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Re: - C14 Decay rates



Larry Cartwright wrote:

*If* the relationship I quoted... is true,
the reaction rate (r) relates to mass (m)
approximately by R = r1/r2 = sqrt(m2/m1).

Yes, sometimes reactions are diffusion limited.
In particular, in dilute solutions you would expect
diffusion to be limiting.

... a bond to a protium atom can be broken as much
as 18 times faster than a bond to a deuterium atom.

That doesn't surprise me a bit.

It helps to think in terms of a "reaction coordinate".
That is, imagine a somewhat abstract coordinate such
that the initial reactants are at point A and the
final products are at point B. Graph the energy
as a function of this coordinate. Typically there
is an energy barrier somewhere between A and B. T
he system has to be thermally activated over the
barrier, or it has to tunnel through it.

Thermal activation depends exponentially on temperature
but only weakly (algebraically) on mass. Tunneling
is independent of temperature but depends exponentially
on sqrt(mass), roughly speaking.

Homework (easy): Read about the ammonia maser in the
big red books, volume 3. Then calculate the inversion
rate for fully deuterated ammonia, ND3.