Hydrolysis Kinetics
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Nucleophilic substitutions are a key organic reaction for A-level chemists. This simple experiment shows how studying the rate of aqueous hydrolysis of a haloalkane allows the order of a reaction to be obtained.
It also illustrates how any device with a 0-1V DC output can be connected to a Philip Harris Universal Interface or DL plus. In this case, a rather ancient WPA meter CMD-40 was dusted down and performed admirably.
Method:
2-chloro-2-methylpropane is hydrolysed in 50/50 ethanol water.
The keys to success in this experiment are complete mixing and discovering the correct range to use. The experiment depends on producing H+ (and Cl-) which will increase the original (small) conductivity.
If you are using a Sensor, calibrate in the normal way.
If you use a Conductivity Meter, proceed as follows:
Connect the 0-1V output to the Interface using a 'Blue Box Lead'. From the Measure menu select Calibrate
and click on the channel which is connected. Click Other. Enter the details:
| Name: Conductivity | Unit: As appropriate |
| Value at 0V: 0 | Value at 1V: As appropriate |
| Resolution: As appropriate | Error: ?? (Enter your own estimate or leave as 0) |
Now set up your experiment. In this case the two samples contained:
| !. 0.5cm3 2-chloro-2-methylpropane in 15cm3 of 50/50 ethanol/water. |
| 2. 1.0cm3 2-chloro-2-methylpropane in 15cm3 of 50/50 ethanol/water. |
Note that this solvent is chosen to ensure miscibility. Set up the Datadisc Recording Screen (use Autorecord or a 20 minute recording), mix the two components in a boiling tube, insert the the probe and start the recording. The second recording can be done immediately after the first (or appended to the first later).
Results
Over the period of the experiment, the conductivity increases in an almost linear fashion as the concentration of reactants scarcely changes. The gradient for the second run is almost double that of the first.
An alternative method for the gradient is shown below. Select Graph:Cursor and press down the left mouse button. Without releasing it, drag to another part of the graph and the separation of the points will be shown. Release the button to fix. In this case I have chosen two well separated points which shows the slight curvature of the graph. For a genuine gradient estimation, of course, it would be wiser in this case to make them closer.
Other haloalkanes
2-bromo-2-methylpropane reacts too quickly to get reliable results by this method, but by judicious use of other haloalkanes it should be possible to get good results. Let us know how you get on.