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When chlorine reacts with cold solution of say potasium hydroxide, the disproportionation goes to lower oxidation states:

$\ce{Cl2 + 2 KOH -> KCl + KClO + H2O} $

Whilst with hot solution the oxidation state of chlorine goes up to $+V$:

$\ce{3 Cl2 + 6 KOH -> KClO3 + 5 KCl + 3 H2O} $

Why is that? Does the heat make the oxidation to higher ox. states easier?

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  • $\begingroup$ Heat makes pretty much everything easier. $\endgroup$ – Ivan Neretin May 23 '16 at 16:17
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The chlorate ($\ce{ClO3-}$) is formed by disproportionation of hypochlorite and hypochlorous acid:

$$\ce{ClO- + 2 HClO -> ClO3- + 2 HCl}$$

Higher temperature promotes the formation of hypochlorous acid through hydrolysis of hypochlorite and therefore speeds up the reaction.

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