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 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.
Chemicals:
0.60g (0.20g used 3 times) of 1-bromobutane. Mass was shown to be sensible by my preliminary experiment. 0.60g is required as the procedure will be performed 3 times, with 0.20g being needed each time.
0.42g (0.14g used 3 times) of 1-chlorobutane. See calculations for mass below. 0.42g is required as the procedure will be performed 3 times, with 0.14g being needed each time.
0.81g (0.27g used 3 times) of 1-iodobutane. See calculations for mass below. 0.81g is required as the procedure will be performed 3 times, with 0.27g being needed each time.
12cm3 (1cm3 added to 4 test tubes, 3 times) of 2.00 moldm-3 aqueous silver nitrate solution.
In allowing the hydrolysis reaction to be observed, the reactants (halogenoalkanes and water) of the hydrolysis reactions are colourless liquids, as are the products (alcohols and hydrogen halides). Therefore it is difficult to observe at what point the halogen has been displaced. The silver nitrate reacts with the hydrogen halide made from the first reaction, producing an insoluble precipitate of a silver halide. The silver halides can easily be seen in the test tubes and each has a distinct colour, allowing the different halide ions to be distinguished from one another. The silver nitrate is not involved in the hydrolysis reaction in any way.
The aqueous state of the silver nitrate solution provides excess water for the hydrolysis reactions meaning they will not be limited by a lack of water.
See calculations for concentration below. 12cm3 is required as the procedure will be performed 3 times and each time 1cm3 is added to 4 test tubes (3 containing the halogenoalkanes and 1 control).
24cm3 (2cm3 added to 4 test tubes, 3 times) of ethanol. Ethanol is a solvent in which both the halogenoalkanes and the aqueous silver nitrate are soluble. It is used to prevent 2 immiscible layers forming when the AgNO3 (aq) is added to the halogenoalkanes.
Calculations:
From my preliminary experiment I know that a reasonable mass of 1-bromobutane to be hydrolysed is 0.20g. Using this I can calculate the mass of 1-chlorobutane and 1-iodobutane needed. For the experiment to be a fair test, the amount (in moles) of halogenoalkane being reacted must remain constant. I will use the equation:
To work out the amount (in moles) that represents 0.20g of 1-bromobutane:
Mr of C4H9Br (4 x 12.0) + (9x1.0) + (1x79.9)
= 0.00146092 mol. of C4H9Br
If the amount of halogenoalkane is to be constant then I must use this number of moles of the other two halogenoalkane compounds. Therefore for each reagent I will use the same equation as above, but rearrange it to make mass the subject and use the number just calculated for the moles value:
For 1-chlorobutane (C4H9Cl):
Mass = moles x Mr
Mass = 0.00146092 x (4x12.0) + (9x1.0) + (35.5)
Mass = 0.14g
For 1-iodobutane (C4H9I):
Mass = moles x Mr
Mass = 0.00146092 x (4x12.0) + (9x1.0) + (127.0)
Mass = 0.27g
I also need to calculate the amount of AgNO3 that reacts with each hydrogen halide compound. Knowing the amount required by the reactions I can then ensure there is excess silver nitrate available, so as not to limit any of the reactions. First I need to work out the amount of each hydrogen halide that is formed in the hydrolysis reaction:
For 1-chlorobutane (C4H9Cl):
C4H9Cl (aq) + H2O (l) = C4H9OH (aq) + HCl (aq)
C4H9Cl : HCl
Ratio 1 : 1
0.00146092 mol. : 0.00146092 mol.
0.00146092 mol. of AgNO3 reacts
For 1-bromobutane (C4H9Br):
C4H9Br (aq) + H2O (l) = C4H9OH (aq) + HBr (aq)
C4H9Br : HBr
Ratio 1 : 1
0.00146092 mol. : 0.00146092 mol.
0.00146092 mol. of AgNO3 reacts
For 1-iodobutane (C4H9I):
C4H9I (aq) + H2O (l) = C4H9OH (aq) + HI (aq)
C4H9I : HI
Ratio 1 : 1
0.00146092 mol. : 0.00146092 mol.
0.00146092 mol. of AgNO3 reacts
As we can see, the same amount of silver nitrate reacts with all three hydrogen halides, so the concentration of silver nitrate solution will need to be the same for each reaction.
As I have chosen to use 1.00cm3 (0.001 dm3) of aqueous silver nitrate solution, I will now work out what concentration this solution would be needed in order to contain 0.00146092 moles:
= 1.46092 moldm-3 of aqueous silver nitrate solution.
