HO_CHEM2056_kinetics_2013_L4-6

Diatomicmoleculesdissociatesorapidly

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ]2 dt SECOND ORDER A*: energized molecule L Grondahl CHEM2056 The Lindemann Mechanism L Grondahl CHEM2056 Gas vs solution phase Change from 2nd to 1st order (low to high pressure): HIGH 1st order The actual pressure at which the change in order is observed is higher the smaller the reacting molecule. Diatomic molecules dissociate so rapidly that the probability of deactivation is negligible and 2nd order is observed to high pressures. A In solution the reacting molecules frequently collide with solvent molecules and for such reactions 1st order is always observed. A LOW 2nd order L Grondahl CHEM2056 L Grondahl CHEM2056 A A The Lindemann Mechanism kk kuni 1 2 k 1 The Lindemann Mechanism k1 A + M A* + M A+M k1 A* + M k-1 M + A* k-1 M + A* k2 A* d [ P] dt kuni M+A k2 A* P k1k 2 [ A][ M ] kuni[ A] k 1[ M ] k 2 kuni k1[ M ] k1k 2 [ M ] k1k 2 k 1[ M ] k 2 k k 2 1 [M ] A*: energized molecule L Grondahl CHEM2056 Summary of collision theory kuni M+A P k1k 2 [ M ] k1k 2 k 1[ M ] k 2 k k 2 1 [M ] 1 1 k 1 kuni k1k2 k1 [ M ] 1 A*: energized molecule k k1 4.16 106 M 1s 1; 1 1.76 105 M 1 k2 L Grondahl CHEM2056 Activated Complex Theory Transition State Theory For second order reactions theory gives an equation which correspond to the observed empirical equation 2BrNO 2NO + Br2 This can be used to estimate the pre‐exponential factor However, issue exists with regards to accounting for steric effects, the P factor cannot be predicted Unimolecular reactions may meet the energy requirement from collision despite being unimolecular L Grondahl CHEM2056 L Grondahl CHEM2056 Nobel Prize in Chemistry 1999 ‫أحمد حسن زويل‬ Awarded to Professor Ahmed H. Zewail, California Institute of Technology, USA “For showing that it is possible with rapid laser technique to see how atoms in a molecule move during a chemical reaction” “For his studies of the transition states of chemical reactions using femtosecond spectroscopy” The Experiment The technique uses laser flashes of 10‐15 seconds, femtoseconds, which is the same as the time scale on which the reactions actually happen One femtosecond is to a second as a second is to 32 million years. This area of physical chemistry has been named femtochemistry. http://www.nobel.se/chemistry/laureates/1999/press.html Ring opening of cyclobutane to yield ethylene Does the reaction from the cyclobutane molecule to two ethylene molecules proceed through a symmetric TS or through an asymmetric IM? In femtosecond spectroscopy the substrates are mixed as beams of molecules in a vacuum chamber. An ultra fast laser then injects two pulses: first a powerful pump pulse that excites the molecule to a higher energy state and thereby starts the reaction second a weaker probe pulse at a wavelength chosen to detect the reaction By varying the time interval between the two pulses the transformation of the original molecule can be monitored The molecule when exited have spectra that may serve as fingerprints The fingerprint and the tim...
View Full Document

This document was uploaded on 04/10/2014.

Ask a homework question - tutors are online