Effusion Rate Question

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Gryhu

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Shouldn't have called this effusion rate, it deals with reactivity.

TBR Gen. Chem. Section VI Passage VII #45

Ok so we're comparing the reactivity of F2 gas with U-235 and U-238. I know the difference between them is 3 neutrons.

Q: What can be said about the reactivity of fluorine gas with U-235 and U-238?

I know that U-235 is lighter than U-238 so its going to be faster. From my understanding, U-235 will have a higher collision frequency with F2 and thus be more reactive than U-238.

I chose B) Fluorine gas reacts more readily with U-235 because its ligher.

The correct answer is C) F2 gas reacts with both equally because the extra neutrons do not significantly affect the chemical reactivity.

I know reactivity deals with electrons so I understand it from that point, but shouldn't it still react more with the U-235 since its faster?

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uranium in its natural state is solid, which may have to be taken into account. in the gaseous state indeed u-235 would be marginally faster.

'reactivity' isn't really a rigorous term though, so i wouldn't be terribly concerned here.
 
Here, the key point the question is bringing up is not which isotope would migrate faster in order to react.

The way it is stated assumes that the specie of fluorine gas is introduced into the same container as the sample of uranium. The key it is testing is that chemical reactivity depends on the valence electrons, which is the same for both species of uranium isotopes as only neutrons differ. Therefore, they have the same reactivity with fluorine.
 
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