4
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Which one of these reactions will occur?

  1. $$\ce{KMnO4 -> K2O + MnO2 + O2}$$
  2. $$\ce{KMnO4 + HCl -> KCl + MnCl2 + Cl2 + H2O}$$
  3. $$\ce{KMnO4 + CO2 \longrightarrow K2CO3 + Mn2O7}$$
  4. $$\ce{KMnO4 + CO2 -> K2CO3 + MnO2}$$
  5. $$\ce{KMnO4 + SO2 -> K2SO4 + Mn2O3 + SO3}$$

And how am I supposed to know which one it is? It's not just this question, I struggle with these types of questions every time. There seem to be a lot of things to take into consideration.

I'm thinking whether it's under 3 or under 4. The oxidation numbers for the reaction 3 don't change at all. But in reaction 4, only the oxidation number of Mn changes (from +7 to +4) and the other oxidation numbers stay the same (if I'm right). So, I think reactions 1, 2 and 5 definitely happen, but I don't know about 3 and 4.

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    $\begingroup$ Indeed, you have to consider many things which together constitute a huge body of knowledge known as chemistry. First of all, check the oxidation states and determine whether a redox reaction is taking place (you must have an oxidizing agent and a reducing agent). See where this gets you. $\endgroup$ – Ivan Neretin Feb 27 '16 at 20:30
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Both 3 and 4 are impossible. $\ce{CO2}$ does not react with the $\ce{KMnO4}$, because $\ce{KMnO4}$ is strongly oxidizing. $\ce{C}$ is in the highest oxidation state and can not be oxidized, hence, it is impossible to react.

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