Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Zichichi on 08/07/2006 09:16:03
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How many energy one needs to produce 1 kg of metallic sodium from NaCl?
The reaction is 2NaCl = 2Na + Cl2
I am sure someone knows this...
I know its cost that is about 0,2 euro/lb,
probably its efficiency is high enough...
nothing is created and nothing is destroyed
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First off, the reaction requires molten sodium chloride so you need to heat the reaction chamber to over 800 degrees C.
Then you would need to run sufficient current (note the standard potential for the redox reaction is around 4 V) through the solution to drive off the chlorine gas using a carbon anode and a steel cathode.
Now 1 kg of sodium is 43.5 moles and the reduction requires one electron so you require 2.6 * 10^25 electrons to create 1 kg of sodium.
Since 1 coulomb is 6.2 * 10^18 electrons you require 4.2 * 10^6 coulombs so at 100 amps it would take around 12 hours to generate the sodium.
In short, not the kind of thing you want to try in your kitchen.
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4Na + 2H2O + O2 = 4(NaOH) + about 1214 kj
4 mol is about 92 g.
1000 g. / 92 g. * 1214 = about 13.200 kj = about 3.7 kwh.
If input energy / output energy is 2 then it needs about 7,4 kwh but
its cost in USA would surpass 0,5 euro(the cost of 1 kg od Na).
Therefore the required energy is surely less than double.
nothing is created and nothing is destroyed
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Delta H of formation of liquid NaCl is -385.92 kJ/mol
(data from: http://webbook.nist.gov/cgi/cbook.cgi?Formula=NaCl&NoIon=on&Units=SI&cTC=on).
1 mol NaCl--> 1 mol Na = 23g.
(1000/23)*385.92 = 16.779kJ.
1 kwh = 3600kJ
16.779/3600 = 4.66kwh
(Of course, the chlorine also produced in the reaction is not thrown away, but it's sold as well).
By the way, NaCl is molten with other salts, for example CaCl2 (60%CaCl2 + 40%NaCl), to reduce the melting point: in this case about 600°C.