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While reading about halogen family I encountered the reaction: $$\ce{NaClO3 + I2 -> NaIO3 + Cl2}$$

I know that that the oxidizing tendency of halogens decreases down the group, so I think that the reverse reaction should have taken place. So why is this reaction taking place?

Is the reason for the above reaction taking place, the higher tendency of iodine to bond with oxygen?

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    $\begingroup$ True, iodine is a much weaker oxidizer than chlorine. Now see, iodine is not the oxidizer at all in this reaction. Quite the opposite, in fact. $\endgroup$ – Ivan Neretin Apr 27 '20 at 11:21
  • $\begingroup$ So , Can I generalize that stability of higher oxidation states of halogen increases down their group...( Also is it because of their decreasing ionisation enthalpy ) $\endgroup$ – user89505 Apr 27 '20 at 11:29
  • $\begingroup$ Yes, you can say so. $\endgroup$ – Ivan Neretin Apr 27 '20 at 11:35
  • $\begingroup$ But it was mentioned in my book that the stability of oxides of halogens is I > Cl > Br...Can you please comment on this as well... $\endgroup$ – user89505 Apr 27 '20 at 11:41
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    $\begingroup$ I don't see any contradiction. True, the stability of compounds with halogens in positive oxidation states (in particular, oxides) increases down the group. $\endgroup$ – Ivan Neretin Apr 27 '20 at 13:25
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The action of iodine is part of a long reaction chain with chlorate, so not a direct action as implied.

I start with iodine and water equilibrium:

$\ce{I2 + H2O <=> HI + HOI }$

Next more complex part of the reaction sequence is discussed here, to quote:

Chlorate ion oxidizes iodide ion to hypoiodous acid and chlorous acid in the slow and rate-determining step:

$\ce{ClO3− + I− + 2 H+ → HOI + HClO2}$

Chlorate consumption is accelerated by reaction of hypoiodous acid to iodous acid and more chlorous acid:

$\ce{ClO3− + HOI + H+ → HIO2 + HClO2}$

More autocatalysis when newly generated iodous acid also converts chlorate in the fastest reaction step:

$\ce{ClO3− + HIO2 → IO3− + HClO2}$

Chlorous acid is unstable, disproportionating to hypochlorous acid and chloric acid:

$\ce{2 HClO2 → HOCl + H+ + ClO3- }$

Also, the known action of hypochlorous acid (even H2O2) on HI liberating iodine:

$\ce{2 HOCl + 2 HI -> 2 HCl + I2}$

Finally, some chlorine evolution:

$\ce{HCl + HOCl <=> Cl2 + H2O }$

Further reference on the complexity of the reaction system, see Page 94, Table 5.2 in this ebook.

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