Electrolysis

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Electrolysis
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Terms used in electrolysis

• Electrolysis is the decomposition of an
  electrolyte in molten state or aqueous solution
  by electricity.
• An electrolyte is a substance which conducts an
  electric current in molten state or aqueous
  solution and is decomposed by electricity.
• The anode is the electrode where oxidation
  occurs. It is the electrode connected to the
  positive terminal of the d.c. supply.
• The cathode is the electrode where reduction
  occurs. It is the electrode connected to the
  negative terminal of the d.c. supply.

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Terms used in electrolysis

• An anion is a negative ion and is attracted to
  the anode.
• A cation is a positive ion and is attracted to
  the cathode.
• An ammeter is an instrument used to measure the
  electric current passing through a circuit.
  Electric current is measured in ampere (A).
• A variable resistor (or rheostat) is used to vary
  the resistance and then regulate the current.

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Electrolysis
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Factors affecting electrolysis

• The position of ions in the electrochemical
  series.
• The concentration of ions in the solution.
• The nature of the electrodes.

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Position of cations in the e.c.s.
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Position of anions in the e.c.s.
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Case 1 Electrolysis of molten lead(II) bromide
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Case 1 Electrolysis of molten lead(II) bromide

• Solid lead(II) bromide does not conduct
  electricity because the ions are not mobile.
• Molten lead(II) bromide contains mobile ions.

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Case 1 Electrolysis of molten lead(II) bromide
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Case 1 Electrolysis of molten lead(II) bromide

• At cathode lead(II) cations receive electrons
  they undergo reduction and discharge to form
  lead atoms.
• Pb2() 2e Pb()

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Case 1 Electrolysis of molten lead(II) bromide

• At anode bromide anions give up electrons they
  undergo oxidation and discharge to form bromine
  atoms.
• 2Br-() Br2() 2e

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Case 1 Electrolysis of molten lead(II) bromide

• Bromine atoms then join in pair to form bromine
  molecules.

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Case 2 Electrolysis of acidified water using
platinum electrodes

• Although water is known to be poor electrical
  non-conductor it actually ionizes slightly to
  give hydrogen ions and hydroxide ions. H2O()
  H(aq) OH(aq)
• Pure acids are covalent compounds. However they
  ionize in water.
• HCl(g) water HCl(aq)
• HCl(aq) H(aq) Cl(aq)

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Case 2 Electrolysis of acidified water using
platinum electrodes
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Case 2 Electrolysis of acidified water using
platinum electrodes

• At cathode hydrogen ions receive electrons they
  undergo reduction and discharge to form hydrogen
  gas.
• 2H(aq) 2e H2(g)

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Case 2 Electrolysis of acidified water using
platinum electrodes

• At anode hydroxide ions give up electrons they
  undergo oxidation and discharge to form oxygen
  gas.
• 4OH-(aq) O2(g) 2H2O() 4e

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Case 2 Electrolysis of acidified water using
platinum electrodes

• 2H(aq) 2e H2(g)
  (1)
• 4OH-(aq) O2(g) 2H2O() 4e (2)
• (1)x2 4H(aq) 4e 2H2(g)
  (3)
• (2)(3) 4OH-(aq) 4H(aq) O2(g) 2H2O()
  2H2(g)
• 4H2O() O2(g) 2H2O()
  2H2(g)
• Overall equation 2H2O() O2(g) 2H2(g)

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Case 2 Electrolysis of acidified water using
platinum electrodes

• Dilute acid is added to provide more mobile ions
  so as to increase the conductivity of the water.
• The concentration of dilute acid increases at the
  end as water is consumed in the electrolysis.

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Case 3 Electrolysis of dilute sodium chloride
solution using carbon electrodes
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Case 3 Electrolysis of dilute sodium chloride
solution using carbon electrodes

• The sodium ions and hydrogen ions move towards
  the cathode.
• At the cathode the position of hydrogen ions in
  the electrochemical series is lower than that of
  sodium ions. Hydrogen ions are preferentially
  discharged (reduced) to form colourless hydrogen
  gas.
• 2H(aq) 2e H2(g)

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Case 3 Electrolysis of dilute sodium chloride
solution using carbon electrodes

• The chloride ions and hydroxide ions move towards
  the anode.
• At the anode the position of hydroxide ions in
  the electrochemical series is higher than that of
  chloride ions. Hydroxide ions are preferentially
  discharged (oxidized) to form colourless oxygen
  gas.
• 4OH-(aq) O2(g) 2H2O() 4e

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Case 3 Electrolysis of dilute sodium chloride
solution using carbon electrodes

• Overall reaction 2H2O() O2(g) 2H2(g)
• Water ionizes continuously to replace the
  hydrogen ions discharged at the cathode. Thus
  there is an excess of hydroxide ions near the
  cathode and the solution there becomes alkaline.
• Water ionizes continuously to replace the
  hydroxide ions discharged at the anode. Thus
  there is an excess of hydrogen ions near the
  anode. The solution there becomes acidic.

