1973-02-10 — Page 22

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真二第張六第日八初月正年丑癸屬夏

WAH KIU YAT PO

六期星

carbonate.

212

CO2

Ca(OH)2

CaCO

Excess CO2 19 passed.

the

milkinegs W

disappear

State

Solubility

Action of

CO2

→ Ca (HCO | | 2

heat

日十月二年三七九一圈公年二十六國民中育教僑僑

Ne2 CO2 + Sio,

Na, $103

Cão

Si02

Casio,

halk and quicklime compared.

1973英文中學會考試題預習專欄

WIN 4X PM W

Uses:

化學科

(十六)

Chemistry (16)

30lution for last weel

D

KUHU

This Is dioxide.

for carbon

In the solvay process for the manufacture of sodium carbona and bicarbonate

In the manufacture of aerated water, which are formed by

forcing carbon dioxide under pressure into solutions of various essences.

varbon dioxide

Preparation

Carbon dioxide is usuall prepared in the laboratory by the action of dilute hydrochloric acid on marble.

Jaco

2H01:

CaCl

The reaction is, however, unsatisfactory if the metallic salt formed is insoluble

or nearly so. For examp7 water

marble.

and dilute sulphuric acid effer- vesce for a few seconds; then the action stops because the spring -soluble calcium sulphate forms a

coating on the marble and the aci has no further contact with the marble. Usually collected over warm water or, by downward deliv- ery.

Hydrochloric acid

In the fire extinguishers. These often contain. sodium

and

bicarbonate solut

xed- oy sulphuric acid. When mi inversion or passing a plunger the two produce carbon dioxide The pressure of the gas forces out a stream of liquid

ZNaHCO2

So Na SO

CO2

Refrigeration --- The solid carbon dioxide (dry ice) 19 used in the preservation of ice cream during transport or sale and, with ether, as refrigerating mixture. Carbonates and bicarbonates Preparation:

Carbonates and bicarbonates of alkali metals

They are made by the ac 1on. of carbon dioxide on alkalis,

CU2+ KOH

H2O

Excess of carbon dioxide will convert the carbonates into bicarbonates -

Properties

Physical

Carbon dioxide is

a colourless gas with only a slight smell. It is mederately

soluble in water. It 19 mar

than air (V.D. = 22). At

ure below 3100, it can be easily Liquified under increased pressur When the liquid is allowed to escape into the air, it solidifier to "carbon dioxide snow" which sublimes without melting at 78°0.

Chemical

a. Action of water:

The solution is slight: acidic, it does not turn blue litmus solution fully red.

H2CO

The acid is dibasic; con- sequently it can form two sets of salts, the normal carbonates (CO) and the bicarbonates (HCO 3" }

Stability of carbon dioxide: Carbon dioxide is very stable to heat; it shows no appreciable decomposition: below 1300°C. and at 1450°C. its degree of dissociati or 19 only about 0.003.

If previously ignitea, magnesium burns in carbon dioxide for a short time. with a spluttering flame, producing white particles 1ts oxide and black carboz particles.

CO2

+ 2Mg

2MgOOL

Sodium and potassium a re burnt in the gas, if previ ously ignited, forming their carbonates and carbon, e.g

4Na

4K

With

300 3002

2Na 00

2K, CO

200

mon carbon, carbon

is formed,.e.g.

With alkalis to give salts or acidic salts, (00, in

excess). B. &••

NaOH

H2O

Na,CO

Na Cu

H2O

2 HaHCO

Wath lime water:

When carbon dioxide 18

bubbed into lime water,

the

latter turns milky insoluble

CO

H

→→→2KHOO

Carbonates and bicarbonates of heavy metals.

The carbonates of heavy metals are insoluble and can be prepared by precipitation; using either sodium or potass

1um carbonate. Mg(NO),

2NaNO

MgCO

Solutions of the bicarbon- ates can be obtained by pass→ ing carbon dioxide into

aqueous suspensions of the carbonates of

metals,

Properties

Bicarbonates

MgCO3

H2O → Mg (HCU {!?

Carbonates

White crystalline solids: Carbonates

All insoluble, except those of Na, K of sodium, potassium and ammonium amorphous: Carbonates of heavy metals (e.g. Ca, Mg, Cu) H

Caco,

3.

(Na,CO, and K2CO, not affected by

•heat)

All soluble..

Ca (HCO3)2) CaCO + H2O CO2 Carbonate may then further decompose and carbon dioxide.

into oxide and 00,

Readily decomposed into carbonate:

Usually decomposed at high temper-- ature giving the oxide,

200 + 000

varoonaves

Action of CO, evolved.

