Elemental analysis and the combustion process

[plzNOCarribbean]

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I have seen lots of questions in regard to the combustion of hydrocarbons and I just want to make sure I understand the process. The TBR books really help you out but I want to get an understand of the different ways their apparatus works.

If someone could please highlight the main thing that is happening, in simple terms, that would be greatly appreciated. I know hydrocarbons react with excess oxygen to yield CO2 and Water vapor, but how is this used to figure out the unknown mass of an unknown hydrocarbon.

On passage V of Chem Book 1 In TBR, book set up had a reaction vessel and different traps. how does it all work? does the hydrocarbon react with oxygen, and then the CO2 and Water vapor that produced are collected afterwords as frozen solids by lowering the temperature? Or, is it just the amount of CO2 gas that is frozen that is measure see how much of the hydrocarbon was oxidized?

 

[loveoforganic]

I don't have what you're looking at on hand, but I think I know what you're referring to. In complete combustion, the hydrocarbon will be completely converted to H2O(g) and CO2(g). If you combust your hydrocarbon in an oxygen rich environment with a "sealed" reaction vessel (lets say gases are forced to pass through a "cold trap" first and then a solution), you have a means of monitoring product formation. The cold trap would be maintained at a temperature below the freezing point of water but above the freezing point of CO2. As gases pass through the cold trap, you will selectively freeze all of your product water (assuming the cold trap is of sufficient size). You can mass this water after the reaction to determine the number of hydrogens in your hydrocarbon.

Next, CO2, being acidic, will react with base. If the solution you make the product gases pass through is basic and of known basicity and volume, you can monitor pH change to determine the number of carbons present in your hydrocarbon. The volume of the solution will not change as you have removed the water gas being produced with the cold trap. Conversely, you could also simply mass the solution before and after, though this could be less accurate due to possibility of dissolving oxygen.

With these two pieces of information (mass H2O, mass CO2), you can determine the ratio of hydrogen and carbon in the compound you combusted.

Edit: http://preparatorychemistry.com/Bishop_Combustion_Analysis.htm good stuff here