So that it will fit into the test tube without any problems
1/50 mole of 1-Chlorobutane = 2.0 c8m3
1/50 mole of 1-Bromobutane = 2.1 cm3
1/50 mole of 1-Iodobutane= 2.28cm3
Hypothesis: The bond strengths of C-Cl, C-Br and C-I are different. Therefore, the energies required to break the bonds are different, resulting in different rates of hydrolysis. Therefore I predict that Iodobutane will have the fastest rate out of the three other liquids this is due to the fact that the bond enthalpy is the smallest out of the three and I also predict that bromobutane will have a faster rate than chlorobutane because it has a smaller bond enthalpy compared to chlorobutane.
Background Information: I will compare the rates of hydrolysis of
1-bromobutane, 1-chlorobutane, and 1-iodobutane.
CH3CH2CH2CH2Hal +H2O CH3CH2CH2CH2OH + H+ + Hal-
The rate of the reaction is followed by carrying it out in the presence of silver ions, Ag+(aq). Silver ions react with halide ions to form a precipitate of silver halide:
Ag+(aq) + Hal- (aq)
Halogenoalkanes are covalently bonded, so they give no precipitate with silver ions. But as the reaction proceeds and halide ions are produced, a white or yellow precipitate of silver halide gradually appears.
Halogenoalkanes are insoluble in water, so the reaction is carried out in the presence of ethanol, which acts as a mutual solvent for the halogenoalkanes, the water and the silver ions.
Also Moving down a group, the atomic number increases. I.e. the nucleus becomes more positive. The distance between the nucleus and valence electrons increases which tends to weaken the force of attraction. Overall, the distance effect is greater; therefore electro negativity decreases down a group.
Bond Bond Enthalpy / Kjmol-1
C-Br 276
C-C 328
C-I 240
Apparatus: Thermostatic water bath 60oc Thermometer
Three test tubes 3 x Stoppers
Test tubes rack Stop clock
Measuring cylinder 10cm3 1m 1-Chlorobutane
Ethanol 1m 1-Bromobutane
Pipette 1m 1-iodobutane
0.05m Silver Nitrate solution
Method: I’ll start of this experiment by accumulating all the apparatus essential to do this experiment.
1) Set the water bath to about 60oC
2) Mark three test tubes A, B and C, so I know which is which. I’ll then
add 2cm3 of ethanol to each of the test tube. The solution ethanol has
be used as a common solvent for the halogenoalkanes and silver
ions, since halogenoalkanes aren’t soluble in water.
3) Add 2.08 cm3 1-chlorobutane to test tube A, 2.14cm3 of
1-Bromobutane to test tube B and 2.28cm3 of 1-Iodobutane to test
tube C. Measure the volumes precisely and accurately using a pipette.
4) Now I’ll place the three test tubes in the water bath, and watch the
temperature rise to 60oC. I can’t let the solutions get any hotter hence
it is flammable.
5) Add 5cm3, 0.05m AgNo3 solution, to each of the test tubes, quickly
and put a stopper on them. Shake them gently and keep them in the
water bath. Start the stop clock as soon as the AgNo3 solution is
added.
6) Make sure you watch the test tubes continuously for 10 minutes and
record the time at which a cloudy precipitate first appears on each
one.
7) You will have to keep the test tubes in the water bath for a further
30mins, observing them every 5mins to see if their are any changes.
Diagram:
Observations: When a silver nitrate solution is poured into the iodobutane, precipitates are formed immediately. However when the silver Nitrate solution is added to Chlorobutane precipitates are formed within a few minutes.
This is due to the fact that the C-I (240) is the weakest between C-Cl (328) and C-Br (276). Iodobutane undergoes hydrolysis at the fastest rate resulting in the fastest rate of formation of silver iodide.
The C-Br bond is stronger than C-I, but is weaker than C-Cl. It undergoes hydrolysis more slowly than iodobutane, resulting in a slower rate of formation of silver bromide.
Lastly, the C-Cl bond is the strongest among the three C-X bonds. Chloride ion is the weakest leaving group and therefore chlrobutane does not undergo hydrolysis easily.
Safety Precaution: During the experiment I will wear gloves, for this reason being, all the organic halogen compounds have harmful vapours and can be toxic by skin absorption. I will also wear safety goggles because some organic compounds can be flammable. At the end of the experiment I will not dispose any waste down the sink due to the fact that it contains harmful substances. Halogenoalkanes are flammable. Their vapours are irritating and harmful, so I will have to avoid inhaling them. Since ethanol is highly flammable. I’ll keep bottles shut when not in use and well away from naked flames. I’ll avoid skin contact because I’ll be wearing gloves and I will not breathe the vapour. I will use the same test tubes with the same diameter. I will ensure that the solutions are not put in the boiling water to avoid evaporation of ethanol.
Conclusion: I have already predicted that 1-iodobutane will react the fastest for the precipitate to materialize and 1-chlorobutane will take the longest period for the precipitate to materialize. At the end of the experiment I will clarify what the results show and the relationship between the 3 halogenoalkanes are and I will show you if my prediction was right or wrong. I will also draw a graph and explain what the graph shows because looking at a graph will illustrate how close the difference was between 1-iodobutane and I-bromobutane. I can also say that I have enjoyed planning this experiment and I hope to do the experiment very shortly.
