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Friday, May 1, 2009

Analysing electrolysis of molten compounds

Malaysia SPM Form 4 Chemistry, Chapter 6: Electrochemistry.

Electrolysis : Decomposing Using the Passage of Electricity

  • There are 2 types of electrochemical cell, namely
  1. Electrolytic Cell
  2. Voltaic Cell (or sometimes is called Galvanic Cell)
  • In electrolytic cell, electric current is flowed through an electrolyte to produce a chemical reaction.
  • In electrolytic cell, electrical energy is converted into chemical energy, and the process is called electrolysis.
  • In voltaic cell, chemical is used to produce electricity.
  • In this cell, chemical energy is converted into electrical energy.



The Electrolytic Cell

  • A suitable apparatus for electrolysis is shown in Figure above.
  • As we can see, the electrode connected to the positive terminal of the cell is positive electrode and is given a name, anode.
  • The electrode connected to the negative terminal of the cell is negative electrode and is called the cathode.


  • When electricity is passed through an electrolyte, chemical reaction happens.
  • In this reaction, chemical is splitting up into 2 new substances.
  • All electrolytes are ionic, which means they are composed of positively and negatively charged ions.
  • On passing an electric current through the electrolyte, these ions move towards the oppositely charged electrode.
  • Most negatively charged ions are non-metal ions, such as oxide (O2-, chloride (Cl-), Iodide (I-), etc.
  • During electrolysis, negatively charged ions move towards the positive electrode(anode). The negative ions lose their electron(s) to the anode, which is positively charged.
  • The electron(s) is then move to the cathode through the external circuit (the wire).
  • The positively charged ions move towards the negative electrode(cathode').
  • These positive ions are metal ions, such as copper (Cu2+), silver (Ag+), lead (Pb2+), etc, or hydrogen (H+).
  • At cathode, positive ions gain electron(s) from the cathode, which has an excess of electrons and therefore an overall negative charge.


  • This process results in the chemical decomposition of the electrolyte. It also allows electrons to travel from the cathode to the anode and hence allows conduction of electricity.
  • During the electrolysis, electrical energy is supplied to the system to produce a chemical reaction.
  • Therefore, during electrolysis, electrical energy convert into chemical energy.

Example 1:Electrolysis of MOLTEN Lead (II) Bromide

  • This is composed of lead(II) ions, Pb2 + , and bromide ions, Br-. Its chemical formula is therefore PbBr2.
  • A suitable apparatus which could be used to carry out this electrolysis is shown in Figure above.
  • The bulb helps to show when electricity is flowing in the circuit, and until the lead(II) bromide is completely molten, the bulb does not light up . This confirms that electrolytes have to be molten for the ions to start to move to the electrodes and thereby conduct electricity.
At the CathodeAt the Anode

Observation

  • When electricity is flowing, a silvery deposit of lead metal forms on the cathode. In fact, as it is molten, it is more likely to drip off in a molten blob.

Observation

  • When electricity is flowing, brown fumes of bromine gas are seen at the anode.

Half equation
Pb2+ + 2e ---> Pb

Half equation
2Br- ---> Br2 + e

Explanation

  • The lead(II) ions, as they are positive, move to the negative cathode, where each ion gains two electrons to form a lead atom.
  • Any reaction at a cathode involved is again in electrons. This is called reduction or more exactly, cathodic reduction .

Explanation

  • The bromide ions, as they are negative, move to the positive anode, where each loses an electron to form a bromine atom.
  • Then two of these newly formed atoms combine to form bromine gas.
  • Any reaction at an anode involves a loss of electrons.
  • In summary, the lead(II) bromide is split into its component elements :

PbBr2 ---> Pb + Br2

Example 2: Electrolysis Of Molten Lead(II) Oxide

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