1973-04-06 — Page 18

日六月四年三七九一懸公年二十六國民華中育教佞華

真二穿張五第二日四初月三年壯発

WAH KIU YAT PO.

報日僑華

五期星

夏橋

1973英文中學會考試題預習專欄

WIG AN MDEX

At cathode

Hydrogen

ΤΟΝ ΘΕΟ

Carbon

platinum

of alkalinity:

Hydrogen

increase

2 vols.

vol. caustic

Hydrogen 1

soda

1.e. 0.0678 em. of copper

deposited

deposited:

Number of coulombs needed

0.23

0001118

137 sec.

117x

化學科

(廿四)

Chemistry

(24)

Solution of last week Part I

6.

13. Electrolysis

AOHOE

An electrolyste is an elect- rovalent compound, which, when molten or dissolved in water, conducts electric current with decomposition. Examples are

•2+ copper sulphate cu2+ so

sulphuric acid 2H SO.

When substances dissolved in solution split up completely or largely into ions, such substances are termed strong electrolystes.

For weak electrolytes, ioni- zation is only slight and most of electrolyte exists in solution in the form of unionised molecules An non-electrolyste is a covalent compound which having no ions, does not conduct electrio current. Examples are chloroform CHC13, and Alcohol C2HOH.

Electrolysis is the decom- position of a compound, when molten or in solution, by the passage of electric current.

Discharge of ions from

mixture,

In a solution containing two or more cations, in the same con centrations and conditions, the ions tend to discharge in the inverse order of their electro- positivity. i.e. the least. electropositive first. For the

cations,

2 Cuf

anions:

OHC1

In this case,

rations may change the order.

electrolysed. Solution

Cathode of

Anode of

Hydrochloric

Caustic soda acid (conc.)

Carbon or platinum.

Platinum

Common salt

platinum or carbon

carbon

platinum

copper

copper.

copper

copper

copper

copper sulphate

sulphate

copper

Farraday's Laws of electrolysis (1) The mass of an element liberat -ed during electrolysis is directly proportional to the quantity of electricity pass- ing.

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 = ampere-second Example: A current of 1.5 A runn ing for 4 min sets free 0.75 gm. of a metal. What weight will. deposited by a current of 2 A. running for 5.5 min?

1.5 x 4 x 60 coulombs deposite

0.75 go.

2 x 5.5 x 60 coulombs deposite

0.75 x 2 x 5.5 x 60

1.5 x 4 x 60

1.375g

(2) The masses of different ele- ments liberated in electrolysis by the same quantity of electriçi ty are directly proportional to the chemical equivalents of the respective elements.

Example 1: What volume of hydro- gen collected at 1600 and 738 mm.

Time=

Electroplating

The important use of electro- lysis is in electroplating, which

is used to obtain both a protect- ive as well as decorative covering for articles.

For example, silver-plating is done by using the metal to be silver plated as the cathode, the anode used being made of pure silver, and the electrolyte, an aqueous solution of sodium argent ocyanide, which gives the fon,

Ag(CN)2

Ag(CN)2

at the cathode: Agt at the anode:

Ag

2CNT

Chromium plating, for in- stance, is not applied directly to iron and steel articles, but over a layer of copper or nickel which is applied to the article first The copper sticks firmly to the surface of the iron, and the chromium to the copper, and in this way, a strong covering is obtained.

The conditions necessary for electroplating are usually very specific.

1. Before immersion in the

electroplating, the cathode should be cleaned.

*

2. The metal should 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 small.

ii) Use an electrolyte in which

only a few of the required. ions are present.

Correct the potential differen- ce of the cell.

4. Carefully to control the

temperature when the plating

is carried out.

Exercise

Part 1

1. When 1 faraday of electricity is passed through a dilute solut- ion of sulphuric acid it liberates

1.1 gm-equivalent of hydrogen

6 x 1023 atoms of hydrogen

SO

relative concent-

pressure will be set free from dilute acid in the same time that 0.83 m. of lead (equivalent 103.5) is deposited from a solut- ion of one of its salts?

A.

D.

E.

山

Anco

Example of electrolysis Dilute

sulphuric acid

H2O

cathode:

2H

SO

H discharges by electron gain 2H+

+ 2e

Hydrogen is liberated. Concentrat -ion of H2SO is decreased.

At anode:

OH discharges by electron loss

20H + 2e

H20 + 301⁄2

Oxygen is liberated. Concentrat ion of H2SO increases.

ects of

Summary of the Effects of elect- rolysis.