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Case 3 Electrolysis of dilute sodium chloride
solution using carbon electrodes

• If a few drops of universal indicator are added
  to the sodium chloride solution the solution
  near the cathode will turn blue while that near
  the anode will turn red.
• The sodium chloride becomes more concentrated as
  water is consumed in the electrolysis.

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Case 4 Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes
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Case 4 Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes

• The copper(II) ions and hydrogen ions move
  towards the cathode.
• At the cathode the position of copper(II) ions
  in the electrochemical series is lower than that
  of hydrogen ions. Copper(II) ions are
  preferentially discharged (reduced) to form brown
  copper metal.
• Cu2(aq) 2e Cu(s)

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Case 4 Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes

• The sulphate ions and hydroxide ions move towards
  the anode.
• At the anode the position of hydroxide ions in
  the electrochemical series is higher than that of
  sulphate ions. Hydroxide ions are preferentially
  discharged (oxidized) to form colourless oxygen
  gas.
• 4OH-(aq) O2(g) 2H2O() 4e

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Case 4 Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes

• Overall reaction
• 2Cu2(aq) 4OH(aq) 2Cu(s) O2(g)
  2H2O()
• Water ionizes continuously to replace the
  hydroxide ions discharged at the anode. Thus
  there is an excess of hydrogen ions near the
  anode. The solution there becomes acidic.
• If a few drops of universal indicator is added
  into the solution red colour appears around
  anode.

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Case 4 Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes

• The blue colour of the solution fades out because
  the concentration of copper(II) ions decreases.
• Copper(II) ions and hydroxide ions are consumed
  in the electrolysis. Hydrogen ions and sulphate
  ions remain in the solution. Thus the solution
  eventually becomes sulphuric acid.

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Case 4 Electrolysis of dilute copper(II)
sulphate solution using carbon electrodes

• After a few minutes cathode is coated with
  copper.
• If the polarities of cells are then reversed
  anode is coated with copper. The factor of
  electrode should be considered as in case 8.

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Case 5 Electrolysis of dilute sodium iodide
solution using carbon electrodes
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Case 5 Electrolysis of dilute sodium iodide
solution using carbon electrodes

• The sodium ions and hydrogen ions move towards
  the cathode.
• At the cathode the position of hydrogen ions in
  the electrochemical series is lower than that of
  sodium ions. Hydrogen ions are preferentially
  discharged (reduced) to form colourless hydrogen
  gas.
• 2H(aq) 2e H2(g)

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Case 5 Electrolysis of dilute sodium iodide
solution using carbon electrodes

• The iodide ions and hydroxide ions move towards
  the anode.
• At the anode the position of hydroxide ions in
  the electrochemical series is higher than that of
  chloride ions. However the concentration of
  iodide ions is much greater than that of
  hydroxide ions. Iodide ions are preferentially
  discharged (oxidized) to form iodine.
• 2I-(aq) I2(aq) 2e

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Case 5 Electrolysis of dilute sodium iodide
solution using carbon electrodes

• Overall reaction
• 2H(aq) 2I(aq) H2(g) I2(aq)
• The solution near the cathode becomes alkaline.
• The iodine produced at the anode dissolves in the
  solution. Therefore a brown colour develops
  around the anode.

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Case 5 Electrolysis of dilute sodium iodide
solution using carbon electrodes

• Hydrogen ions and iodide ions are consumed in the
  electrolysis. Sodium ions and hydroxide ions
  remain in the solution. The solution eventually
  becomes sodium hydroxide solution.

