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Electrochemistry
Is the study of the relationship between electrical energy and chemical reactions.
It deals with the chemical action of electricity and the production of electrical energy by chemical reactions.
All redox reactions involve transfer of electrons from the oxidised substance to the reduced substance.
For example, zinc reacts with copper (II) ions as follows:
Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)
Explanation of Electrolysis
Electrolysis is the process of decomposing (breaking down) an ionicsubstance, called an electrolyte, into simpler substances using electricity. The chemical reaction of electrolysis occurs when an electric current is passed through a solution containing ions (ions are charged atoms, they have more or less electrons than protons which causes an imbalance, the overall charge can be negative or positive).
For the electrolyte to conduct electricity, it must be:
The process of Electrolysis: Electrolysis of an ionic substance
The ionic substance is heated until it melts.
The ions are able to move freely after the ionic substance is melted.
The power supply is connected and the electrodes are charged.
The ions move to the oppositely charged electrode (the negative electrons move to the positive electrode, called the anode, and the positive electrons move to the negative electrode, called the cathode).
The electrodes give/take electrons from the ions which makes the ions neutral.
The ions become atoms (because they are neutral) and are deposited onto the electrode.
Reactions occur at electrodes.
tetrachloromethane CCl4
trichloromethane CHCl3
pure water H2O
sugar solution C12H22O11
molten sulphur S
ammonia solution NH3
aqueous ethanonoic acid CH3COOH
aqueous sulphurous acid H2SO3
aqueous carbonic acid H2CO3
aquous nitric acid HNO3
aquous hydrochloric acid HCl
aqueous potassium hydroxide KOH
aqueous sodium hydroxide NAOH
copper(II) sulphate solution CuSO4
Electrolysis of Molten Compounds
Example
Electrolysis of molten PbBr2
To make molten lead(II) bromide, PbBr2, we strongly heat the solid until it melts. To electrolyse it, pass current through the molten PbBr2.
What happens:
Ions present: Pb2+ and Br–
Reaction at Anode
Br- loses electrons at anode to become Br atoms. Br atoms created form bond together to make Br2 gas.
2Br–(aq) –> Br2(g)+ 2e–
Reaction at Cathode
Pb2+ gains electrons at cathode to become Pb atoms becoming liquid lead (II).
Pb2+(aq) + 2e– –> Pb(l)
Overall equation
PbBr2(l) –> Pb(l) + Br2(g)
Electrolysis of Aqueous Solution
Aqueous solutions contain additional H+ and OH– ions of water, totaling 4 ions in the solution :
2 from electrolyte, 2 from water.
Only 2 of these are discharged.
Electrolysis of aqueous solutions use the theory of selective discharge.
At cathode
At anode
Examples
A. Concentration Solutions
Electrolysis of Concentrated NaCl
What happens:
Ions Present: Na+, H+, OH– and Cl–
Reaction at Anode
Overall Equation
2HCl(l) –> H2(l) + Cl2(g)
Note: any cation and anion left undischarged in solution forms new bonds between them.
E.g. in above, leftovers Na+ and OH– combine to form NaOH.
B. Very Dilute Solutions
Electrolysis of Dilute H2SO4
What happens:
Ions Present: H+, OH– and SO42-
Reaction at Anode
Since only water is electrolysed, the sulfuric acid now only becomes concentrated.
Electrolysis using different types of electrodes
Inert Electrodes are electrodes which do not react with electrolyte or products during electrolysis.
Eg. platinum and graphite.
Active Electrodes are electrodes which react with products of electrolysis, affecting the course of electrolysis.
Eg. copper.
A. Electrolysis of CuSO4 Using Inert Electrodes (e.g. carbon)
What happens:
Ions Present: Cu2+, H+, OH– and SO42-
Reaction at Anode
Ions Present: Cu2+, H+, OH– and SO42-
Reaction at Anode
Reaction at Cathode
Impurities of anode falls under it.
Electroplating
Electroplating is coating an object with thin layer of metal by electrolysis. This makes the object protected and more attractive.
Object to be plated is made to be cathode and the plating metal is made as anode.
The electrolyte MUST contain plating metal cation.
Plating Iron object with Nickel
Reaction at Anode
Reaction at Cathode
Overall Change
Uses of Electroplating.
A VIDEO ABOUT ELECTROPLATING
Creation of Electric Cells by Electrolysis
A Simple cell or an Electric cell is a device that converts chemical energy into electrical energy, and it consists of 2 electrodes made of 2 metals of different reactivity.
