CHAPTER 6 · PDF filemohd faisol mansor/chemistry form 4/chapter 6 105 2. State the meaning of...

mohd faisol mansor/chemistry form 4/chapter 6

100

CHAPTER 6 ELECTROCHEMISTRY

Electrolytes are

substances that can

conduct electricity when

they are in molten state

and aqueous solution.

This due to the present of

free moving ions in the

electrolytes.

Electrolysis is a process

whereby compounds in

molten or aqueous state

are broken down into

their constituent

elements by passing

electricity through them.

Non-electrolytes are

substances that can not

conduct electricity when

they are in all state. This

because non-electrolyte

exist as molecule which

means contain no ions.

Example

2NaCl (l) 2Na (s) + Cl2 (g)

Sodium Metal

Chlorine Gas


101

Electrolytic Cell (molten state)

a) Electrode attach to positive terminal (battery) =

b) Electrode attach to negative terminal (battery) =

c) Anion (negative ion) discharged at electrode =

Anion will _______________ electrons.

d) Cation (positive ion) discharged at electrode =

Cation will ______________ electrons.

e) Electron flow from ___________________ to ___________________

f) Electrolytic Cell will change the _______________ energy to

________________ energy.

[ Draw the apparatus of electrolysis molten sodium chloride ]


102

Product of Electrolysis

Ion

discharged

at Cathode

Observation Half-equation Product

All ion metal

except

Copper ion

Copper ion

Hydrogen

ion

Ion

discharged

at Anode

Observation Half-equation Product

Oxide ion

Chloride ion

Bromide ion

Iodide ion

Hydroxide

ion

Gas Test

Hydrogen

gas

Oxygen gas

Chlorine gas


103

Electrolysis Molten Lead(II) Bromide

1. State the ion consists in the electrolyte.

2. Which electrode is

a) Cathode =

b) Anode =

3. Which ion will be discharged at

a) Cathode =

b) Anode =

4. State the observation at

a) Cathode =

b) Anode =

5. State the product formed at

a) Cathode =

b) Anode =

6. Write the half equation at

a) Cathode =

b) Anode =

7. Draw the electron flow on the diagram above.

P

Q


104

1. State the ion present in the following electrolyte. Predict the

products from the electrolysis of some molten compound and

write the ionic equation involved.

a) Magnesium oxide

b) Copper(II) chloride

c) Lead(II) iodide

Exercise


105

2. State the meaning of the following terms.

a) Anode b) Cathode c) Electrolysis

3. A molten oxide, R2O3 is electrolysed using carbon electrodes.

a) Draw a labeled diagram to show the set-up of apparatus for

the electrolysis.

b) What ions are present in the electrolyte? Write the formulae for

the ions present in the electrolyte.

c) State the ions move to each of the electrodes during

electrolysis.


106

d) Write half equation of the reaction at each of the electrodes.

e) Name the substances formed at each of the electrodes.

f) Label the flow of electron in the diagram (a).


107


108

Electrolysis of Aqueous Solution

Aqueous solution consists of four types of ions. Two ions from the

compound and two ions from the water.

Example:

Molten sodium chloride Sodium chloride solution

Generally, there are 3 factors that may influence the selective of ions

during electrolysis of an aqueous solution

1. Position of ions in the electrochemical series

2. Concentration of ions in the electrolytes

3. Types of electrodes used in the electrolysis


109

Position of ions in the electrochemical series (ECS)

The ions that are lower in the ECS will selected to be discharged.



a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


[ Draw the apparatus of electrolysis sodium chloride solution ]


110

1.

For the electrolysis of copper(II) sulphate solution,

(a) State all the ions that are present in the electrolyte.

(b) State the ions in (a) which discharged to the

i) anode :

ii) cathode :

(c) Write a half equation for the reaction at the

i) anode :

ii) cathode :

(d) The blue colour of copper(II) sulphate solution fades if the

electrolysis is carried for a long period of time. Explain why.

