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We observe that nitrate ion has a trigonal planar geometry according to its lewis structure and the VSEPR theory. Its resonance hybrid consists of three contributing structure -three probable double bond organisations. According to my knowledge, we would expect that pi bond to be distributed equally over the molecule and thus, the dipole moments would cancel each other out. My question is, why such an ion would dissolve in water even though it is non-polar?

M.A.R.
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Pınar
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  • You realize that nitrate is an ion, right?! – M.A.R. Mar 25 '15 at 16:07
  • So it dissolves in water just because it is an ion? – Pınar Mar 25 '15 at 16:08
  • A common assumption is "all ions must be polar". Is it true? – Pınar Mar 25 '15 at 16:11
  • I hate courses that provide a short view - no offence. What is polarity compared to a negative charge on species? – M.A.R. Mar 25 '15 at 16:12
  • It is a result of electronegativity difference. – Pınar Mar 25 '15 at 16:14
  • See the comments in this relevant question. When we mention polarity, we are usually referring to electric dipoles. In that sense, some ions may be electric monopoles without electric dipoles and hence could be called non-polar ions (though they could still be expected to have strong ion-dipole interactions with water and hence be soluble). Benzene, on the other hand, has no electric monopole or dipole, but has a significant electric quadrupole, even if it is also called non-polar. – Nicolau Saker Neto Mar 25 '15 at 16:16
  • Just to clearify, nitrate ion is a non-polar specie, right? – Pınar Mar 25 '15 at 16:20
  • I can't recall ever reading anyone refer to ions as polar or non-polar, so I don't know for sure. The term may not be usually applied to ions. – Nicolau Saker Neto Mar 25 '15 at 16:22
  • @Pınar If, as I think, the problem here is reasoning its solubility, notice that polarity results in electron concentrations' difference, while an ion totally has an extra charge. I never had to worry about the fact that an ion is polar or not. – M.A.R. Mar 25 '15 at 16:24

1 Answers1

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Don't overthink it.

The oxygens have a partial negative charge in nitrate from resonance. The protons in water have a partial positive charge from dipoles. Positives attract negatives.

Molecule polarity becomes an issue when discussing neutral molecules interacting with neutral molecules. Molecular polarity is one thing, and bond polarity is another. Transformations / arrangements / alignments of molecules such that the dipoles (the things that make a molecule or bond 'polar') align to maximize interaction are certainly important for dissolution, but when a species is a bona fide free ion, water will tend to dissolve it.

Lighthart
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