Chemistry Form 4: Chapter 6 - Electrolyte

Electrolyte is a chemical substance which conducts electricity in the molten or aqueous state.

Electrolytes are able to conduct electricity because there are freely move ions in the molten or aqueous state.

Examples of electrolyte are salt solution, dilute acids and dilute alkalis

Chemistry Form 4: Chapter 6 - Electrolysis of Lead (II) Bromide

Electrolysis is a chemical process where a substance in its molten state or in an aqueous solution decomposed by electric current.

This involves conversion of energy from electrical energy to chemical energy.

Electrodes are the strips of metals inserted into electrolyte for conduction of electricity. he metal electrode connected to the positive terminal of the battery is called the anode (+). The metal electrode connected to the negative terminal of the battery is called the cathode (-).

Electrolytic cell is the complete set-up for electrolysis. This consists of the vessel containing the electrolyte, anode, cathode, battery and wires.

At the Cathode At the Anode

Observation

When electricity is flowing, a silvery

deposit of lead metal forms on the

cathode. In fact, as it is molten, it is

more likely to drip off in a molten

blob.

Observation

When electricity is flowing,

brown fumes of bromine gas are

seen at the anode.

Half equation Pb

2+ + 2e ---> Pb

Half equation 2Br

- ---> Br2 + e

Explanation

The lead(II) ions, as they are positive,

move to the negative cathode, where

each ion gains two electrons to form a

lead atom.

Any reaction at a cathode involved a

gain in electrons. This is called

reduction or more exactly, cathodic

reduction .

Explanation

The bromide ions, as they are

negative, move to the positive

anode, where each loses an

electron to form a bromine atom.

Then two of these newly formed

atoms combine to form bromine

gas.

Any reaction at an anode

involves a loss of electrons.

Chemistry Form 4: Chapter 6 - Electrolysis of Concentrated of Sodium

Chloride Solution

Electrolysis of dilute sodium

chloride solution Electrolysis of concentrated

sodium chloride solution At anode :

Anions : OH- and Cl

- OH

- ions are selectively

discharge according to the

position of ions in the

Electrochemical Series, ES. Gas bubbles are formed which

lighted up a glowing wooden

splinter. This gas is oxygen. 4OH

- ---> 2 H2O + O2 + 4e

At anode :

Anions : OH- and Cl

- Cl

- ions are discharged

instead of OH- because of

their higher concentration

in the electrolyte. A greenish-yellow gas

with a pungent and

choking smell is released.

The gas turns the blue

litmus paper red and then

white. 2Cl

- ---> Cl2 + 2e

At

cathode: Cations : H

+ dan Na

+ H

+ ions are selectively

discharge according to the

position of ions in the ES. Gas bubbles are formed. When

a lighted wooden splinter is

placed near the mouth of the

test tube, a ‘pop’ sound is

produced. Hydrogen gas is

produced 2H

+ + 2e ---> H2

At

cathode: Cations : H

+ dan K

+ H

+ ions are selectively

discharge according to the

position of ions in the ES. Gas bubbles are formed.

When a lighted wooden

splinter is placed near the

mouth of the test tube, a

‘pop’ sound is

produced. Hydrogen gas

is produced 2H

+ + 2e ---> H2

Chemistry Form 4: Chapter 6 - Types of Electrodes Affect Product of

Electrolysis

Carbon dan platinum electrodes are inert because both of these materials do not react with the electrolytes or the product of electrolysis.

Other electrodes except carbon and platinum can react with the electrolytes.

Chemistry Form Four : Chapter 6 - Electrolysis of copper (II) sulphate

Electrolysis of copper (II) sulphate

using carbon electrode. The bulb is lighted indicates that copper (II) sulphate is an

electrolyte which can conduct electricity. Carbon electrode is inert which does not react

with electrolyte or product of electrolysis.

At anode : Hydroxide ions ions are selectively

discharged. Gas bubbles are formed which lighted up a glowing wooden splinter. This

gas is oxygen.

