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Author: Subject: Cyanide periodic trends
sceptic
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[*] posted on 13-2-2023 at 21:18
Cyanide periodic trends


Looking over the melting points for the alkali metal cyanides, I noticed that further down the alkali metal column they were, the higher the melting point seemed to be. For example, lithium cyanide melts at 160 degrees Celsius, while sodium cyanide melts at 563.7 degrees, and potassium cyanide at 634.5 degrees.

I would have expected the trend to go in the opposite direction, since there should be stronger attraction between ions of the same charges when the ionic radius is smaller. What would cause the cyanides to reverse this trend?
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Yttrium2
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[*] posted on 14-2-2023 at 08:14


Quote: Originally posted by sceptic  
Looking over the melting points for the alkali metal cyanides, I noticed that further down the alkali metal column they were, the higher the melting point seemed to be. For example, lithium cyanide melts at 160 degrees Celsius, while sodium cyanide melts at 563.7 degrees, and potassium cyanide at 634.5 degrees.

I would have expected the trend to go in the opposite direction, since there should be stronger attraction between ions of the same charges when the ionic radius is smaller. What would cause the cyanides to reverse this trend?


melting point / boiling point go up as we move down and right on the periodic table? why? what trend?

care to elaborate on that second paragraph. is it ionic radius or atomic radius that you are thinking of, -- or both!?

Sounds like a General Chemistry question I would have been able to answer years ago. Perhaps I still could If I had my notes or references/book. -- I really shouldn't have left them atop that mountain.

-- Too young to say the above!!




[Edited on 2/14/2023 by Yttrium2]
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B(a)P
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[*] posted on 14-2-2023 at 12:30


It is an interesting observation, though it does not continue past potassium.
I noticed the same trend for azides some time ago and did a little research, but couldn't figure it out. I would be keen to hear an intelligent answer on this.
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DraconicAcid
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[*] posted on 14-2-2023 at 12:47


You can expect melting points to decrease as the size of the ions increases, just from a look at Coulomb's Law- larger ions gives a larger distance between them, and therefore a weaker overall attraction.

However, this assumes that these ionic compounds have the same crystal structure. I'm pretty sure that NaCl and CsI, for example, have different structures- the ions in NaCl are six-coordinate, whereas those in CsI are eight-coordinate. That's going to completely change the lattice energy calculation.

Also, for a stable crystal, you want the ions to match in size. If you have a tiny cation rattling around in a space that would better fit a larger one, that's going to muck up the lattice energy calculations as well.

I'm not sure if that qualifies as what B(a)P hopes for as an intelligent contribution, but I'll happily be corrected by someone more knowledgeable than I.




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[*] posted on 14-2-2023 at 13:12


DraconicAcid your responce is well considered and you are clearly knowledgable on the topic, so for me it absolutely qualifies as an intelligent contribution, thanks very much for the responce:).
I had a similar line of thinking, but got stuck when looking further into it I discovered that sodium cyanide has the same crystal structure as sodium chloride, as does potassium cyanide, so I would have expected the sodium/potassium chloride trend to match the sodium/potassium cyanide trend. Where I ended up, and this is just a guess, is that for azides the bond between the alkali metal and the azide ion was much less ionic then the equivalent alkali metal/halide bond and so therefore something else is at play.
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