(h) Hence, or otherwise, indicate which of the terminals A and B. is the positive pole of the battery..

Solution to Q.9.2

十八國民華中

育教日十初月二十年午庚籠夏

報日僑

五期星日五廿月一(一九九一年十八國民華中

Solution

n(Cu) deposited ionic charge on X

(a) Give" a

n(Y) deposited

3x(60x12) 96500

0.224 F

ionic charge on Cu

brief explanation of the. pink colour observed around the electrode P.

become mob electricity.

and

32)

conduct

1.e.

0.03. 0.02

臺華郵輪 ®

1991 中學會考預習專欄 明德出版社

of Pb 207.

MILL & DALE PRESS

2e 2mÒ1: 2F.

Pb

1mal

.207g

Question 9.1

0.224F.

忘假期

華之旅程

Chemistry (17)

mass of Pb deposited

substances

0.224

207 X

10

(d) What is the magnitude of

current passed?

the:

興獻:

無時

絕不加價。

¿先生親〨策劃 生將隨團出發:

康身

Unit 9: Continued

Rate of Electrolysis..

N. LAH

The rate of electrolysis depends mainly on the rate of electron flow (the magnitude of the current): and. the rate of ion flow in the electrolyte. Thus, the rate of electrolysis: wj13. be increased by..

(a) using a larger em.. smaller resistance

"external circuit,

or

the

(b) placing the electrodes riearer

to each other

(c) using electrodes with larger

surface area.

BD GIFE

(d) using a

more concentrated

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solution of the electrolyte..

7. Calculations on Electrolysist

7,4.0 = 1 x t

Qis.the quantity of electricity in: coulombs(C).

1. Is the electric current. amperes(A),

t is the time In seconds(s) of: which the current has been passed..

7.2 A faraday{F} denotes the quantity

of electricity carried by one mole:

of electrons.

IF 96500€.

7.3 Faradays

First Lawn of Electrolysis; the mass of an element dissolved off, or liberated at can electrode during electrolysis is directly. proportional to the quantity of electricity: passed : - through the electrolyte.

Example 1

If a current of 3A is passed for 12 min throught fused Tead(11): bromide; what will the mass of lead be deposited?

23.18 g

Example 2

How long has a current of 5A been passed when 672 cm3

of oxygen,

measured at st.p., is obtained at the anode during the electrolysis of dilute sulphuric acid? Solution

Molar.voluine of gas at s.t.p. is 22.4 dm.

40H è

(4X96500)C

22.4 dm3

at s.t.p.

0.672 dm'

at s.t.p.

(4×96500)

11580C - 11580

0.672

38.6 min

| 7.4 Faraday's Second Law

Electrolysis: ‘the number of males of the different elements dissolved same off or deposited by the .quantity of electricity is inversely. proportional to the

·charges on their respective dons. For example, since the charges on "Cult and Ag+ ions respectively are In the ratio 2:1,so, the same quantity of electricity would deposit Cu and Ag in the mole ratio. of 1:2

Example 3

The same amount Cof. "electricity deposits 1.905 gof. copper a 1.04`g of a metalocess ctively. What is the ion

送?

of X is 52.)

Solution

Let X have an ionic charge of +x .

1.905 63.5 0.03 mol

n{Y} deposited 1-94 = 0.02.mol

By Faraday's.

2nd Law electrolysis, we have"

財團徵求

(a) Classify

the following

into conductors,

electrolytes and non-electrolytes.

A: potassium bromide

B: sugar

C: aqueous sulphur dioxide solution D:graphite:

E hydrochloric acid,

F: iodine crystal

G: ethano? (CH CHżCH2OH)

Htetrachloromethane (CC);

I sulphur

J: Liquid ammonia

(b).

Battery

Bulb

beaker

Sodium chloride crystals.

In the above set-up the bulb' does not light up at : first; however when water is added into the beaker.. the bulb. begins to light up.

(1) What are the composing

units of sodium chloride? (2): Give a brief. explanation of

the. observation.

11. How could you make the bulb 1ight up without adding water?.

Question 9.2

| The circuit below is for carrying out electrolysis on 5 M sodium chloride

| solution, using canaon electrodes, and

(c) what is the gas collected over.the.

celectrode:g?

Write the ionic equation of its formation-

(e) What: wound happen at the electrode

:S?

Give the fonic equation of the heaction.

(f) Would there be any change in the

(a) Phenolphthalein turns pink due to the presence of the hydroxide ions (OH). The continual discharge of H* (aq) at the electrode P_shifts the equilibrium H20{2}

H(aq) + OH(aq) to the right, thus leaving behind an excess of OH ions which

...then phenolphthalein pink.

turn

Since

(b) B is the positive pole.

H(aq):1s_discharged at electrode: P. so, the electrode P:must be the cathode and the electrode: () the anode..

colour intensity of the copper(II) | (c) Chlorine gas. sulphate solution during the electrolysis? Explain briefly.

(g) What effect ' would each of.the.

following changes have on. the rate. of electrolysis?

Using .current.

3 smaller

electric

ii. Placing the electrodes Rand S.

closer to each other.

(h) If the copper electrodes R and S

are replaced by two platinum- electrodes M and N respectively..

What would be discharged at the electrode N?

Give.the ionic equation of the : reaction:

ii. Mention

copper(11)

two changes of the sulphate solution.

during the electrolysis. Expalin: briefly.

Solution to Q.9.1

(a) Conductors: D.

Electrolytes: A, C, E, Non-electrolytes: 8, FS

I, 3.

of sodium ions {Na*} and. chloride ions (C17), {2}_In_solid. state, the jons:

5 M.copper(L1) saphate solution, using (b) i, (T) Sodium chloride is composed copper electrodes. A few drops of. "phenolphthalein had been added to thexam

soditum chloride" Solution. When steady direct current was passed for <6 min 26s, the gas collected over the" electrode P occupied 22.4 cm, measured. at s.t.p., and the solution around the electrode :P turned pink.

5 M sodium

chloride-

solution

phenolphthát

+

lein

: 5'n copperU }

www: sulphate solution Rheostat Battery:

mobile, so sodium

der does not conduct icity and the bulb does not light up..

·water, "the ions of

·sodium: chloride are. dissociated and becomes mobile there als an electric'

Mowing

and the DATB light up.

to

ii. Heat to melt the crystals. In

moften state, the jons anso.

201 (aq)

+ Cl2(g) + 2e (d) Molar volume of gas at s.t.p.

·22.4dm)

2H+ 2e

Zmol 2F

:1 ma1

22.40m at s.t.p.

22.4cm at s.t.p.

(2 x6408) x 96500c

X

22400

1930

1.93

* 6x60+26

0.5A

(e) Copper is dissolved

electrode S.

Cu2+(aq) ———— Cu(s) + 2e'

{f}'No. The amount of Cu(ad) deposited at electrode Ris replaced by the 'same amount of cu2+(aq)

The formed at the electrode S.

concentration

remains unchanged, so, there is no change in its colour intensity.. (g)i. The rate is decreased,

ti. The rate is increased.

(h) 1. Oxygen

40H(aq)

收購不交吉 高價收購

ii. (1) The

2H20{0)+02(9)+2€ ̄

of

blue colour Cuson(aq) fades off. due to the deposition of Cu2+(aq) at Glectrode M

(2) The pH of the solution:

The pH of t

decreases. because the

discharge of OH (aq)at electrode N shifts the equilibrium H,0(2.)

H*(aq) + DH (aq): to

right, leaving behind an

excess of h*taq), so,

becomes more "acidic.

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