electrochemistry
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Chapter 6
Electrochemistry
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A. Electrolytes & Non-electrolytesElectrolytesSubstances that can conduct electricity and undergo chemical changes
Non-electrolytesSubstances that cannot conduct electricity
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Electrolyte
Non-electrolyte
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Why electrolyte at molten and aqueous state can conduct
electricity?
BECAUSEThere presence of free moving ions
in the electrolyte
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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
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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.
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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
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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
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Electrolysis of molten compound
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Electrolysis of molten compoundPbBr2
LiClMgBr2
PbI2ZnOCuCl2Molten Potassium oxideMolten Magnesium oxideMolten Potassium iodide
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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
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Position of ions in the Electrochemical SeriesThe ions that are lower in the
electrochemical series will be selected discharged
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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
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1 mol dm-3 of copper(II) sulphate using carbon as electrodes
1 mol dm-3 of copper(II) sulphate solution
CarbonCarbon
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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
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2 mol dm-3 of hydrochloric acid, HCl using platinum as electrodes
2 mol dm-3 of hydrochloric acid, HCl
PlatinumPlatinum
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Anode :
Cathode :
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Types of electrodesElectrolysis of copper(II)
sulphate, CuSO4 solution using copper electrode
Electrolysis of silver nitrate, AgNO3 solution using silver electrode
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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
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Anode :
Cathode :
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0.02 mol dm-3 of silver nitrate, AgNO3 using silver as electrodes
0.02 mol dm-3 of silver nitrate, AgNO3
SilverSilver
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D. Electrolysis in IndustriesThe most industrial application of
electrolysis:1. extraction2. purification3. electroplating of metals
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E. Voltaic CellsChemical energy to electrical
energyExamples of chemical cell:
simple voltaic cell Daniell cell dry cell alkaline cell lead-acid accumulator
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Simple Voltaic Cell
Two different metals being immersed into an electrolyte and connected by wire
V
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Daniell cell – has two types
V
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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.
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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
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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
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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
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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.