JO TOH

any dilute

acid

Effect of

solution

adding.

of Meso

How to

distinguish

CaCO3 + 2HC1→→→→CaC1, + 1,0 + 00,

Immediate ppt. because MgCO, is insoluble.

1. Action of gentle heat,

eig

but not the carbonate, would give

2. MgSO test.

NaHCO, + HCl-

Bicarbonates

CO2 evolved.

NaCl + H2O + CO,

NO ppt. because Mg(HCO,), 19

soluble.

On boiling with water, the bicarbonate

ppt. on boiling because Mg(HCO2), becomes MgCO3 + H2O + CO2

off carbon dioxide.:

Calcium carbonate and related compounds

Calcium carbonates occurs in nature as chalk, limestone, and marble.

Effect of heat: Calcium carbonate when heated gives quicklime.

Cao + CÓ

CO2

Caco

Action of water on quick-lime ((slaking process)

When water is added to quick- line, there is usually a hissing sound; great heat is developed; steam is given off, the lime cracks and puffs up and finally crumbles to a powder about thre times as bulky. This is slaked lime:

If insufficient water is addeć to slaked lime to dissolve all of the latter to form lime-water, then a milky liquid is obtained which contains line-water with solid particles of slaked lime suspending in the liquid. This milky liquid is called milk of lime.

Uses of Time:

1. For soil treatment

e) In agriculture to keep down the acidity in the soil. b) It makes the clay particles

stick together to form big ones making the soil porous, thus letting the water pass easily

2. Mortar 19 a mixture of quick lime, water and sand.

CaO + H2O

Ca(OH)2 + CO

Ca(OH)

→ CaCO2+ H2O

3. Cement is a mixture of calcium silicates, and aluminates made by strongly heating a mixture of limestone and clay. When mixed with water, it gradually sets and hardens.

4. Calcium carbide is made by strongly heating a mixture of quioklime and carbon in an elect- ric furnace.

Dao + 30 - Gel,

of

UU

5. Common glass is made by fusing soda-soh and quicklime with excess of sand.

Chalk

white

No ta

quick-lime

white

Burns the tongue

unpleasant and

bitter taste.

Decomposes when No change on hea

heated

Addition or water has no effect

No effect on moist litmus. Effervesces

when HC1 19 added, owing to evolution of CO.

-ing

Much heat produc ed on addition

of water

Moist litmus is turned blue. Much heat is pra duced, but no effervescene.

CaO + H2O

Ca(OH)2

Ca(OH)

+2HQ

CaCl 2,0

Manufacture of sodium' carboNETA

by solvay process.

In this process soɑium bicar- bonate is made first, by saturate ing a solution of brine with ammonia and then treating it with carbon dioxide,

First stages Ammoniation of brin

NH OH

Second stage: Carbonation

CO2 + HO H2CO3 + NH OH

NH, HCO3 + H20

NHHCO + NaCl —NH CI Мансо

3

Sodium bicarbonate separates out as a white ppt. which is drawn of! filtered and washed free from: ammonium compounds

Third stage: Conversion to sodiu

carbonat 2NaHCO

Na2 CO3 + H20 + CO2 Crystallisation from hot water produces the decahydrate, washing soda, Na, CO 10H20

Economics of the solvay process” This process is an excellent ex- ample of modem methods in which waste is kept down to a minimum. The raw materials are brine and Lime-lons.

CaCO3

Cao + CO,

A part of the carbon dioxide is recovered by heating of sodium bicarbonate and is returned to the Parbonating tower.

The line resulting from the heat ing of limestone is used for treat -ing the ammonium chloride

CaO + 2NH C1 - CaCl2 + 2NH +H,

The principal weakness of the process of its failure to utilise the chlorine of the sodium chloride used. This is lost in the forme as calcium chloride for which there is only slight demand Exercise

Part I

The following equation repre- sents the decomposition of marble.

CaCO

CaO 4 CÙ2

What volume of the carbon a10x- ide would be obtained if 10 gm of marble were used in the decom- position at N.T.P.?

560 cm

B, 1,120 cm C. 2,240 cm

200

E.22,400 cm

2. A burning splint la extinguish- ed while a burning magnesium ribbon continues to burn 19 a gas jar containing carbon dioxiu. We would therefore be justified in saying that

A. Carbon dioxide is a good

oxidising agent.

B. Carbon dioxide is a support

-er of combustion.

C. Magnesium is a powerful

reducing agent.

Magnesium has a stron

affinity for carbon..

E. Magnesium should not stored

near a carbon dioxide type

fire extinguisher

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