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Author Topic: Problem of the week - 12/11/2012  (Read 22626 times)

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Rutherford

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Re: Problem of the week - 12/11/2012
« Reply #15 on: November 26, 2012, 07:48:42 AM »

I got an idea here. It is obvious that A/B and D/F are isomers. As the compounds are trigonal bipyramidal it was hard to find the appropriate names of these isomers, they can't be cis/trans nor mer/fac. Then I found that stereoisomers of trigonal bipyramidal molecules can be axial and equatorial, so I believe that they much differ in stability and therefore in their chemical properties. The only problem is that I don't know which form is more stable and which is covalent and ionic.
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Dan

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Re: Problem of the week - 12/11/2012
« Reply #16 on: November 26, 2012, 08:15:00 AM »

The trigonal bypyramidal compounds will always have 2 axial and 3 equatorial substituents, so you can't get isomers for XY5.

The trigonal bipyramidal compound could be represented as:


Think about the polyhedra in the hint I posted.
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Rutherford

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Re: Problem of the week - 12/11/2012
« Reply #17 on: November 27, 2012, 03:08:10 AM »

I didn't mean for XY5. I meant it for compounds A/B and D/F. There is more that one possible space arrangement for XY4Z.
I can't see how the tetrahedron and the octahedron are related to the geometry of these molecules, so I will leave this.
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Dan

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Re: Problem of the week - 12/11/2012
« Reply #18 on: November 27, 2012, 03:49:35 AM »

I can't see how the tetrahedron and the octahedron are related to the geometry of these molecules, so I will leave this.

Hint: empirical vs molecular.
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Rutherford

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Re: Problem of the week - 12/11/2012
« Reply #19 on: December 08, 2012, 07:01:38 AM »

As no one tried this and as I am really curios about the answer, I will try it again.
You said empirical vs molecular, and on the picture there were a tetrahedron and an octahedron. I came to the idea that the molecules combine, so that B and F make molecular crystal cells which is the reason why they are solids.
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Dan

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Re: Problem of the week - 12/11/2012
« Reply #20 on: December 10, 2012, 12:12:42 AM »

Quote
I came to the idea that the molecules combine

You're so close now. How could XY5 (represented as a trigonal bipyramid) combine to form a tetrahedron and an octahedron?

Hint: How many vertices do these polyhedra have?
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Rutherford

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Re: Problem of the week - 12/11/2012
« Reply #21 on: December 10, 2012, 01:53:05 AM »

Trigonal bipyramid has 5, tetrahedron 4 and octahedron 6. From two trigonal bipyramids, one tetrahedron and one octahedron can be formed. If  XY5 would have to make a tetrahedron, then it will lose one Y so it will become a cation (XY4+).  If XY5 would have to make a octahedron it has to accept a Y so it will become an anion (XY6-). These two can combine to make an ionic compound XY4*XY6 or X2Y10.
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Dan

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Re: Problem of the week - 12/11/2012
« Reply #22 on: December 10, 2012, 02:35:23 AM »

Correct, phosphorous(V) halides can undergo autoionisation.

Se for example: http://en.wikipedia.org/wiki/Phosphorus_pentachloride#Structure

So, can you propose structures for B and F?
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Rutherford

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Re: Problem of the week - 12/11/2012
« Reply #23 on: December 10, 2012, 02:44:42 AM »

Do they overlap?
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Dan

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Re: Problem of the week - 12/11/2012
« Reply #24 on: December 10, 2012, 06:21:30 AM »

Do they overlap?

I don't understand your question.
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Rutherford

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Re: Problem of the week - 12/11/2012
« Reply #25 on: December 10, 2012, 06:33:15 AM »

Because of the positive charge on one P atom and the negative on the other one, a ionic bond will be formed. As the ionic bond is shorter than the covalent bond, the tetrahedron and the octahedron should overlap, but then the electron pairs would repel each other much, so I am not sure about the structure.
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kaliaden

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Re: Problem of the week - 12/11/2012
« Reply #26 on: January 04, 2013, 09:48:21 PM »

It's easy to find the empirical formula of each compound but how exactly did you calculate the molecular masses of X, Y and Z?

So far, i get that C is PCl5 and E is PF5

A and D are covalent and could auto-ionise to form B and F (which are ionic) respectively
I think it would be easier for a chlorine atom to move between two molecules of A and F since the phosphorus-fluorine bond would be stronger (PF5 doesn't auto-ionise but PCl5 does)
So B would be [PClF3]+[PCl3F3]- and F would be [PCl3F]+[PCl5F]- right?
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