AS Assessed Practical Plan – Rates of hydrolysis of halogenoalkanes investigation

A-Level: Chemistry

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I have been asked to design an experiment to examine how the rate of hydrolysis of halogenoalkanes varies with respect to the C-X bond. I will be varying the C-X bond using the halogens chlorine, bromine and iodine (C-Cl, C-Br or C-I) to see which of these bonds gives the fastest rate of hydrolysis. Other factors that affect the rate of reaction are temperature, concentration of reagents, pressure, surface area and the use of a catalyst, however I will make sure that these remain constant, in order to ensure that only the chosen variable (the halogenoalkanes) is causing any difference to the rate of hydrolysis.

Predictions:

Firstly I predict that during warming reactions will occur in all test tubes except the control. I predict that the liquid in these test tubes will change from transparent to cloudy causing the ‘cross’ on each test tube to no longer be visible. I predict that this will be due to an insoluble precipitate being formed in each test tube. Furthermore I predict that this precipitate will be either silver chloride, silver bromide or silver iodide depending on the halogenoalkane used and that as these precipitates will be formed at different rates, meaning the ‘crosses’ will disappear at different times.

I predict that as the halogen is changed down the group, i.e. from chlorine to bromine to iodine, the rate of hydrolysis will increase. This is because the bond enthalpy of a carbon-iodine bond is lower than that of a carbon-bromine bond, which is subsequently lower than that of a carbon-chlorine bond. This is shown by the following extract from the data website - http://www.webelements.com/webelements/elements/text/C/enth.html:
C-X Bond Bond Enthalpy (kJ mol-1)
C-Cl 397 ±29
C-Br 280 ±21
C-I 209 ±21

The enthalpies of the C-X bonds decrease from C-Cl to C-I due to the difference in size of the halogen atoms. As 7 group is descended, the size of the halogen atom increases, i.e. iodine is a much larger atom than chlorine. The larger the halogen atom is the less the bond overlap with the carbon atom, resulting in a longer C-X bond. The longer the bond, the weaker the bond and this means less energy is required to break the long chlorine-iodine bond than the short carbon-bromine or even shorter carbon-chlorine bonds. Therefore I predict that iodine will be displaced in the least amount of time, followed by bromine and then chlorine.

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