Experiment 12

1. Experiment 12 Kinetics – Hydrolysis of 2-chloro-2-methylpropane

2. Objectives • Determine the rate coefficient for the hydrolysis of 2-chloro-2-methylpropane • To use the Arrhenius equation • Determine the activation energy for the hydrolysis of 2-chloro-2-methylpropane

3. Introduction • Hydrolysis of 2-chloro-2-methylpropane is an example of a substitution reaction • Hydroxide attacks the carbocation after chlorine dissociates from the molecule

4. Introduction • The rate of a reaction is the change in reagent/product conc. over time • By measuring the concentration of reagents/products at various points during the reaction the rate can be obtained

5. Introduction • Hydrolysis of 2C2MP is dependant on the [reagent] only • Rate is directly proportional to [reactant] • k is the rate coefficient

6. Introduction • The rate law indicates that the reaction depends on [reactant] • The reaction is first order • In this case k has units of s-1

7. Introduction • Rate laws can provide much information regarding the mechanism of reactions • The rate-[reactant] relationship must be determined experimentally • Manipulation of the rate law gives

8. Experimental • Perform the hydrolysis reaction at three different temperatures and note the time taken for each reaction • We can thus calculate the rate coefficient and the activation energy for the reaction

9. Experimental • Pipette 3 ml of 0.1 M 2C2MP solution to a test tube • Pipette 0.3 ml of 0.1 M NaOH solution into another test tube and add 6.7 ml of water • Add 2 drops of bromophenol blue indicator • Clamp in a water bath/ice bath and record the temperature and exact time (t0)

10. Experimental • Add the 2C2MP soln to the NaOH soln and swirl to ensure complete mixing • When the indicator turns from blue to yellow record the reaction time • Repeat twice – values should be within 10% of each other – and get the average

11. Experimental • Perform the hydrolysis reaction at room temperature, in an ice/water bath and in a water bath at ~35°C • Knowing the rate coefficient at three temperatures allows us to use the Arhennius equation to calculate the activation energy of the reaction

12. Results & Calculations • Calculate the rate coefficient k from • Calculate k at each of the three temperatures – can then use Arhennius eqn to plot a graph giving EA/R as slope

13. Arhennius equation A graph of ln k against 1/T gives a line with slope –EA/R and intercept of ln A

14. Drawing the graph • Graph must fill the page • Label the axes, include units and values on the axes • Plot the data as a point surrounded by a small circle that represents the possible error • Draw a straight line which best fits the data points • Use points from either end of the line to calculate the slope – minimizing the error

15. Report • Introduction • Observations • Results & Calculations • Graph • Discussion & Conclusions

16. Physical Chemistry Section • NO PRE-PRACTICAL TALKS FOR THE NEXT 4 PRACTICALS • This week and next week you will be in your usual labs • The 3 weeks thereafter you will rotate between the experiments from week to week • Hand your reports your usual demonstrator as before