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We were taught that nitric acid can oxidize $\ce{Cl-}$ to $\ce{Cl2}$, but sulfuric acid cannot. Is this due to its smaller size, or structure, or what?

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The nitrate ion is a strong oxidant whereas the sulfate ion is a milder oxidant that only shows oxidizing properties in high concentrations. The oxidizing atom in question is the oxygen atom in the acid, not the hydrogen ion. This is because hydrogen ion is not very electrophilic, which is why non-oxidizing acids cannot dissolve copper: even copper has a higher affinity to electrons than hydrogen.

In a nitrate ion, the molecular structure is inherently not very stable, as the nitrogen atom has to bear a positive charge to maintain the molecular structure. Besides, nitrogen is in itself a very electronegative element, so that makes the nitrate ion even more unstable, hence its oxygen atom readily reacts with other atoms or molecules that oxygen can oxidize, including chloride ions:
$\ce{Cl- + O -> OCl-}$
$\ce{H+ + OCl- <=> HOCl}$
$\ce{H+ + Cl- <=> HCl}$
$\ce{HCl + HOCl <=> H2O + Cl2 }$

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  • $\begingroup$ Oxygen in these acids is in lowest ox. state so it can oxidate anything. $\endgroup$ – Mithoron Mar 15 '15 at 14:32
  • $\begingroup$ @Mithron isn't the formal oxidation state of oxygen here -2? Otherwise are you talking about partial charge? $\endgroup$ – busukxuan Mar 15 '15 at 15:06
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    $\begingroup$ Sorry I meant it cannot because it's -2. $\endgroup$ – Mithoron Mar 15 '15 at 15:08
  • $\begingroup$ @Mithoron not entirely. Formal oxidation state serves as no more than a reference for determining stability of compounds, while the reality isn't too accurately reflected by this. It is indeed true that the nitrogen atom takes a much greater oxidizing role than oxygen here, the reaction still takes place through the oxygen atom as if oxygen is the oxidant here, and the stability of whatever product is formed is determined by oxygen's oxidative power and not nitrogen's. $\endgroup$ – busukxuan Mar 15 '15 at 16:04

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