However I want to ensure that there is excess silver nitrate available, so I will use a solution of concentration 2.00 moldm-3 to achieve this.
Method:
1) Draw a cross directly onto the side of each test tube using the same marker pen. Ensure that the cross is drawn in the same place each time by using a ruler to measure its position. The centre of each cross should be 2cm from the bottom of the test tube. The crosses are drawn low down on the test tubes to make sure they will be fully covered by the liquid. If they were drawn too high the top parts of the crosses would not be covered and would therefore always be visible. It is important that the cross is drawn in the same place each time as a test tube’s shape means its diameter varies with its height. If one cross was drawn higher than the others the distance between the two sides of the test tube would be greater, meaning more precipitate would be needed to obscure it from view than in the other tubes, which would be unfair.
2) Apply a small label to the top of each test tube stating its number; either 1, 2, 3 or 4. Label each stopwatch similarly and place the stopwatches next to their corresponding test tubes. The labels stop any confusion about which halogenoalkane is contained in which tube and which stopwatch is timing which tube. Without this it would be particularly easy to confuse the halogenoalkanes as they are all colourless liquids before their hydrolysis and the person carrying out the practical may not have knowledge of the different silver halide precipitate colours.
3) Taking 4 measuring cylinders, add aqueous silver nitrate solution to each using a measuring syringe so they all contain 1cm3. Place the cylinders to one side, they are required later. Using a measuring cylinder increases the accuracy of the measurements.
4) Using an electronic balance measure out the masses of the halogenoalkanes. For each compound place the beaker that will contain the sample onto the balance and press the zero button first. Using a dropping pipette, add the compound to the beaker until the balance reads the mass required (see below):
1-chlorobutane = 0.14g
1-bromobutane = 0.20g
1-iodobutane = 0.27g
Using an electronic balance increases the accuracy of the measurements.
5) Pour the halogenoalkanes into the test tubes as listed below:
Test Tube 1 = 1-chlorobutane.
Test Tube 2 = 1-bromobutane.
Test Tube 3 = 1-iodobutane.
Test Tube 4 (control) = add no halogenoalkane.
Put a bung into each test tube. Bunging the test tubes when access is not required increases the safety of the experiment. The halogenoalkanes are harmful by inhalation, so using bungs will minimise exposure.
6) Using one measuring cylinder put 2cm3 of ethanol into the 4 test tubes. Using a measuring cylinder increases the accuracy of the measurements.
7) Slowly heat the solutions, then start one of the stopwatches. When the stopwatch reads 5 minutes reset it and continue with the procedure. Leaving the test tubes for a few minutes allows their contents to equilibrate.
8) Remove the bungs. Access is required.
9) Working quickly and using 1 measuring cylinder per test tube, pour 1cm3 of silver nitrate into each tube. Start the stopwatch that corresponds to each test tube the moment the silver nitrate is added. Stopwatches must be started the moment the silver nitrate is added to obtain accurate recordings of the amount of time needed for the hydrolysis reaction to complete. Premature or late starting would mean the times shown at the end of the experiments would not accurately reflect the rate with which the halide ions were displaced.
10) Put the bungs back in and shake each test tube twice. Shaking ensures complete mixing. Each test tube must be shaken the same number of times so that shaking does not become a variable (it must not be shaking that has any effect on the rate of hydrolysis).
11) Arrange the test tubes so that the sides with the crosses drawn on are furthest away from you. Look through each tube. The moment a cross becomes invisible, stop the stopwatch corresponding to the test tube number. Continue with this until all the crosses have become invisible, or 20 minutes has elapsed since the last silver nitrate was added. The halogenoalkanes should react well before 20 minutes, so this extra time is given to demonstrate that the control will never react, even when left for much longer than the other test tubes. A time of 20 minutes is sensible as it allows the experiment to be completed in a practical amount of time, whilst still giving adequate time to the control.
12) Record any observations of the test tube contents. Detailed and well presented observations will allow proper analysis of the results.
13) Record the time shown on each stopwatch in the relevant column of a results table.
14) Repeat the above procedure a further 2 times. To increase the reliability of the results.
Bibliography
My Student Course Notes – “Chains and Rings” and “Atoms, Moles and Stoichiometry”.
Chemistry 1, Cambridge University Press, Brian Ratcliff - Pages 138, 139.
http://www.webelements.com/webelements/elements/text/C/enth.html