liquid formed from burning dry hydrogen is water.

thalle

nel Rydrogen

Calliz

6. Acids bases and salts, Definition of an acid

Infer of cat wall

cendised hola'

An acid is defined as a com- pound containing hydrogen, which may be partially or wholly replac- ed by a metal or with a metallio oxide, hydroxide or carbonate. ¤•S•Mg + 2H01→→→→→→→→ MgCl2 + Hy

ZnO + H SO→ ZnSO4 + H2O NaOH + HCl

CuCO + H2SO

NaCl + HO.

>CuSO+CO2+H20

The best and simplest definit -ion of an acid, is given in terms of ions.

An acid is a compound which an aqueous solution produces hydrogen or hydroxonium ions as the only electropositive ions- le.g.

> F20* + €1 ̄

hydroxonium

1on

CH COOH + H2O→H ̧0*+ (CH ̧CCO)

Basicity

The basicity or an acid is the number of hydrogen ion (H) that can be formed from one mole- tule of the aoid. For example,

Hydrochloric acid

HCI-——-> H

Nitric acid,

HNO

Sulphuric acid

01

WAH KIU YAT PO

Preparation of acids

報旧僑茶

(1) Acid anhydrides on water

An acid anhydride is an oxide or non-nutaL which dissolves in water to form an acid such as S03

H2SO4 HCO3

002 + H2O

(2) Precipitation or Double decom- position

BaCl + H2SO

H2S + Pb(NO3)2 H2S+ Cuso

2HNO

black

Cus+ H2SO4

black (3) By the formation of a volatile acid.

NaHSO

HC1

KHSO

NaCl + H2S04 KNO3 + H2SO

(4) Synthesis:

H+ 012

Properties:

1) Sour taste

(2) Tum Litmus red (3) Neutralize alkalis:

+ HNO3

HCL + NaOH — NaCl + H_0

(4) Liberate carbon dioxide from

carbonate:

CaCO3

+ HCl → CaC1, + H2O +

(5) Corrosive action on metals

Metala above hydrogen in the Electrochemical Series displace hydrogen from dilute hydrochloric acid and dilute sulphuric acid. The violence of the reactions de- creases from top downz.

Copper and metals below. nyarogen in the series though do not directly replace hydrogen from dilute acid, yet they do replace it indirectly when acids react wi with the oxides, hydroxides and

carbonates of the metals: Definitions of Bases and Alkalis

A base is an oxide or hydro- oxide of a metal or ammonium, it contains oxide (0) or hydroxyl (OH) ions which reacts with acid. to form a salt and water only.

An alkali is a base that is soluble in water, and forms hydro- xyl ions as the only negative charged ions.

Strong and weak alkalis

The strong alkalis are elect- rovalent compounds, are completely ionized even in the solid state, and contain no molecules, e.g. caustic soda, caustic potash and slaked lime.

Ammonium hydroxide is a weak alkali, it is largely covalent, exists as molecules with only a few ions, and its ionization is reversible.

Preparation or Bases.

(1) oxides of the metals

a) By heating or burning a metal

in air or oxygen. Example:

2Mg + 0

ZMgO

b) By heating the hydroxide car-

bonate or nitrate of a poly- valent metal

Example:

Cu(OH)

Caco

2Pb (NO3/2

HO

cau + 003

2Pb04NO

(2) Hydroxides or the metals.

(monobasic)

(monobasic)

a) Insoluble hydroxides-

by

(α1 basic)

precipitation (double decom- position)

Example:

2NaOH + CuSONa, 50+Cu(OH),

(tribasic)

soluble hydroxides-

(blue

ppt.) by the

action of water on the corres:

e

Example:

2Na + 2H2O→ 2NaOH +

Phosphosphoric acid

H2SO 2H

H3PO4 →→→ 3H+ + PO,

Stron

ong and weak acids

Strong acids are completely Lonized in dilute solution..

HNO

HSO

NO

2H + 50 =

Weak acids are only slightly ionized in dilute solution and exist largely as molecules and their ionization is reversible,

H2003 CH3COOH

Oxy-acids

2H +

CO3

CHCOO

Acids containing oxygen as sulphuric acid, (H2SO or nitric doid, (HNO,) etc. are called oxy- 40ide.

ponding metal or its oxide.

Na20 + H20 → 2HaUH

Properties.

soapy feeling.

(2) Action on the indicators: Titmus-------blue Phenolphtholein--- -pink nethyl orange---- -yellow (3) Caoacuty to neuvralyse acids (4) With ammonium salts on heating

to give ammonia:

NH OL Nath NH2T+H2O+NaC1

(5) Solution of alkalis will pre- cipitate the insoluble hydro-

日七十月一年三七九一曆公年二十六國民藝中育敦儒菊

三期星

xides of other metals from solution of their salts, e.g.

FeCl3 + 3NaOH →→ Fe(OH) V+ зNaC1

reddish- brown ppt

(6) Strong corrosive action to-

wards the organic substances. Neut

is the combination of the oxide or hydroxyl ions of a base with the hydrogen ions of an acid to form, water and a salt is also formed. e.g..