At anode

Chlorine

oxygen 1 vol.

decreases. of

alkalinity

chlorine vol.

pue ueixo

@4

H2SO

copper

dissolved

weight of hydrogen

Height of lead-

Equivalent of hydroge

Equivalent of lead.

weight of hydrogen

·0.83

103.5

weight of hydrogen 0. 00802-

As 1

weighs 0.09 gm. at

0.00802 lat

0.09

this weight occupies

.f.p. 0.08910

At 160 and 738 mm pressure the volume of gas will be

X

·760 738

cm3

97.1 cm

8.91.x

Example 2: A current passed through solutions of silver nit- rate and copper (II) sulphate liberates 0.23 gm. of silver. If the atomic weights of silver and copper are 108 and 63.5 respective -1y, how much copper will be de- posited? If one coulomb deposits 0.001118 gm. of silver, for how long mist a current of 1.5 A flow? I gm. atom of Ag is deposited in the same time as 0.5 gm-atom of Cu.

-108 gm. of Ag is deposited in the same time as

63.5 gm. of Cu

0.23 gm. of Ag will be deposit -ed at the same time ast

63.5 x 0.23 gm. of Cu

X 108

mole of oxygen gas gm. of oxygen

23

molecules of oxygen

An anode is always

the positive electrode the electrode at which current leaves a cell 3. the electrode at which

electrons leaves a cell 4. the electrode at which

oxidation takes place the negative electrode 1, 2

3. Oxygen is not always liberated at the anode when sulphuric acid- is electrolysed. The product is dependent upon the

1. nature of the anode.

2. nature of the cathode

3. temperature of the electroly

tė 4 current density:

concentration of the acid.

A. 1, 2, B. 1, 3, 5) C. 2, 4, 5 D. 1, 2, 3, E. 1 3

4. If a chloride ion carries a single charge, how much electrici- ty would be needed to liberate 2 gm-molecules of chlorine?

A. 1 faraday

B. 2 faradays

C. 4 faradays

D. 2 coulombs. E. 4 coulombs

5. Two fused metal salts were

electrolysed in series. One salt

was lead (II) iodide. The other

was a compound of an unknown metal Y." After a while 6.9 gm.

of lead had been deposited and

If the atomie 0.5 gm. of metal Y. masses of lead and the metal Y are 207 and 27 respectively the charge on ions of Y will be

A. impossible to calculate B. equal to the charge on lead

ions

C. one D. three E four.

6. A current of 5 amperes was passed through a silver voltameter for 10 minutes.

and the silver ion is Ag. What weight of silver in grans is deposited?

(1 faraday

of silver is 100 The atomic weight

P.

96500 coulombs)

x 10 x 60 x 108

96500

10 x 108 96500

x 10 x 965:00

108

10 x 60 x 96500.

108

x:10 x 60

7. Which of the following would be the most suitable material for use as an anode in the electroly- sis of hydrochloric acid?.

P. Lead Q.carbon

R. copper S. silver

T. zinc

8. In the electrolysis of copper sulphate solution using copper electrodes,

P. only copper ions and sulphate ions are found in solution the concentration of the electrolyte remain constant:

R. each copper ion is discharged

by one electron at the cathode

S. the copper anode slowly.

increases in weight

T. the hydroxyl ions are select

ively discharge

9. In electrolysis, the passage⠀ the current is always solely res- ponsible for

F. the migration of ions towarda

the electrodes

the original ionization of the solution

R. the original dissociation of

the electrolyte

S. the liberation of two gases T. none of these.

10. Select the non-electrolyte

P. fused sodium chloride Q. molten silver.

R. copper sulphate solution S. molten sodium chloride T. silver nitrate solution

Part II

By passing a current of 0.65 A for 35 minutes through water, copper, and silver coulometers the following weights of ele ments were liberated: 0.0143 gm of hydrogen, 0.114 gm. of oxyg en, 0.449 gm. of copper, and 1.542 gm. of silver.

(1) Show that this results

agree with Faraday's second

law; (ii) Calculate the electrochem- ical equivalent of silver.

(H=1.008, 0= 16,

Cu 63.5, Ag = 107.9)

a) Define: electrolyte,

electrolysis, anode, cathode...

b) Draw the apparatus you would use to electrolyse water and to collect the products. c) What substance had not alter -ed in quantity during the electrolysis, and how would you verify this fact experi- mally?

a) Describe what you would.

observe and explain what happens when a solution of copper sulphate is electro- lysed between copper elect- rodes.

b) Draw a labelled diagram of

the apparatus you would use to carry out the electroly -sis of aqueous sodium chloride. Explain what happens at each electrode.