To sum up, the measurements going to be taken in this experiment will be the time taken for precipitate to form which blocks out the colour of the black background paper. Note that the unit of the measurements will be in seconds.
Control variable
There are several factors that affect the rate of precipitate forming and they are:
- Concentration of silver nitrate used
- Concentration of halogenoalkanes used
- Volume of ethanol used
All these factors have to be kept constant in order to prove a fair test and five accurate and reliable results. They will be explained further in the fair test section.
Fair test
In order to produce reliable and accurate results for data analysis, the following fair test precautions should be taken into consideration:
- The concentration of halogenoalkanes should stay constant. In order words, the moles of halogenoalkanes used for this experiment have to be the same such that within a fixed volume of solution, the same number of moles of halogenoalkanes is found. This is to ensure that the chance of the halogenoalkanes molecules getting collide with the water molecules is the same for each set of experiment.
- The concentration and volume of silver nitrate solution used in the experiment should keep constant. This is to ensure within the fixed volume of solution mixture, the number of silver ions is the same.
- Volume of ethanol used is also important to prove the test is fair. As halogenoalkanes and silver nitrate cannot be dissolved in water, forming layers of solution, ethanol is used as a common solvent such that both solute can be dissolved for reaction to happen. However, if the amount of ethanol used in the experiment is not the same, for example, less ethanol is used for a particular test tube some of the halogenoalkanes may not be dissolved.
Apparatus
The apparatus used in this experiment will be as follows:
Test tubes x 5 Test tube rack x 1
Graduate pipette x 5 Large beaker x 1
Thermometer x 1 Stopwatch x 1
N.B graduate pipette is used as it has measuring interval such that the volume of the solution can be accurately measured. The graduated pipette is having a volume of 2cm3 and with 0.2cm3 interval such that to give an accurate measurement of the volume of solution used.
Safety precautions
As the halogenoalkanes is a highly volatile substances which is flammable and is toxic when in contact with skin, the following safety precautions have to be taken to prevent serious damage:
- To ensure there is not Bunsen burner placing near the halogenoalkanes as they are highly flammable, if they are spilled onto the naked flame, it may cause fire. It is also needed to be aware that when discarding the halogenoalkanes solution, it should be done in the fume cupboard to carry away any harmful and toxic gas.
- Gloves should be worn if the mixture is going to be touched as it is toxic and will cause irritation when touching it.
Solution used in this experiment
Three halogenoalkanes will be used in the experiment accompanied by the other solution and they are as follows:
- 1-chlorobutane
- 1-bromobutane
- 1-iodobutane
- Ethanol
- Aqueous silver nitrate
Experimental diagram
Amount of solution used in this experiment
As this experiment is carried out in a laboratory bases, the amount of solution used will be extremely small. In order to keep a fair test, as mentioned as above, the mole of halogenoalkanes used should be the same. 0.01 mole of halogenoalkanes used in this experiment is considered as suitable. The actual amount of halogenoalkanes used in this experimnent can be calculated using their density and mass of 1 mole. The detail calculation are as shown below:
(calculation which has done in the book, just copy it is fine)
＊The amount of ethanol used will be 2cm3, this is to ensure all the halogenoalkanes has been dissolved thoroughly for the hydrolysis reaction.
＊The amount of aqueous silver nitrate solution used in this experiment will be 2cm3, this is to allow sufficient silver ions found in the solution for the formation of silver halide precipitate. The concentration of silver nitrate used will be 0.1 mol/dm3.
Control experiment
In order to prove the test is fair, it is decided to do a control set up. This is test tubes containing only silver nitrate solution and ethanol. This is to prove that without halogenoalkanes solution, the silver nitrate will not change colour. However, if it does change colour, this refers that there are errors occurring in the experiment which the results obtained may not be accurate enough for analysis.
Method
The method of the experiment is outlined as below:
- Gather all the equipments for experiment according to the apparatus list and the solution list.
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In each test, place in 2cm3 of ethanol as a common solvent. After that, add in the correct amount of halogenoalkanes in each test tube, which have been calculated in the above section) using graduated pipettes. A control has been set up with only 2cm3 of ethanol added.
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In a separate test tube, place in 10cm3 of silver nitrate solution.
- In the beaker, add in suitable volume of water which able to cover the mixture in the test tube. Place in all the test tube, wait for 10 minutes, this is to allow the temperature of the solution to be equilibrium (i.e the mixture in the 4 test tubes are having the same temperature). Measure the temperature in the water bath using a thermometer. Place a black colour paper at the back of the test tubes.
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Add in 2cm3 of silver nitrate, shake using inversion method to ensure the silver ions are even disturbed over the solution. At the same, start to the stopwatch.
- Stop the stopwatch once the solution changes from colourless to cloudiness and the black colour of the paper cannot be seen. This indicates the end point of the reaction. Record the time needed for the complete hydrolysis process.
- The experiment may have to be repeated for 2 more times to eliminate any potential errors and to obtain a average result for accurate analysis.
Analysis of results
After obtaining sufficient results from the experiment, an average time taken for the hydrolysis process for each halogenoalkanes will be calculated. A table will be drawn to show the results for comparison. The shorter the time needed for the precipitate formation, the faster the rate of hydrolysis that particular halogenoalkane has. The table can also be used to show the rate of hydrolysis for each type of halogenoalken by using the equation of rate = 1 / time.
