Category: Concept map

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Hall Herold Electrochemical Extraction of Aluminium

Highlights

  • Principle
  • Extraction of aluminium
delta G = change in free energy, E = emf, n = number of electrons, F = faraday

Electrochemical reactions

Key points

  • Metal oxides undergo reduction in presence of carbon to become metals.
  • They can undergo reduction thermodynamically and electrochemically.
  • Reactive metals are difficult to be obtained by thermodynamic process.
  • Delta G = change in Gibb’s free energy. Its negative value indicates the greater feasiblity of the reaction.
  • E = cell emf. When it is positive, delta G becomes negative.
  • Hall Herold extracted aluminium from its oxide in the molten form by electrochemical process.
#Item Description
1Electrolytic bathIron tank lined with carbon
2Cathodecarbon lining
3Anodecarbon blocks suspended in the bath
4Electrolyte20% alumina, molten cryolite, 20% calcium chloride
5Electrodic reactionAt cathode reduction occurs.
At anode oxidation occurs.
Electrolytic Bath Design

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Second law of thermodynamics

Second law of thermodynamics is about the extent and direction of energy transformation.

i) Kelvin’s statement

It is impossible to obtain a continuous supply of work from a body
by cooling it to a temperature below the coldest of its surroundings.

ii) Clausius statement

It is impossible for a self acting machine unaided by any external agency to transfer heat from a body at a lower temperature to another
body at a higher temperature.

(iii) Kelvin – Planck’s statement
It is impossible to construct a heat engine operating in a cycle, that
will extract heat from a reservoir and perform an equivalent amount of
work.