Sodium propanoate, CH₃CH₂COONa, can be turned into two different alkanes…
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Sodium propanoate, , can be turned into two different alkanes. A chemist takes two samples of it, each of mass . The first sample is dissolved in water and electrolysed by Kolbe's method until all the propanoate has reacted. Gases come off at both electrodes. The second sample is mixed with soda lime and heated, and one gas comes off. A friend wants to make the alkane of the first sample by another route: the Wurtz reaction of bromoethane with sodium in dry ether. Take the molar volume at STP as . Atomic masses in : H 1, C 12, O 16, Na 23, Br 80.
- What mass of alkane forms in the electrolysis?
- What total volume of gas, at STP, comes off at the anode?
- What volume of gas, at STP, comes off at the cathode?
- What mass of alkane does the second sample give with soda lime?
- What mass of bromoethane does the friend need to get the same mass of alkane as in (a), if the Wurtz reaction is complete?
Write the balanced equation for each route before you calculate.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. of , with . Each sample is .
(a) Butane. Kolbe's method: . At the anode each propanoate ion loses an electron and then , giving an ethyl free radical. Two ethyl radicals join to make butane. So , and the mass is .
(b) Anode gases. Butane and carbon dioxide both leave at the anode: . The volume is .
(c) Cathode gas. Hydrogen forms at the cathode, one mole for every two moles of salt: .
(d) Ethane. , with CaO present. One mole of salt gives one mole of ethane: .
(e) Bromoethane. , in dry ether. We need of , with : .
Answer: (a) . (b) . (c) . (d) . (e) .
Check. A mass balance over the Kolbe equation. The left side is of salt and of water, in all. The right side is of butane, of , of and of NaOH, also .
Watch out. Kolbe's method doubles the alkyl group, so the propanoate gives butane, not propane. Soda lime removes one carbon, so it gives ethane. And the anode gives two gases: counting only the butane gives for (b).
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