© MHS 2009 1

Chapter 6: Electrochemistry

1. Electrolyte Electrolytes are substances in molten state or aqueous solution that can conduct

electricity due to the presence of free moving ions

2. Non-electrolytes Non-electrolytes are substance that cannot conduct electricity either in molten state or

aqueous solution.

3. Electrolysis Electrolysis is a process whereby compounds in molten or aqueous state are broken

down into their constituent elements by passing electricity through them.

4. Electrolytic cell The electrolytic cell is the set of apparatus needed to conduct electrolysis.

It consists of a battery, an electrolyte and two electrodes.

5. Electrode Electrodes are electrical conductors.

Graphite or platinum is usually used as electrodes because they are inert, they do not

react with electrolyte or the products of electrolysis.

6. Anode The electrode which is connected to the positive terminal of an electric source.

Negatively charged ions (anions) in the electrolyte are attracted to the anode.

7. Cathode The electrode which is connected to the negative terminal of the batteries.

Positively charged ions (cations) in the electrolyte are attracted to the cathode.

8. Electrolysis of

Molten

Compounds

Molten compound:

A molten compound consists of one type of cations and one type of anion only.

In solid state, ions do not move freely but are held in fixed positions in a lattice.

In molten electrolyte, the ions move freely.

During electrolysis, the negative ions or anions move to the anode.

The positive ions or cations move to the cathode.

A new substance is then formed at each electrode.

Example: Electrolysis of molten lead (II) bromide, PbBr2.

PbBr2 is an ionic compound. It consist Pb2+ and Br-.

In solid PbBr2, these ions do not move freely but are held in fixed positions in a lattice.

When it melts, the ions are free to move.

During the electrolysis of molten PbBr2, Br- are attracted to the anode.

At the anode, Br- undergo discharge whereby each of these ions releases an electron to

form a neutral bromine atom.

Two bromine atoms combine to form a bromine gas, Br2 molecule.

Thus, Br2 is released at the anode.

Half equation: 2Br-(l) Br2(g) + 2e-

At the cathode, Pb2+ undergo discharges whereby each of the ions accepts two

electrons to form a lead atom.

Thus, lead metal is formed at the cathode.

Half equation: Pb2+(l) + 2e- Pb(s)

Combining the two half equations, we get the overall equation. Pb2+(l) + 2Br-(l) Pb(s)

+ Br2(g)

© MHS 2009 2

Example

1.

2.

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Molten NaCl

Pb2+ + 2e- Pb

2Br- Br2 + 2e-

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Molten PbBr2

© MHS 2009 3

3.

4.

9. Electrolysis of Aqueous Solutions

An aqueous solution is produced when a solute is dissolved in water. An aqueous solution of a salt consists of 2 types of cations (cations of the salt and hydrogen ions, H+), and 2

types of anions (anions of the salt and hydroxide ions, OH-).

H+ and OH- are always present together with the ions produced from the dissociation of salts in aqueous

solutions. This is because water dissociates partially to form H+ and OH-.

H2O H+ + OH-

There are three factors that may influence the selective discharge of ions during the electrolysis of an

aqueous solution.

i. Position of ions in the electrochemical series

- The ions that are lower in the electrochemical series will be selectively discharged

- .

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Molten lead (II) oxide

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Molten zinc iodide, ZnI2

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ii. Concentration of ions in the electrolytes

- If the concentration of a particular ion is high, the ions is selectively discharged.

iii. Types of electrodes used in the electrolysis

The common materials used as electrodes are carbon and platinum because they are inert.

Example: Electrolysis of copper (II) sulphate, CuSO4 solution.

CuSO4 Cu2+ + SO42-

H2O H+ + OH-

If carbon is used as the electrodes, OH- ions are discharged at the anode because of the position of OH- ion

in the electrochemical series.

If copper is used as the anode, both SO42- ions and OH- ions are not discharge.

Instead the copper anode dissolves by releasing electrons to form copper (II) ions, Cu2+. Hence, the mass of

anode decrease.

Copper acts as an active electrode here because it takes part in the chemical reactions during electrolysis.

1.

2.

3.

4.

5. 6.

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Silver nitrate, AgNO3 solution Dilute copper (II) chloride CuCl2

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Copper (II) sulphate, CuSO4 solution

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Zinc iodide, ZnI2 solution

© MHS 2009 5

7.

8.

