Lecture 32 : Theories Of Reaction Rates : Collision Theory
32.3
The Harpoon Mechanism.
For reaction involving strongly electropositive and electronegative elements we find that can be greater than . This is due to the harpoon mechanism in which the electrostatic attraction enhances the reaction cross section. When the reactants K and Br2 approach each other in the reaction,
K + Br2 KBr + Br
(32.23)
at a certain close distance, an electron of K (the harpoon) is captured by Br2 to give Br2-. Once the ionic intermediates K+ and Br2- are formed, they are strongly attracted towards each other and move towards one another with greater velocity (enhanced reactivity) leading to a larger value of .
Let us understand the above description quantitatively by studying the potential energy diagram of K + Br2 and K+ + Br2- as a function of the separation between K and Br2. The diagram is shown in Fig 32.5.
Figure 32 . 5 The harpoon mechanism for the K + Br2 reaction.
can be greater than 
. This is due to the harpoon mechanism in which the electrostatic attraction enhances the reaction cross section. When the reactants K and Br2 approach each other in the reaction,
KBr + Br 
