Question

Which of the following is\are combination reaction?
(i)$2KCl{O_3} \to 2KCl + 3{O_2}$
(ii) $MgO + {H_2}O \to Mg{\left( {OH} \right)_2}$
(iii) $4Al + 3{O_2} \to 2A{l_2}{O_3}$
(iv) $Zn + FeS{O_4} \to ZnS{O_4} + Fe$

Answer 1

Hint: Combination reactions involve the formation of a new compound with the combination of the two or more reactants. These reactions are usually exothermic. Combination reactions can occur in compounds that have the same ionic charge as well as different ionic charge.

Complete step by step answer:
Combination reaction refers to the synthesis reaction where the products of this reaction are formed by the combination of two or more reactants. The reactants usually release energy on combination in order to be able to pair with a new atom.
In the first reaction, we have $2KCl{O_3} \to 2KCl + 3{O_2}$ where in the potassium chlorate undergoes a decomposition reaction and hence gets divided into two compounds that is potassium chloride and oxygen. Therefore, it is not a combination reaction.
In the second reaction, $MgO + {H_2}O \to Mg{\left( {OH} \right)_2}$ the magnesium oxide reacts with water and forms magnesium hydroxide which is in the end a combination of magnesium oxide and water. Thus, this reaction is a combination reaction.
In the reaction, $4Al + 3{O_2} \to 2A{l_2}{O_3}$ aluminium combines with oxygen to form aluminium oxide. This is a combination reaction as oxygen is combined with aluminium.
The last reaction involves the disproportionation reaction of zinc. This means that one compound is a reducing agent and the other is the oxidising agent. In this reaction, Zinc is the reducing agent as it undergoes oxidation and iron is the oxidising agent. This is not a combination reaction.

So, the correct answer is Option ii,iii .

Note: The reaction where the reactants form bonds with each other are called combination reactions.There will not be splitting of reactants as seen in decomposition reactions. The reactants will also not have a designated role of being a reducing agent or oxidising agent as seen in redox reactions.