At cathode : Copper ions are selectively discharged. Shiny brown metal deposited

around cathode. This metal is copper.

Electrolysis copper (II) sulphate solution with copper electrodes

At cathode : Copper ions are selectively discharged. Shiny brown metal deposited,

makes the cathode thicker. This metal is copper.

At anode : Copper electrode eroded and became thinner. Copper anode dissolved to form copper ions.

Chemistry Form Four: Chapter 6 - Electroplating of copper metal

1) Anode --> plating metal = copper

2) Cathode --> object to be electroplated = iron key

3) Electrolyte --> salt solution that contains electroplating metal ions = copper (II)

sulphate

cathode: A brown metal deposited on the surface of the iron key.

anode: Copper anode becomes thinner.

Chemistry Form 4: Chapter 6 - Purification of Metal

The anode is a impure copper. The cathode is a pure copper.

When electricity is passed through the cell, the copper is dissolved at anode where Cu2+ ions go into solution. Anode copper becomes thinner.

At cathode, copper ions are selectively discharge and deposited. Cathode copper becomes thicker.

The blue intensity of solution remains unchanged because concentration of copper (II) ions in the solution remains unchanged. The rate at which copper enter into electrolyte from anode is equal to the rate at which the copper ions discharged at cathode.

Chemistry Form 4: Chapter 6 - Extraction of Aluminium From its Ore

The ores of aluminium is called bauxite. Bauxite is a composition of aluminium oxide, (Al2O3). The position of aluminium in the reactivity series indicates that, it could not be extracted by reducing the oxide using carbon or carbon monoxide. Electrolysis of molten aluminium oxide ("alumina") will, however, give aluminium and oxygen.

The function of cryolite is to lower the melting point of aluminium oxide.

Aluminium metal forms at the cathode electrode and sinks to the bottom of the tank, where it is tapped off.

Oxygen forms at the anode electrodes. This oxygen reacts with the carbon of the anode electrodes, forming carbon dioxide, and they gradually burn away. Consequently, the anode electrodes have to be replaced frequently.

Chemistry Form 4: Chapter 6 - Daniell Cell

Daniell cell is also known as Galvanic cell which using same working principle as voltaic cell. It converts chemical energy to electrical energy. Daniell cell is a part of a battery consisting of two metals joined by a salt bridge or a porous pot.

More electropositive metal acts as negative terminal which undergoes oxidation process (release electron)

Less electropositive metal acts as positive terminal which undergoes reduction (gain electron).

Electron flow from negative terminal to positive terminal. The function of salt bridge or porous pot is to allow the movement of ions

to pass through it to complete electrical circuit. Another function is to separate two electrolyte solutions.

Examples of salt bridge are sodium chloride or potassium nitrate.

At Negative Terminal: More electropositive metal ( Zinc ) Observation: Zinc plate dissolves or becomes thinner, its mass decreases Half chemical equation : Zn ------> Zn

2+ + 2e

At Positive Terminal: Less electropositive metal ( Copper ) Observation: Copper plate becomes thicker Half chemical equation : Cu

2+ + 2e Cu

Chemistry Form Four: Chapter 6 - Displacement of Metal from Salt

Solution

Electrochemical series can be constructed by using displacement reaction of metal from

salt solution. A more electropositive (reactive) metal can displace a less electropositive

metallic ion (less reactive) from its salt solution.

For example: Reaction between magnesium and copper (II) sulphate produces copper

and magnesium sulphate.

Chemistry Form 4: Chapter 6 - Constructing Electrochemical Series based

on the Potential Difference between Two Metals

In a voltaic cell, two different metals are used to create a potential difference. A higher metal in the electrochemical series will be made negative terminal. A lower metal in the electrochemical series will be made positive terminal. Conversion of energy is chemical energy to electrical energy.

Metals which are further apart in the electrochemical series produce a higher voltage than metals which are close to each other.

By arranging the metals in order of their voltage readings, you can build up the

electrochemical series.