Exercise

Carbon electrode

Copper(II)

sulphate solution


111

2.

For the electrolysis of dilute sulphuric acid,

a) State all the ions that are present in the electrolyte

b) State the ion in (a) which discharged to

i) anode

ii) cathode

c) Write half equation for the reaction at the

i) anode

ii) cathode

d) Explain why the concentration of dilute sulphuric acid increases

gradually during the electrolysis

3. Base on the answer 1(c) and 2(c), name the process that occur

at the

a) anode

b) cathode

Dilute sulphuric

acid

Carbon electrode


112

Concentration of ions in the electrolytes

If the concentrations of particular ions are high, the ion is selectively

discharged



a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


[ Draw the apparatus of electrolysis concentrated sodium chloride solution ]


113

1.

Diagram above show the apparatus set up for the experiments of

electrolysis using two different concentration of hydrochloric acid.


i) Experiment A :

ii) Experiment B :

Exercise

Carbon

electrode

Dilute Hydrochloric

acid solution

Concentrated

Hydrochloric acid

solution

Experiment A

Carbon

electrode

Experiment B


114

b) State the ion in (a) which discharged to anode and cathode in

i) Experiment A :

ii) Experiment B :

c) Write half equation for the reaction at the anode and cathode in

i) Experiment A :

ii) Experiment B :

d) State the observation occur at cathode and anode in

i) Experiment A :

ii) Experiment B :

e) State the product formed at cathode and anode in

i) Experiment A :

ii) Experiment B :


115

Types of electrodes used in the electrolysis

If using the active electrode at anode, ions that are present in the

electrolytes are not discharge. Instead the active electrode will

corrodes and dissolves in the electrolytes.



a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


a) Cathode =

b) Anode =


[ Draw the apparatus of electrolysis silver chloride solution using silver electrodes ]


116

1.

Diagram above show the apparatus set up for the experiments of

electrolysis using two different electrodes immersed in copper(II)

sulphate solution.


i) Experiment A :

ii) Experiment B :

Exercise

Carbon

electrode

Copper(II) sulphate

solution

Experiment A

Copper

plate

Experiment B

Copper(II) sulphate

solution


117

b) State the observation occur at anode and cathode in

i) Experiment A :

ii) Experiment B :

c) Write half equation for the reaction at the anode and cathode in

i) Experiment A :

ii) Experiment B :

d) Explain the observation on the colour of copper(II) sulphate

solution in

i) Experiment A :

ii) Experiment B :


118

Electrolysis in Industry

Most common

application:

i) Extraction of metal

ii) Purification of metal

iii) Electroplating

1) Extraction of Metal

Extraction of aluminium

from aluminium oxide.

2) Purification of Metal

In purification:

The impure metal is made to be the

anode

The cathode is a thin layer of pure metal

3) Electroplating

Electroplating is a process to coat

one metal onto another metal.

The purposes of electroplating

onto metal are:-

i) Make it look more attractive

ii) more resistant to corrosion

In electroplating :

object to be electroplated

as the cathode

anode is the metal used for plating

Electrolyte is a solution of the

compound of the electroplating

metal

Copper nugget


119

1) Ion present in electrolyte =

2) Ion discharged

a) Cathode =

b) Anode =

3) Observation

a) Cathode =

b) Anode =

4) Half equation

a) Cathode =

b) Anode =

5)Function of cryolite, Na3AlF6

Extraction of Metal

[ Draw the apparatus of electrolysis for extraction of aluminium from aluminium oxide ]


120


2) Ion discharged

a) Cathode =

b) Anode =

3) Observation

a) Cathode =

b) Anode =

4) Half equation

a) Cathode =

b) Anode =

5) Colour changes of electrolyte

Purification of Metal

[ Draw the apparatus of electrolysis for purification of impure copper ]


121


2) Ion discharged

a) Cathode =

b) Anode =

3) Observation

a) Cathode =

b) Anode =

4) Half equation

a) Cathode =

b) Anode =

5) Colour changes of electrolyte

Electroplating

[ Draw the apparatus of electrolysis to electroplate key by using copper as electrode ]


122

Voltaic Cells

A simple voltaic cell can be made by dipping two different

types of metals in an electrolyte

Electron flow from one metal to another metal through the

connecting wire in the external circuit.

