Electrolysis Revision Notes

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  • Created by: nic
  • Created on: 24-03-13 15:31

ELECTROLYSIS OF AQUEOUS SODIUM CHLORIDE

Use either carbon or platinum electrodes

at - ve electrode:

at +ve electrode:

overall reaction:

Note: The solution around the negative electrode becomes alkaline because of the increased hydroxide ions.

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ELECTROLYSIS OF AQUEOUS COPPER (II) SULFATE

Use platinum electrodes!!! (carbon is not suitable as it will react with the oxygen produced at the positive electrode)

At + ve electrode:

At - ve electrode:

Overall reaction:

Note 1: the blue colour of the solution gradually fades, and eventually becomes colourless as copper (II) ions are removed from solution.

Note 2: the copper (II) sulfate solution gradually changes into sulfuric acid, since the copper (II) ions are replaced by hydrogen ions.

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ELECTROLYSIS OF DILUTE SULFURIC ACID

Use  platinum electrodes!!! (Carbon electrodes will react with the oxygen produced at the positive electrode)

at -ve electrode:

at +ve electrode:

overall reaction:

Note: As the water is decomposed, the solution of sulfuric acid gradually becomes more concentrated.

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ELECTROLYSIS OF MOLTEN COMPOUNDS

Molten compounds: electrolysing a compound containing only two elements can be predicted as follows:

  • The metal is formed at the negative electrode
  • 
  • The non-metal is formed at the positive electrode

e.g.Electrolysis of molten lead (II) bromide

At + ve electrode:

At - ve electrode:

Bromine forms as a gas and Lead forms as a liquid because the temperature of the molten electrolyte is above the boiling point of bromine and the melting point of lead.

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ELECTROLYSIS TERMS

ELECTROLYSIS = a chemical change caused by passing an electric current through a compound which is molten or in solution.

ELECTROLYTE = a substance that undergoes electrolysis. Electrolytes all contain ions.

PANC = Positive Anode Negative Cathode.

positive ions = CATIONS (because they are attracted to the cathode).

Positive ions move to the cathode where they are discharged by gaining electrons.

negative ions=ANIONS (because they are attracted to the anode)

Negative ions move to the anode where they are discharged by giving electrons to the electrode.

Remember OILRIG (Oxidation Is Loss, Reduction Is Gain of electrons)

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ELECTROLYSIS OF SOLUTIONS USING CARBON ELECTRODES

1. Product at negative electrode is either hydrogen or a metal

2. Product at positive electrode is either oxygen or another non-metal (e.g. chlorine)

  • if metal is higher in the reactivity series you get hydrogen produced instead of the metal.
  • if metal is below hydrogen (e.g copper, silver, gold) in the reactivity series you get the metal produced.
  • if you have reasonably concentrated solutions of halides (chlorides,bromides or iodides),you get the halogen (chlorine, bromine or iodine).
  • with other common negative ions, you get oxygen.

If you tried to electrolyse a covalent compound (molten or solution) there wouldn't be a current flow because there aren't any ions.

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