Isotope Exchange Reaction

Isotope Exchange Reaction

When an homogeneous isotope exchange reaction
AX + BX* → AX* + BX
is considered, the rate of the reaction can be given by the equation-
[(a+b)/ab ]RT = −ln(1 − F)
where, a = [AX] + [AX*]
b = [BX*] + [BX]
R = Rate constant
F = Fraction exchanged in time 't'
When ln(1 − F) is plotted against time 't', a straight line passing through the origin is generally obtained. If there is a positive intercept in the ordinate, it indicates the exchange is either catalysed or follows two mechanisms, one of which is very fast.
Example- Exchange between PCl₅ and Cl₂ dissolved in CCl₄. The three planar (equatorial) chlorine atoms exchanged instaneously whereas the chlorine atoms at the vertices exchange slowly.
Similarly, if there is a negative intercept in the ordinate, it indicates that there is an induction period before the start of the exchange reaction.
Example- radiation induced exchange between Br⁻ and BrO₃⁻. This reaction having a period of induction varying from 6 to 36 minutes.

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