I understand that a combination reaction is one where two substances combine to form a third substance, like with $$\ce{2Na + Cl2 -> 2NaCl}.$$ I do not understand why $$\ce{CaO + SO2 -> CaSO3}$$ is not a combination redox reaction. After all, it is creating a third substance, so why is it not a combination redox reaction?
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2 Answers
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To determine whether a reaction is a redox reaction, you first have to find the oxidation number of each element in the reactants and products. You do this by either counting the number of electrons in each bond and assigning them to the most electronegative atom in the bond, or by memorizing some rules and applying them. When you do that for this reaction, you should get the following:
\begin{align} \ce{ &&CaO + SO2 &-> CaSO3}\\ \ce{Ca:&& +2 &-> +2}\\ \ce{O: && -2, -2 &-> -2}\\ \ce{S: && +4 &-> +4} \end{align}
Since the oxidation number hasn't changed for any element, no reduction or oxidation has taken place, and this is not a redox reaction.
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This question can be solved the easiest if we break it down to its fragments. The requirements of a combination redox reaction are that the reaction must be a redox reaction and a combination reaction at the same time.
The question of whether this is a combination reaction or not is pretty easy: we get one compound from the original two, so yes, it is a combination reaction.
However, this is not a redox reaction, because no particles' oxidation number changes, so no oxidation/reduction actually happens.
So in conclusion this is a combination, but not a redox reaction, so it doesn't qualify as a combination redox reaction.
