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How can one determine the oxidation number of nitrogen in $\ce{KCN}$?

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    $\begingroup$ Could you elaborate on what you already know? $\endgroup$ – ManishEarth Feb 6 '13 at 2:13
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KCN is a ionic compound consisting of $\ce{K+}$ and $\ce{CN-}$. You can know that the cyanide ion will have a negative charge, because a potassium ion always has a positive. Also the elements in the cyanide ion are more electronegative - that means they will have the negative oxidation number.

So we know that since potassium has an oxidation state of +1, the cyanide ion has an overall oxidation state of -1. As stated above, ions have oxidation states corresponding to their charge. But you want to know what the oxidation state of the NITROGEN atom has.

There are rules you can use for polyatomic ions (ions with multiple atoms in it), such as $\ce{CN-}$. Nitrogen is more electronegative than carbon. Thus it will have the negative oxidation state (think: it's better at grabbing electrons, which are negatively charged). It is 3 columns away from being a noble gas, thus its oxidation state it -3. The overall oxidation state of the ion is -1, thus carbon must have an oxidation state of +2.

To sum up: K = +1, C = +2, N = -3. If you're trying to balance an equation though, you don't need to look at C and N individually. You just look at the $\ce{CN-}$ as a whole and the $\ce{K+}$.

How did I know $\ce{CN-}$ was an ion? Since potassium is a metal, and only forms ionic bonds. C and N do not form ionic bonds, they bond covalently, thus they act as a single ion in ionic compounds. Does that make sense? Other than that it's just something you learn to recognize with time!

Read rules for determining oxidation states here: http://www.tmcleod.org/cgi-bin/chem1/wiki.cgi?action=browse&diff=3&id=OxidationNumber

There's a comment to this answer explaining what causes electronegativity. A general rule of thumb is that electronegativity increases as you go towards higher group number and low period number. You can study it for yourself in the picture below:

Electronegativity in periodic table
(source: wikispaces.com)

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    $\begingroup$ Some of this is a little too general and hand waving. Potassium doesnt always have a charge. Potassium metal for example. Also "more to the right of the periodic table" is not a sufficient description of electronegativity. The cause of electronegativity is the relatively similar shielding of the nucleus across a group whilst the number of protons (and hence charge on the nucleus) increases across the group. This increased positive charge with similar shielding causes N to be more attractive to electrons than C. $\endgroup$ – Horba Feb 6 '13 at 11:17
  • $\begingroup$ This doesn't look like Pauling scale in your picture, which one is it then? $\endgroup$ – Mithoron Feb 26 '15 at 17:05
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The oxidation state of $\ce{N}$ is effectively zero, while the oxidation state of $\ce{C}$ is -1. You may rationalize this by looking at the molecular orbitals (MOs) of the cyanide ion. Carbon and nitrogen are sp-hybridized and triply bonded. As the MOs are filled, the bond MOs are filled first followed by the lone pair MOs, one for N and one for $\ce{C}$. Both lone pairs are much lower in energy than the $\sigma^\ast$, $\pi^\ast$, or $lp^\ast$ MOs.

Alternatively, you may follow the 8-electron rule and use the fact, that carbon and nitrogen are covalently bonded. This also gives a -1 oxidation state for carbon.

This behavior is also consistent with various reactions for cyanide, which acts as a nucleophile binding with the carbon instead of nitrogen.

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  • $\begingroup$ You're talking about oxidation mumber or formal charge? $\endgroup$ – Mithoron Feb 26 '15 at 17:01

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