HCL NaOH HO + Na01 Salts:

A salt is a compound formed - by replacing part or all of the replaceable atoms of hydrogen in a molecule of an acid by metallic atoms or by the ammonium radicals. Classification:

1) Normal salts. These are salts

produced when the whole of the replaceable hydrogen of an acid has been replaced by a metal. e.g.

NaCl: Na250, and Na.

34

(2) Acid salts. These are salts

produced when only part of tne replaceable hydrogen of an acid has been replaced by a metal, eig.

NaHSO KHSU 4.

and NaHCO

Carbonates

Carbonates of polyvalent met

18 decomposes to metallic oxide nd carbon dioxide liberated, e.g.

CaCO

→CaO + CO2 3:80000.

)

POCO 3

40000

PbO

+

(c) Some salts which do not decom bose

sodium chloride when heated strongly, it crackles and melte at a temperature about 80000. potassium chloride when heated strongly, it will fuse at a temperature of about 700°C. Exercise (Part 1)

1. A hydroxide is also called an

alkali if it is

A. Insoluble in water

B. Soluble in water"

C. Neutrilised by an acid

D. Obtained from a heavy metal E. Obtained from a transition

Ifce of blue litmus paper. is placed in a liquid and it tums red we can conclude that the liquid 19

acidic

not acid10:

alkaline

D.

not alkaline

E.

etc.

Basic Salts. A basic salt is the compound formed when only part of the hydroxyl groups of a base is replaced by an acid group, e.g.

Pb(OH)NO and B; (OH)2No 3-

(4) Double salts. A double. Salt is

a salt formed by the union of two simple salts. eig. Chrome alum: T

K2SO4AL¿ (504) 3 - 2411 0

Ammonia alum (NHSO

paration:

Normal salt

A12 (504)3 •

a) by neutrilisation::

2NaOH+H SO

b) by double decomposition:

Pb(NO3)2 + H2SO→ FÚSO4+2HNO3

Cuso

c) from an insoluble base:

Chio + HSO Pb(OH)

2

d) displacement:

Zn + H SO

e) by oxidation

3Pb8HNO.

ZnSO

4

Pb(NO3)2+H2O

3Pb(NO.

NO3)2+4

+2NO

(f) by direct combination:

2Na + C12

Hg + S

(2) Acid salts:

2Na01

HgS

In the preparation of the acid salt requires twice as much acid or half as much alkali as the preparation of the nomal salts. For examples:

2NaC1+ H2SO4

(full amount) NaCl + H 504-

(half amount) KNO3 + H 50

(full amount) KNO+ H2SO

(half amount)

(3) Basic Salt

Na SO+ 2HC1

(Normal salt) NaHSO + H01

4

(acid salt) K2504

+2HNO3 (normal salt) +HNO

KASO

(acid salt).

A basic salt may be prepared by treating the base with insuffic -ient acid to convert the whole of the base into – the normal salt.e.g B; (OH) + HNO →→ B; (OH) NO2+H Õ

372

In general, a basic galt is one formed by combination of the normal salt with the oxide or hydroxide of the metal.

Action of heat on some salts (a) Hydrates

Blue vitriol

CuSO, 5H 0

Ouso

10000.

4H20

Cuso

H20

CuSO

4CuSO

6500 2602

Washing soda

Na2 CO. 10H0-> Na, CO 3 + 10H2O NaCO3 melts to a colourless

liquid at 85200.

neutral

Which of the hydroxides, listed

below is insoluble in water

but dissolves in aqueous sodium hydroxide? Y

A. potasium hydroxide

D.

Iron (II) hydroxide. Calcium hydroxide

Lead (II) hydroxide Ammonium hydroxide.

Acetic acid is a weak acid because

it cannot Turn the litmus red

it is an organic acid

it is not an oxidizing acid, it porduces a lot of hydro- gen ions in solution

it produces a few hydrogen ions in solution.

Which of the following salts can be prepared by double de- composition?

A. Lead chloride

B. Ammonium nitrate C. Sodium chloride) D. Potassium nitrate E. Sodium carbonate.

When ammonium hydroxide is add- ed to a solution of zinc sulph ate a precipitate is formed, which will dissolve in excess of ammonium hydroxide. This is because

P. zance nyɑroxide is an alkali

zinc forms a complex anion zinc forms a complex cation Ammonium hydroxide forms a complex anion

Ammonium hydroxide Terms & complex cation

which one of the following

:

metals can liberate hydrogen £ from both an acid and an aika- line

H. ITOL

silver

P. mercury

copper zino.

When excess sulphur dioxide is passed into sodium hydroxide, the product formed is

P. sodium sulphito

Q. sodium hydrogen sulphite

R. sodium sulphate

S. sodium sulphide

T. sodium thiosulphato

Which of the following salts is an acid salt?

P.

NaSO4 HCO3

R. KIICO,

3.

CII COONA

10. Direct combination 18 most use

ful in the preparation of

P. insoluble metallic sulphide

adid salts

R. basic salts.

crystalline magnesium chloride

T. anhydrous chlorides

Part II

1. White notes on the following:

The care to be taken in heating

(1) bydrated salts alld. (ii) liquids in rest tubes Adding anthonium hydroxide Solution dropa by drop to copper sulphate solution till it is in excess

c) Crystal of lead nitrate are

heated in a bulb tube

2. State fcur methods of prepar-

ing salts, illustrating sach with an equation.