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Case 6 Electrolysis of conc. sodium chloride
solution using carbon electrodes
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Case 6 Electrolysis of conc. sodium chloride
solution using carbon electrodes

• The sodium ions and hydrogen ions move towards
  the cathode.
• At the cathode the position of hydrogen ions in
  the electrochemical series is lower than that of
  sodium ions. Hydrogen ions are preferentially
  discharged (reduced) to form colourless hydrogen
  gas.
• 2H(aq) 2e H2(g)

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Case 6 Electrolysis of conc. sodium chloride
solution using carbon electrodes

• The chloride ions and hydroxide ions move towards
  the anode.
• At the anode the position of hydroxide ions in
  the electrochemical series is higher than that of
  chloride ions. However the concentration of
  chloride ions is much greater than that of
  hydroxide ions. Chloride ions are preferentially
  discharged (oxidized) to form chlorine gas.
• 2Cl-(aq) Cl2(g) 2e

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Case 6 Electrolysis of conc. sodium chloride
solution using carbon electrodes

• Overall reaction
• 2H(aq) 2Cl(aq) H2(g) Cl2(aq)
• Water ionizes continuously to replace the
  hydrogen ions discharged at the cathode. Thus
  there is an excess of hydroxide ions near the
  cathode. The solution there becomes alkaline.
• The chlorine gas formed at the anode dissolves in
  the solution. The solution there becomes acidic
  and has a bleaching effect.

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Case 6 Electrolysis of conc. sodium chloride
solution using carbon electrodes

• Hydrogen ions and chloride ions are consumed in
  the electrolysis. Sodium ions and hydroxide ions
  remain in the solution. Eventually the solution
  becomes sodium hydroxide solution.

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Case 7 Electrolysis of conc. sodium chloride
solution using mercury electrodes
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Case 7 Electrolysis of conc. sodium chloride
solution using mercury electrodes

• The sodium ions and hydrogen ions move towards
  the cathode.
• At the cathode the position of hydrogen ions in
  the electrochemical series is lower than that of
  sodium ions. However sodium ions are
  preferentially discharged (reduced) to form
  sodium metal. The sodium metal formed dissolves
  in the mercury to form a sodium amalgam.
• Na(aq) e Hg(l) Na/Hg(l) sodium amalgam

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Case 7 Electrolysis of conc. sodium chloride
solution using mercury electrodes

• The sodium amalgam then reacts with water to form
  sodium hydroxide and hydrogen.
• 2Na/Hg(l) 2H2O(l) 2NaOH(aq) H2(g) 2Hg(l)

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Case 7 Electrolysis of conc. sodium chloride
solution using mercury electrodes

• The chloride ions and hydroxide ions move towards
  the anode.
• At the anode the position of hydroxide ions in
  the electrochemical series is higher than that of
  chloride ions. However the concentration of
  chloride ions is much greater than that of
  hydroxide ions. Chloride ions are preferentially
  discharged (oxidized) to form chlorine gas.
• 2Cl-(aq) Cl2(g) 2e

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Case 7 Electrolysis of conc. sodium chloride
solution using mercury electrodes

• Overall reaction
• 2Na(aq) 2Cl(aq) 2Hg(l) 2Na/Hg(l)
  Cl2(g)
• Sodium ions and chloride ions are consumed in the
  electrolysis. Thus the sodium chloride solution
  becomes more and more dilute.
• This reaction is very important in the
  manufacture of chlorine bleaching solution.

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Case 8 Electrolysis of dilute copper(II)
sulphate solution using copper electrodes
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Case 8 Electrolysis of dilute copper(II)
sulphate solution using copper electrodes

• The copper(II) ions and hydrogen ions move
  towards the cathode.
• At the cathode the position of copper(II) ions
  in the electrochemical series is lower than that
  of hydrogen ions. Copper(II) ions are
  preferentially discharged (reduced) to form brown
  copper metal. Cu2(aq) 2e Cu(s)

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Case 8 Electrolysis of dilute copper(II)
sulphate solution using copper electrodes

• The sulphate ions and hydroxide ions move towards
  the anode.
• At the anode the position of hydroxide ions in
  the electrochemical series is higher than that of
  sulphate ions. However copper is a stronger
  reducing agent than hydroxide ions and thus more
  easily oxidized. The copper anode dissolves to
  form copper(II) ions (oxidized).
• Cu(s) Cu2(aq) 2e

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Case 8 Electrolysis of dilute copper(II)
sulphate solution using copper electrodes

• Overall reaction
• Cu(s) Cu(s)
• anode cathode
• The net effect is the transfer of copper from the
  anode to the cathode. The rate at which copper
  deposits on the cathode is equal to the rate at
  which the copper anode dissolves.
• Increase in mass of cathode decrease in mass
  of anode

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Case 8 Electrolysis of dilute copper(II)
sulphate solution using copper electrodes

• The concentration of copper(II) ions in the
  solution remains the same. The blue colour of the
  solution does not change.

