Comparing the rate of the hydrolysis of different hal- groups of halogenoalkanes Watch

David.Williamz
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Comparing the rate of the hydrolysis of different hal- groups of halogenoalkanes

Outline of the method:

-1cm3 ethanol, 2 drops of a halogenoakane (1-Chlorobutane, 1-bromobutane and 1-Iodobutane)

-1cm3 0.1 moldm3 silver nitrate solution in the above solution

-record time it takes to form a precipitate



The problem is that the reaction for 1-bromobutane occurs instantly with the above given concentrations/volumes/measurements.

The 1-chlorobutane does not even show a white ppt. as it is meant to (apparantly this would take hours and hours) so thats a major contrast compared to the 1-bromobutane.

The 1-iodobutane forms a ppt. in about 5-6 seconds which is reasonable to measure.

My solution was:

Try the reactions out with less silver nitrate and the same conc. of the halogenoalkanes; I tried 0.75cm3, 0.5cm3, 0.25cm3. This made the 1-iodobutane react slower however the 1-bromobutane still reacted instantly. The 1-chlorobutane still of course did not react in any reasonable time.

I tried out using more (just to make sure that was a main factor) and less drops of the halogenoalkanes with more and less silver nitrate solution, and the experiment was still too difficult to be able to record a time that a ppt. forms reasonably.

This experiment is supposed to be on of the main ones in my investigation of the rate of hydrolysis of halogenoalkanes however its very unreasonable so far from my experience.

I wondered if anyone could suggest another way to measure the rate of hydrolysis of halogenoalkanes in terms of changing the halogen in the halogenoalkane, or to suggest further things I can do to try and resolve my current experiment?

thanks
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Kyri
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First of all, your concentrations of haloalkanes in each reaction aren't constant the way you've done the experiment so far. Each haloalkane has a different molecular weight and a different density so they will have a different number of moles in one drop. You should be using the same number of moles of each haloalkane so you have the same concentrations in mol/dm3 otherwise the rates of each reaction are not comparable.

Also, haloalkanes tend to have fairly high densities (much more dense than water). If you're using one of those plastic pipettes most schools have then one drop in 1 cm3 of ethanol would actually be fairly concentrated so I'd suggest using more than 1 cm3 of ethanol. Perhaps try 5 cm3 or if that's too fast, try 10 cm3. I also would have suggested using less silver nitrate but you say you've already done that. Another simple way to reduce the rate of reaction is to reduce the temperature so you could try doing these reactions in an ice/water bath.
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shengoc
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(Original post by Kyri)
First of all, your concentrations of haloalkanes in each reaction aren't constant the way you've done the experiment so far. Each haloalkane has a different molecular weight and a different density so they will have a different number of moles in one drop. You should be using the same number of moles of each haloalkane so you have the same concentrations in mol/dm3 otherwise the rates of each reaction are not comparable.

Also, haloalkanes tend to have fairly high densities (much more dense than water). If you're using one of those plastic pipettes most schools have then one drop in 1 cm3 of ethanol would actually be fairly concentrated so I'd suggest using more than 1 cm3 of ethanol. Perhaps try 5 cm3 or if that's too fast, try 10 cm3. I also would have suggested using less silver nitrate but you say you've already done that. Another simple way to reduce the rate of reaction is to reduce the temperature so you could try doing these reactions in an ice/water bath.
kyri, no disrespect or anything, but wouldn't it be difficult to maintain a reasonable comparison if the temperature is not maintained constant? perhaps an electronically set ice bath, if that exists(have only used warm one, not cold ones), and surely working temperatures would have affected the rate.

not that i have better ideas, though, lol:confused:
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Kyri
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(Original post by shengoc)
kyri, no disrespect or anything, but wouldn't it be difficult to maintain a reasonable comparison if the temperature is not maintained constant? perhaps an electronically set ice bath, if that exists(have only used warm one, not cold ones), and surely working temperatures would have affected the rate.

not that i have better ideas, though, lol:confused:
That is a very valid point and would certainly be an issue if doing this reaction on a larger scale using grams of material. A regulated heat bath would of course give more accurate results so that's an added advantage of an ice/water bath which would maintain the temperature of the water at 0 degrees. I don't think much heat would be produced anyway when using one drop of each compound, especially if diluted further with more than 1 cm3 of ethanol. I think it's more important to get the concentrations constant, and besides in these sorts of courseworks the students always need some sort of improvement to the experiment to talk about
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David.Williamz
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Ah in one of the activity sheets from last year this was actually an experiment and it says to use 0.01mol silver nitrate which is a 1/10 of the one I have been using, hopefully this thing will work soon !
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