Chapter 6

Chapter 6Electrochemistry

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

Non-electrolytesSubstances that cannot conduct electricity

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 cellThe set of apparatus needed to conduct electrolysis

ElectrolyteCathodeAnode+-STEPS OCCUR DURING ELECTROLYSISMovement 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 productObservationConfirmatory testChlorine gasGreenish-yellow gas bubbles releasedChanges moist blue litmus paper to redBromine gasBrown gas releasedIodine Purple gas releasedOxygen gasColourless gas bubble releasedPlace a glow wooden splinter near the mouth of test tubeThe glowing wooden splinter light upMetal (all metal)The mass of electrode decreasedCopper metalBrown solid formed

Important note: ObservationCathode (negative electrode)Electrolysis productObservationConfirmatory testAlmost all metal (except copper metal)Grey solid formedThe mass of electrode increaseNo test for metalsCopper metal Brown solid formedHydrogen gasColourless gas bubbles releasedPlace a lighted wooden splinter near the mouth of test tubeA pop sound heard/produced

Electrolysis of molten compoundPbBr2LiClMgBr2PbI2ZnOCuCl2Molten Potassium oxideMolten Magnesium oxideMolten Potassium iodide

Electrolysis of Aqueous SolutionsFactors that affect the electrolysis of an aqueous solutionPosition of ions in the Electrochemical SeriesConcentration of ions in the electrolytesTypes 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

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

0.02 mol dm-3 of hydrochloric acid, HCl PlatinumPlatinum

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

Concentration of ionsIf the concentration of a particular ions is high, the ions is selectively dischargedTIPS:

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

Types of electrodesElectrolysis of copper(II) sulphate, CuSO4 solution using copper electrodeElectrolysis of silver nitrate, AgNO3 solution using silver electrode

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

0.02 mol dm-3 of silver nitrate, AgNO3 SilverSilver

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

D. Electrolysis in IndustriesThe most industrial application of electrolysis:extractionpurificationelectroplating of metals

E. Voltaic CellsChemical energy to electrical energyExamples of chemical cell:simple voltaic cellDaniell celldry cellalkaline celllead-acid accumulator

Simple Voltaic CellTwo 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.