Question

Formation of $S{O_3}$ from $S{O_2}$ and ${O_2}$ is a (combination/decomposition) reaction.

Answer 1

To know the answer, we must know the difference between combination and decomposition reaction. Both of them are opposite of each other. If two reactants combine to form a new compound, it is a combination and the reversible reaction of it can be referred to as a decomposition reaction.

Complete step-by-step answer:

When two or more reactants combine with each other so as to form a new product, we call it a combination reaction and it is also known as synthesis reaction. Whereas a decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances which are the products ultimately.
Formation of $S{O_3}$ from $S{O_2}$ and ${O_2}$ clearly shows that sulphur dioxide and oxygen combine with each other so as to form sulphur trioxide as a new product. Therefore, it is a combination reaction. It is not a decomposition reaction as it does not involve breaking of any compound into two or more simpler substances.
This process of formation of $S{O_3}$ from $S{O_2}$ and ${O_2}$ is called the Contact process. It is a modern method used by industries in order to manufacture sulphuric acid. During this process, sulphur dioxide and oxygen are passed over a hot catalyst namely vanadium pentoxide and unite to form sulphur trioxide. This combines with water and produces sulphuric acid.
$2S{O_2}(g) + {O_2}(g) \to 2S{O_3}(g)$
$S{O_3}(g) + {H_2}O(l) \to {H_2}S{O_4}(l)$
The role of catalyst vanadium pentoxide is to oxidize sulphur dioxide to sulphur trioxide at high temperatures. We can also state that this reaction is a reversible reaction i.e. the reverse reaction or breakdown of sulphur trioxide back to its reactants may also occur.

Note: This combination reaction is exothermic, which means heat is released during the process. This states that the backward reaction would be favoured at higher temperatures. That is why the vanadium pentoxide catalyst takes ${450^o}C$ to work. Rise in temperature increases the rate of reaction.