In a simple cell, the MORE REACTIVE metal/electrode is ALWAYS designated the NEGATIVE electrode
The anode (negative electrode) is made of more reactive metal. This is because they have more tendency of losing electrons.
The cathode (positive electrode) is made of less reactive metal.
The further apart the metals in the reactivity series, the higher the voltage created.
The electrons in a simple cell will ALWAYS flow from the NEGATIVE electrode (made of the MORE reactive metal) to the POSITIVE electrode.
A simple electric cell using zinc and copper
The electrons then flow from the zinc rod to the copper rod through the external circuit. At the copper rod, reduction occurs – the hydrogen ions in solution accept these electrons to form hydrogen gas;
The magnitude of the voltage (potential difference) is related to the positions of the two metals in the reactivity series. The further apart the two metals, the larger will be the potential difference (voltage) produced.
The reaction at the anode is oxidation and the reaction at the cathode is reduction.
The reaction at the anode is oxidation and the reaction at the cathode is reduction.
They move from anode to the cathode in the external circuit.
Factors affecting electrolysis
Concentration
Type of electrode
Terms used in electrolysis
These are substances which allow an electric current to pass through them e.g. metals and graphite.
Any substance which allows the electric current to pass through it, is called an electrical conductor. The substance can be solid metallic in nature or can be in fused state as well but the flow of energy in form of electrical charge takes place only when there is less resistance to the flow of this charge. The lesser the resistance the higher the flow electrical charge through a solid body and more the number of free ions in fused state, easier the flow of charge in it.
In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of charge through them.
“Conductor” implies that the outer electrons of the atoms are loosely bound and free to move through the material.
Most atoms hold on to their electrons tightly and are insulators. In copper, the valence electrons are essentially free and strongly repel each other.
Any external influence which moves one of them will cause a repulsion of other electrons which propagates,
“domino fashion” through the conductor.Simply stated, most metals are good electrical conductors, most nonmetals are not. Metals are also generally good heat conductors while nonmetals are not.
Note
In a metal or graphite’s, flow of current is movement of electrons
Are materials which do not allow an electric current to flow through them
Using a burette
12.90 12.40
15.80 28.65
21.55 49.75
6.50 40.75
33.80 39.70
Calculations involving moles in solution
What is the morality of solution prepared by dissolving 56gms of potassium hydroxide with water and solution made up of 1 litre?
K o H
R.F.M of KOH = (1×39) + (1×16) + (1×1)
=39+16+1
=56
Morality = 1
Procedures in titration apparatus (burette), pipette 25 or 20cm3, 2 conical flasks, white tile, 2 beakers.
Wash apparatus with water.
Remove as much water as possible from the beakers and collect solution. (100 mls of each solution –acid in a glass beaker and alkali or carbonate solution in plastic.
Rinse burette with a little of the solution, it will be used to measure
Do the same with a pipette.
Fill the burette up to above the 0 mark (about full) if it doesn’t start from zero.
In case you use a bead burette remember to remove the air bubble
Set your burette so the lower meniscus is between (somewhere) 0 and 1
Clamp burette in vertical position and record the initial burette reading.
Note: All burette reading must be recorded to 2 decimal places.
Pipette solution and transfer it into clean conical flasks and add two drops of indicator.
Titrate the first solution by running solution into it from the burette until the colour of the indicator changes.
Record the final burette reading.
Find the volume of solution you have used by subtracting the initial reading from final reading.
Before you start the 2nd experiment make sure you have enough solution to take you through that experience.
Exp 2
You are provided with solution BA, and BA2
Solution
BA1 = 0.1M (solution of acid HNX
BA2 = 0.1M sodium hydroxide
You are required to determine the stoichiometry (reaction mole ratio) of the acid. Base
Procedure
Pipette 20/25cm3 of BA2 into a clean conical flask and add to it 2 drops of pthenothalene indicator.
Titrate the solution with BA, from the Burette.
The end point is when the colour of the indicator just turns colourless
Report titration until you got consist result and record them in the table below
Litre values to be averaged =
= 23.80cm3
Calculations
100cm 3of acid contains 0.1 M
1cm3 of acid contains
1050cm3 of acid contains x 10.50
= 0.00105 moles
The number of moles of sodium hydroxide used.
100cm 3of NaoH contains 0.1 Moles
1cm3 of NaoH contains
1050cm3 of NaoH contains x 25.0
= 0.0025 moles
Hence determine the stoichiometry for the reaction between acid and base.
HnX: NaoH
:: 2
1:2
State the basicity of the acid n
n=2 : Basicity = 2
ASSIGNMENT : ELECTRO CHEMISTRY Assignment MARKS : 10 DURATION : 1 week, 3 days