Homework 6A15(a)

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Katherine Fitzgerald 1A
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Homework 6A15(a)

Postby Katherine Fitzgerald 1A » Sun Jul 28, 2019 11:51 am

We are asked to identify the product for the Lewis acid-base reaction PF5 + F- ---> ?

I don't understand why these would join together. While P has the capacity for an expanded octet, doesn't it "prefer" to have only 5 bonds? when the extra F- attaches to form PF6, it has a charge of -1 overall. I can see that the free F- is in a "preferable" state when bonded (it's formal charge returns to zero)... is it more energetically favorable to have the -1 charge on the PF6- than F-? If so, why?

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Re: Homework 6A15(a)

Postby Chem_Mod » Sun Jul 28, 2019 7:08 pm

Reading this question, it appears they are just using these molecules and ions just as tools to help teach the concept of Lewis acid and bases. The book probably picked these as they are fairly complex lewis structures that also test your knowledge of expanded octet and formal charge. Even though it may possibly be unfavorable, the question is not asking you to determine if it is or not, and also even if it is unfavorable, it could still occur in nature. Take for instance a weak acid with a very small Ka value, so little will dissociate in water because it is not as favorable compared to a strong acid as HCl, but nevertheless some of the acid will still dissociate.

Of course, when drawing lewis structures draw them with the most stability that you can, but in this problem it is just telling you to draw these certain lewis structures, whether or not they are favorable.

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