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I have a solution of only sulfate ions (no other positive ions belonging to the sulfate ions, because the positive ions are attached to the intermediate in a catalytic reaction).

I want to convert the sulfate ions to sulfur trioxide by using $\ce{V^5+}$ ions...

So what if $\ce{V^5+}$ ions could be present and could gain 2 electrons from one sulfate ion to produce $\ce{V^3+}$ and $\ce{SO3}$ (however I'm not even sure if $\ce{SO3}$ would be formed)? Here the two negatively charged oxygen atoms will each lose 1 electron to the $\ce{V^5+}$ ion, so the oxidation state of the central sulfur atom is still $+6$, but the oxidation states of the 2 oxygen atoms are $0$.

Could this be possible? And if so, would it be $\ce{SO3}$ that is formed, or other sulfur oxides? The mechanism

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    $\begingroup$ Please notice that the oxidation of sulfur in the sulfate ion and and sulfur trioxide it the same; the latter is just the anhydride of the corresponding acid of the first. $\endgroup$
    – Klaus-Dieter Warzecha
    Jan 1, 2014 at 15:32
  • $\begingroup$ And how can they then be converted to each other (sulfate->so3)? $\endgroup$
    – user2117
    Jan 2, 2014 at 13:03

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A good analogy to your proposed reaction is the formation of carbon dioxide above a solution of carbonate salts. Like sulfur trioxide, carbon dioxide is an acid anhydride. The acid for sulfur trioxide is sulfuric acid, $\ce{H2SO4}$. The acid for carbon dioxide is carbonic acid, $\ce{H2CO3}$.

Dry carbonate salts like sodium carbonate can be made to release $\ce{CO2}$ by heating to high temperatures. For example, calcium carbonate decomposes to calcium oxide and $\ce{CO2}$ at around 800 °C to 900 °C. Since such high temperatures are required, this technique does not work with aqueous solutions at ambient temperatures. (The water would boil off way before gas is released.)

Thus, sulfur trioxide evaporation from sulfate salts would be even more disfavored at room temperature than carbon dioxide evaporation from carbonate salt solutions, because of sulfur trioxide's affinity for water is much higher than $\ce{CO2}$'s.

Also, the reaction mechanism you have drawn is not a dehydration, but a reduction of the sulfate. That would not form sulfur trioxide, but a reduced sulfur compound, such as sulfite. Sulfur dioxide gas evaporation from above sulfite salts is probably more favored than sulfur trioxide evaporation, but that isn't saying much. Either one is still highly disfavored under neutral or alkaline pH.

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