More electropositive metal will release electron, thus act as

the negative terminal. Less electropositive metal will accept

electron and act as the positive terminal.

Continuous flow of electron produces an electric current.

Simple zinc-copper

Voltaic

Zinc more reactive than

copper

Zinc will act as terminal

________________, and

copper will act as

terminal ______________.

Zinc will release electron

to form Zn 2+.

Half equation:

Cu 2+ ions from copper(II) sulphate solution receive

electron to form copper metal.

Half equation :

Overall equation:

The further the distance between the position of two

metals is in ECS the bigger the cell voltage.


123

Different Types of Voltaic Cells

Two types of voltaic cell:

1) Primary cells: non rechargeable cell

Example: Daniell cell, dry cells, alkaline cell

2) Secondary cells: rechargeable cells

Example: Lead-acid accumulator, Nickel-cadmium

Daniell Cell 1

1. Used salt bridge

Salt bridge contain inert

ions or salt that does not

react with electrolyte.

Example:

Daniell Cell 2

2. Used porous pot

Porous pot has fine pores

that allow ions flow through.

What is the function of salt

bridge and porous pot in

Daniell Cell?


124

1. For the simple voltaic cell that you see at the diagram

a) State how electricity was produced.

b) What are the chemical changes that occur at the

magnesium ribbon and the copper plate.

c) Write the half equation for the changes that occur at each

the electrode.

d) What is the direction of electron flow from terminal to

another through the external circuit.

Exercise

Mg

Cu

Magnesium sulphate

solution


125

Electrochemical series (ECS)

The electrochemical series (ECS) can be constructed by two

method:

a) The potential difference (voltage difference) between pairs of

metal.

b) The ability of metal to displace another metal from its salt solution.

Tendency of

metal to release

electrons to

form ions

increases

Tendency of cation

to receive

electrons to form

metals

increases


126

The potential difference (voltage difference) between pairs of metal.

The bigger the voltage value the further apart their position.

The metal act as negative terminal is placed at higher position

in electrochemical series (ECS).

How to determine the positive/negative terminal?

Example: The voltaic cells are constructed as shown in the figure. The

voltmeter reading of the cell I is 1.1 V while that of cell II is

2.5 V.

Arrange the metals in descending order in the

electrochemical series.

Cell 1

Cell 2

P

P

Q

R


127

The ability of metal to displace another metal from its salt solution

If the M can displace metal N from an aqueous N salt solution, then:

i) Metal M is more electropositive than metal N

ii) Metal M is placed at a higher position than metal N in the ECS

Example: Zinc and copper(II) sulphate sulphate solution

Observation :

Half-equation :

The Important of ECS

ECS can be used to determine:

The terminal of voltaic cell

The standard cell voltage

The ability of a metal to displace another metal from its

salt solution.


128

1. The diagram shows an electrolytic cell. The left section of the cell

(S) is a source of electricity to drive the right section (T) of the

cell.

a) State the change of energy in cell S

b) i) For cell S, state the positive terminal of the cell

ii) Explain your choice for b(i)

c) i) State what has happened at the negative terminal

ii) What process has happened in this electrode

Exercise

Copper Zinc Aluminium

Copper(II) Sulphate Zinc Sulphate

Aluminium

Sulphate

S T


129

d) Explain why the color of copper(II) sulphate remain unchanged

e) Determine the anode of cell T

f) Write down the half equation for the cathode in cell T

g) What will happened if the aluminium in cell S is replaced by copper

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