Chapter 6

Electrochemistry

A. Electrolytes & Non-electrolytesElectrolytesSubstances that can conduct electricity and undergo chemical changes

Non-electrolytesSubstances that cannot conduct electricity

Electrolyte

Non-electrolyte

Why electrolyte at molten and aqueous state can conduct

electricity?

BECAUSEThere presence of free moving ions

in the electrolyte

B. Electrolysis of Molten Compounds

ElectrolysisA 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

CathodeAnode

+ -

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: ObservationAnode (positive electrode)

Electrolysis product Observation Confirmatory test

Chlorine gas Greenish-yellow gas bubbles released

Changes moist blue litmus paper to 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 tubeThe glowing wooden splinter light up

Metal (all metal) The mass of electrode decreased

Copper metal Brown solid formed

Important note: ObservationCathode (negative electrode)

Electrolysis product Observation Confirmatory test

Almost all metal (except copper metal)

Grey solid formedThe 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 tubeA “pop” sound heard/produced

Electrolysis of molten compound

Electrolysis of molten compoundPbBr2

LiClMgBr2

PbI2ZnOCuCl2Molten Potassium oxideMolten Magnesium oxideMolten Potassium iodide

Electrolysis of Aqueous SolutionsFactors 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 SeriesThe 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

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

1 mol dm-3 of copper(II) sulphate solution

CarbonCarbon

Concentration of ionsIf the concentration of a

particular ions is high, the ions is selectively discharged

TIPS:

ANODEIon selectively discharged based on CONCENTRATION of ions

CATHODEIon 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

PlatinumPlatinum

Anode :

Cathode :

Types of electrodesElectrolysis 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

CopperCopper

Anode :

Cathode :

0.02 mol dm-3 of silver nitrate, AgNO3 using silver as electrodes

0.02 mol dm-3 of silver nitrate, AgNO3

SilverSilver

D. Electrolysis in IndustriesThe most industrial application of

electrolysis:1. extraction2. purification3. electroplating of metals

E. Voltaic CellsChemical energy to electrical

energyExamples 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

V

F. The Electrochemical SeriesThe 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.