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Comparing a chemical cell and an electrolytic cell
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Comparing a chemical cell and an electrolytic cell
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Comparing a chemical cell and an electrolytic cell
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Commercial uses of electrolysis

• Manufacture of hydrogen chlorine and sodium
  hydroxide and bleaching solution
• Refining of copper
• Electroplating
• Extracting reactive metals
• Aluminium anodization

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Manufacture of bleaching solution
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Manufacture of bleaching solution

• At the anode 2Cl(aq) Cl2(g) 2e
• At the cathode Na(aq) e Hg(l)
  Na/Hg(l)
• sodium amalgam
• The sodium amalgam then flows into a second cell
  and reacts with water to form sodium hydroxide
  hydrogen and mercury.
• Mercury is then recovered and then pumped back
  into the reaction chamber.
• 2Na/Hg(l) 2H2O(l) 2NaOH(aq) H2(g)
  2Hg(l)

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Manufacture of bleaching solution

• This process also produces waste which contains
  poisonous mercury compounds. These waste products
  will cause serious pollution problems if they are
  discharged into rivers and seas.

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Refining of copper
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Refining of copper

• Copper ore contains a few impurities mostly
  silver gold platinum iron and zinc reduce
  the electrical conductivity of copper
  significantly.
• anode impure copper
• cathode very pure copper
• electrolyte copper(II) sulphate solution and
  sulphuric acid

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Refining of copper

• Iron and zinc are more reactive than copper.
  They form ions more readily than copper.
• At anode iron and zinc give up electrons first.
  Then copper gives up electrons to form copper(II)
  ions.
• Zn(s) Zn2(aq) 2e
• Fe(s) Fe2(aq) 2e
• Cu(s) Cu2(aq) 2e

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Refining of copper

• Impurities such as silver gold and platinum
  settle at the bottom of the container.
• At the cathode the position of copper(II) ions
  in the electrochemical series is lower than that
  of hydrogen ions. Copper(II) ions are
  preferentially discharged (reduced) to form brown
  copper metal. Cu2(aq) 2e Cu(s)

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Refining of copper

• Overall reaction
• Cu(s) Cu(s)
• anode cathode
• Refer to case 8

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Electroplating
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Electroplating

• Electroplating is the coating of an object with a
  thin layer of a metal by electrolysis.
• Cathode object to be plated
• Anode plating metal
• Electrolyte a solution of a compound of the
  plating metal

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Electroplating

• Objects may be electroplated with copper nickel
  chromium gold or silver.
• Typical example electroplating of copper

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Pollution problems of electroplating

• The electroplating industry produces many toxic
  waste by-products.
• acids and alkalis
• cumulative poisons of heavy metals and ions (such
  as nickel chromium and mercury)
• toxic cyanides.

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Solutions

• Controlling the pH value of effluents
• The pH value of acidic effluents can be
  controlled by adding sodium carbonate.
• The pH value of alkaline effluents can be
  controlled by adding sulphuric acid.
• Treatment of heavy metal compounds
• Add sodium hydroxide solution to the effluents to
  form insoluble metal hydroxides. The solid is
  then filtered off.

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Solutions

• Treatment of poisonous chromium waste
• Poisonous chromium(VI) compounds are reduced to
  non-toxic chromium(III) compounds by sodium
  sulphite. Sodium hydroxide solution is then added
  to the chromium(III) compounds to form solid
  chromium(III) hydroxide. The solid is then
  filtered off.

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Extraction of reactive metals

• Reactive metals such as K Na Ca Mg and Al are
  extracted from its ores by electrolysis of molten
  metal ores.
• Metal ions are attracted to the cathode and
  reduced to form metal.
• Mn(l) ne- M(s)

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Anodization of aluminium

• Aluminium oxide is a protective oxide layer. It
  does not react with acids and alkalis.
• However natural occurring aluminium oxide layers
  are thin and unevenly distributed.
• Anode Al
• Cathode circular sheet of steel
• Electrolyte dilute sulphuric acid

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Anodization of aluminium

• At anode
• 4OH-(aq) O2(g) 2H2O() 4e
• Oxygen is then reacted with aluminium anode to
  form a thick protection oxide layer.
• 4Al(s) 3O2(g) 2Al2O3(s)
• Aluminium oxide can be dyed.