![[Halogenation of Propanone - Mechanism A]](../images/four1a.gif)
of Consumption
of Iodine
Approximation
Production of Pe
Steady State
Approximation
of Consumption
of Iodine
Assumption
(since the reversible reaction is slow and the iodination reaction is fast)
Rate Expression
HALOGENATION OF PROPANONE
Review
| Overall Reaction | Pk + I2 →I + H+ + I- |
| Proposed Mechanism | |
| Overall Rate of Consumption of Iodine | -d[I2]/dt = + k2[Pe][I2] |
| Steady State Approximation | d[Pe]/dt = 0 |
| Overall Rate of Production of Pe | d[Pe]/dt = + k1[H+][Pk] - k-1[Pe][H+] - k2[Pe][I2] |
| From Applying the Steady State Approximation | [Pe]ss = + k1[H+][Pk] / (k-1[H+] + k2[I2]) |
| Overall Rate of Consumption of Iodine | -d[I2]/dt = + k1k2[H+][I2][Pk] / ( k-1[H+] + k2[I2]) |
| Simplifying Assumption | k-1[H+] << k2[I2] (since the reversible reaction is slow and the iodination reaction is fast) |
| Final (Simplified) Rate Expression | -d[I2]/dt = + k1[H+][Pk] |
Conclusion
The final result of the derivation of an appropriate rate expression for the proposed mechanism (A) was:
-d[I2]/dt = + k1[H+][Pk]
The rate expression obtained from the experimental data (for the Year 2 Halogenation of Propanone experiment) could now be compared with the derived rate expression. Based on this comparison, a determination could be made as to whether or not the proposed mechanism was consistent with the experimental results.
For the write-up of the Halogenation of Propanone experiment, the final derived rate expression for the proposed mechanism (A) must be cited.
In addition, the Halogenation of Propanone write-up must include the derivation of an appropriate rate expression for the second proposed mechanism (B). Details about Mechanism B can be found in the Halogenation of Propanone lab script. Remember to include the intermediate steps of the derivation.
References
Note
If there are any problems, either with this Tutorial or with the write-up for the Year 2 "Halogenation of Propanone" practical, please contact Col Flowers.