Topic 2 - Predicting the products of electrolysis

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  • Created by: E456
  • Created on: 17-04-17 09:34

UNIVERSAL BEHAVIOUR

  • The positive ions in the electrolyte will always migrate towards the negative electrode, or cathode. 
  • The negative ions in the electrolyte will always migrate towards the positive electrode, or anode.

CHANGING THE CATHODE

  • Since the cathode is where metals or hydrogen are discharged, the material from which the cathode is made usually has very little effect on the products of electrolysis.

CHANGING THE ANODE

  • The anode, if it is made of a suitable material, may go into solution as positive ions, donating electrons into the circuit. 
  • Alternatively, a suitable negative ion may be discharged and donate its electrons to the circuit.
  • There is no general rule for predicting the outcome, but there are a number of key examples which you should know for IGCSE.

EXAMPLE 1: Electrolysis of copper sulphate solution with graphite or platinum electrodes.

Since carbon cannot form a positive ion and go into the solution, a negative ion must be discharged at the anode. Similarly platinum is ‘noble’, and does not easily form positive ions. Therefore in both these instances the OH  ion will be discharged at the anode.

Copper is deposited at the cathode as expected. Since Cu2+ ions are being discharged to form copper metal and hydroxide ions are being removed, in time there will be an excess of sulphate and hydrogen ions left in the solution.

EXAMPLE 2: Copper sulphate solution electrolysed with copper electrodes.

The copper anode goes into solution as copper ions and at the cathode, copper ions are discharged. In this case, the strength of…

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