10

Electrolysis in Industries

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Concentrated potassium chloride, KCl

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Concentrated copper (II) bromide, CuBr2

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Ag C

Ag2SO4

Ag Ag+ + e-

Ag+ + e- Ag

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A

Ni Ni

NiSO4

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Extraction of metals

- Reactive metals such as aluminium and magnesium can be extracted from their ores be electrolysis.

- Aluminium can be extracted from its ore, molten aluminium oxide, Al2O3 using carbon electrodes.

- In this process, a substance known as cryolite, Na3AlF6 is added to aluminium oxide, Al2O3 to lower its

melting point.

Purification of metals

- Pure copper and silver can be obtained through the process of electrolysis.

- In the purification of copper, the impure copper is made to be the anode while the cathode is a thin layer of

pure copper.

Electroplating of metals

- In electroplating of metals, electrolysis is used to coat one metal onto another metal.

- In the process of electroplating, a more expensive or attractive metal such as silver or gold is coated onto

the object to make it look more attractive and more resistant to corrosion.

11

Voltaic Cells

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- A simple voltaic cell consists of two different metals immersed into an electrolyte. Chemical energy is

converted to electrical energy in the cell.

- The more electropositive metal (metal that is higher position in the ECS) will release electron Negative

terminal

- The less electropositive metal (metal that is lower position in the ECS) will be positive terminal.

- The electron flow form negative terminal to positive terminal. Cation which is lower at ECS will be

discharged.

12 Daniell Cell

- In a Daniell cell, zinc and copper are used as electrodes. Each electrode is immersed into a different

electrolyte. The electrolytes are connected by a salt bridge or a porous pot.

12 The Electrochemical Series (ECS)

The porous pot and salt bridges are: i. to allow the flow of ions so

that the circuit is completed

ii. to prevent the two aqueous solution from mixing

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- The electrochemical series is an arrangement of metals based on the tendency of each metal atom to

donate electrons.

- The electrochemical series can be constructed based on the potential difference between two metals, and

the ability of a metal to displace another metal from its salt solution.

- The electrochemical series is used to determine the terminals and voltage of a cell. It is also used to predict

the ability of one metal to displace another metal from its salt solution.

- The further the two metals are in the ECS, the greater the voltage produced by the cell.

The Advantages Disadvantages of Various Voltaic Cells

Cell Advantages Disadvantages

Daniell cell Easily set up in the laboratory Wet cell – electrolyte easily split

Voltage cannot last

Dry cell No spillage

Small in size

Easily carried about

Produces regular current and voltage

Obtained in different sizes

Does not last

Cannot be recharged

Leakage can occur if cell cannot be used

anymore

Alkaline cell Lasts longer than dry cell (10 x)

Produces a higher and more regular current

Leakage can occur if cell cannot be used

anymore

Expensive

Cannot be recharged

Mercury cell Small in size

Produces regular current for a longer period

of time

Lasts a long time

Very expensive

Cannot be recharged

Mercury that is produced is poisonous

Lead-acid

accumulator

Can be recharged

Produces a high voltage (12V) for a long

period time

Produces a high current (175A) suitable for

a heavy duty

Spillage of acid can occur

Big in size

Heavy, difficult to be carried about

Expensive

Loses charge if not used for long

Nickel-

cadmium

cell

Can be recharged up to 500 times

Dry cell no spillage

Smaller than accumulator - portable

Expensive

Transformer needed for recharging cell

Electrolytic cell Voltaic cell

© MHS 2009 9

Similarities

Contains an electrolyte

Consist of an anode and a cathode

Electron move from the anode to the cathode in the external circuit (connecting wires)

Positive ions and negative ions move in the electrolyte

Chemical reactions involve the release or acceptance of electrons

Differences

Characteristics Electrolytic Cell Voltaic Cell

Energy change Electrical energy chemical energy Chemical energy electrical energy

Electric current

and reaction

Electric current results in a chemical

reaction

Chemical reaction produces an electric

current

Electrode /

Terminal

Cathode: Negative terminal

Anode : Positive terminal

Cathode: Positive terminal

Anode : Negative terminal

Flow of electron Electron flow from the positive electrode

(anode) to the negative electrode

(cathode)

Electrons flow from the negative electrode

(anode) to the positive electrode (cathode)

Negative terminal Cation receives electrons from the

cathode (negative terminal)

Electron are released at the negative

terminal

Positive terminal Anion release electrons to the anode

(positive terminal)

Electrons are received by the positive

terminal

Types of

electrodes

Same or two different types of metal, or

graphite electrodes

Two different types of metal

anion/-ve ion

cation/+ve ion

ammeter

- +

cathode / -ve electrode

anode / +ve electrode

- +

- +Battery

A