12 Questions
What type of electrodes are used in the Downs cell?
Inert
What material is the anode made of in the Downs cell?
Graphite
What prevents iron, the cathode material in the Downs cell, from being oxidized?
Continual supply of electrons from the power supply
What occurs at the anode in the Downs cell?
Oxidation of anions
Why is calcium chloride added to molten sodium chloride in the Downs cell process?
To lower the melting point
Which metal is mentioned as being difficult to reduce from Na+ ions to the metal form?
Sodium
What was Humphry Davy's contribution to the isolation of sodium?
He was the first person to isolate sodium using electrolysis.
Why is a mixture of sodium chloride/calcium chloride used as the electrolyte in the Downs cell?
To reduce the energy costs by lowering the melting point of sodium chloride.
Why should there be no contact between Cl2 and Na in the Downs cell?
They will lead to the loss of products from the electrolysis.
Why is it more cost-effective to use aqueous electrolytes over molten electrolytes?
It takes more energy to maintain molten electrolytes than aqueous ones.
What role does an iron mesh screen play in the Downs cell?
To separate the products at the anode and cathode.
In general, what is one of the main disadvantages of using a molten electrolyte in electrolysis?
It requires more energy to maintain the molten state of the electrolyte.
Study Notes
The Downs Cell
- The Downs cell is used to produce sodium and chlorine in commercial quantities.
- The cell uses inert (unreactive) electrodes: a graphite anode and an iron cathode.
- At the cathode, Na+ ions are reduced to sodium metal: Na+(l) + e- → Na(l)
- At the anode, 2Cl- ions are oxidized to chlorine gas: 2Cl-(l) → Cl2(g) + 2e-
- The overall cell reaction is: 2Na+(l) + 2Cl- → 2Na(l) + Cl2(g)
Molten Electrolytes
- In the electrolysis of a molten electrolyte, anions are oxidized at the anode and cations are reduced at the cathode.
- Water is not present to interfere with the desired reactions.
- Calcium chloride is added to molten sodium chloride to lower its melting point, but it does not affect the overall cell reaction.
History of Sodium Isolation
- Sodium was first isolated by electrolysis in 1807 by Humphry Davy.
- Davy electrolysed molten sodium hydroxide to isolate sodium.
- He was the first person to isolate sodium, potassium, calcium, barium, magnesium, boron, and strontium using electrolysis.
Advantages and Disadvantages of Molten Electrolytes
- The advantage of using a molten electrolyte is that there is no water present to interfere with the desired reactions.
- The main disadvantage is that the process requires much more energy to maintain the electrolyte in a molten state.
Operating the Downs Cell
- The electrolyte is a mixture of sodium chloride and calcium chloride in a 1:2 ratio, which lowers the melting point of sodium chloride from 801 °C to about 600°C.
- An iron mesh screen is used to keep the products at the anode and cathode apart.
- The construction and operation of the Downs cell minimizes contact between the two products to prevent them from reacting to re-form sodium chloride.
Explore the use of molten electrolytes in commercial electrolytic cells, specifically focusing on the Downs cell that produces sodium and chlorine. Learn about the inert electrodes used in the cell, such as graphite for the anode and iron for the cathode.
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