Chapter 6

Electrochemistry

A. Electrolytes & Non-electrolytes

Electrolytes

Substances that can conduct electricity and undergo chemical changes

Non-electrolytes

Substances that cannot conduct electricity

Electrolyte

Non-electrolyte

Why electrolyte at molten and aqueous state can conduct electricity?

BECAUSE

There presence of free moving ions in the electrolyte

B. Electrolysis of Molten Compounds

Electrolysis

A process where compounds in molten or aqueous state are broken down into their constituent elements by passing electricity through them

Electrolytic cell

The set of apparatus needed to conduct electrolysis

Electrolyte

Cathode Anode

+ -

STEPS OCCUR DURING

ELECTROLYSIS

Movement of ions to the

electrodes.

Discharges of ions at the

electrodes.

Graphite or platinum

is usually used as

electrodes because

they are inert.

Important note: Observation Anode (positive electrode)

Electrolysis product Observation Confirmatory test

Chlorine gas Greenish-yellow gas bubbles released

Place moist blue litmus paper into test tube Moist blue litmus paper turn red Bromine gas Brown gas released

Iodine Purple gas released

Oxygen gas Colourless gas bubble released

Place a glow wooden splinter near the mouth of test tube The glowing wooden splinter light up

Metal (all metal) The mass of electrode decreased

Copper metal Brown solid formed

Important note: Observation

Cathode (negative electrode)

Electrolysis product Observation Confirmatory test

Almost all metal (except copper metal)

Grey solid formed The mass of electrode increase

No test for metals

Copper metal Brown solid formed

Hydrogen gas Colourless gas bubbles released

Place a lighted wooden splinter near the mouth of test tube A “pop” sound heard/produced

Electrolysis of molten compound

Electrolysis of molten compound

• PbBr2 • LiCl • MgBr2 • PbI2 • ZnO • CuCl2 • Molten Potassium oxide • Molten Magnesium oxide • Molten Potassium iodide

Electrolysis of Aqueous Solutions

Factors that affect the electrolysis of an aqueous solution

• Position of ions in the Electrochemical Series

• Concentration of ions in the electrolytes

• Types of electrodes used in the electrolysis

Position of ions in the Electrochemical Series

• The ions that are lower in the electrochemical series will be selected discharged

K+

Na+

Ca2+

Mg2+

Al3+

Zn2+

Fe2+

Sn2+

Pb2+

H+

Cu2+

Ag+

F-

SO42-

NO3-

Cl-

Br-

I-

OH-

The Electrochemical Series

Ease of discharge increases

0.01 mol dm-3 of copper(II) sulphate using carbon as electrodes

1 mol dm-3 of

copper(II) sulphate

solution

Carbon Carbon

Concentration of ions

• If the concentration of a particular ions is high, the ions is selectively discharged

TIPS: ANODE Ion selectively discharged based on CONCENTRATION of ions CATHODE Ion selectively discharged based on position of ions in Electrochemical Series

2 mol dm-3 of hydrochloric acid, HCl using platinum as electrodes

2 mol dm-3 of

hydrochloric acid, HCl

Platinum Platinum

Anode : Cathode :

Types of electrodes

• Electrolysis of copper(II) sulphate, CuSO4 solution using copper electrode

• Electrolysis of silver nitrate, AgNO3 solution using silver electrode

0.02 mol dm-3 of copper(II) sulphate, CuSO4

using copper as electrodes

0.02 mol dm-3 of

copper(II) sulphate,

CuSO4

Copper Copper

Anode : Cathode :

0.02 mol dm-3 of silver nitrate, AgNO3

using silver as electrodes

0.02 mol dm-3 of silver

nitrate, AgNO3

Silver Silver

D. Electrolysis in Industries

• The most industrial application of electrolysis:

1. extraction

2. purification

3. electroplating of metals

E. Voltaic Cells

• Chemical energy to electrical energy

• Examples of chemical cell:

simple voltaic cell

Daniell cell

dry cell

alkaline cell

lead-acid accumulator

Simple Voltaic Cell

Two different metals being immersed into an electrolyte and connected by wire

V

Daniell cell – has two types

• Use porous pot

• Use salt bridge

Use porous pot

Use salt bridge

F. The Electrochemical Series

• The Electrochemical Series is series of element, arranged according to the order of decreasing tendency to released electrons.

• Or the greater the tendency to donate electrons, the more electropositive is the metal and the higher it is in the Electrochemical Series.

Principles Used:

• Metals are arranged according to the tendency of their atoms to release electron.

• More the tendency of their atoms to release electron, the higher located it is in the series.

• Elements located at HIGHER part of the Electrochemical Series are more electropositive and have higher tendencies to release electrons to form positive ions.

• Example : magnesium is more electropositive than copper in Electrochemical Series

The Electrochemical Series can be constructed based on:

Metals are arranged according to their tendency to release electrons to form positive ion (cation).

• metal has a higher tendency to release electron placed a the higher position in Electrochemical Series.

• the metal act as negative terminal

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

• If metal is able to displace another metal from its salt solution, this metal is placed at the higher position in Electrochemical Series

The potential difference between two metals.

• The further apart between two metals in the Electrochemical Series, the greater the potential difference between them.

• The greater the voltage produced by the cell.