Question

Which of the following are combination reactions?

(i)$2{\mathrm{KClO}}_3\stackrel{\mathrm{heat}}{\to }2\mathrm{KCl}+3{\mathrm{O}}_2$

(ii) $\mathrm{MgO}+{\mathrm{H}}_2\mathrm{O}⟶\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$

(iii) $4\mathrm{Al}+3{\mathrm{O}}_2⟶2{\mathrm{Al}}_2{\mathrm{O}}_3$

(iv) $\mathrm{Zn}+{\mathrm{FeSO}}_4⟶{\mathrm{ZnSO}}_4+\mathrm{Fe}$

• A. (i) and (iii)
• B. (iii) and (iv)
• C. (ii) and (iv)
• D. (ii) and (iii)

Answer 1

The correct option is D

(ii) and (iii)

Explanation for the correct option:

(D) (ii) and (iii)

Reaction of Magnesium oxide with water:

• As Magnesium oxide $\left(\mathrm{MgO}\right)$ and water mix to generate Magnesium hydroxide $\left(\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\right)$, it is a combination reaction.
• The chemical reaction can be depicted as:

$\mathrm{MgO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)⟶\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)$

Reaction for the formation of Aluminium oxide:

• As Aluminium $\left(\mathrm{Al}\right)$ and Oxygen $\left({\mathrm{O}}_2\right)$ mix to generate Aluminum oxide$\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$, it is once again a combination reaction.
• The chemical reaction can be depicted as:

$4\mathrm{Al}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)⟶2{\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)$

Explanation for the incorrect options:

(A) (i) and (ii)

(i) $2{\mathrm{KClO}}_3\left(\mathrm{aq}\right)\stackrel{\mathrm{heat}}{\to }2\mathrm{KCl}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)$

• It is a reaction of breakdown.

(ii) $\mathrm{MgO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)⟶\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)$, it is a combination reaction

• As Magnesium oxide $\left(\mathrm{MgO}\right)$ and water mix to generate Magnesium hydroxide $\left(\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\right)$, it is a combination reaction.

(B) (iii) and (iv)

(iii) $4\mathrm{Al}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)⟶2{\mathrm{Al}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)$

• As Aluminium $\left(\mathrm{Al}\right)$ and Oxygen $\left({\mathrm{O}}_2\right)$ mix to generate aluminum oxide$\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$, it is once again a combination reaction.

(iv) $\mathrm{Zn}\left(\mathrm{s}\right)+{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)⟶{\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+\mathrm{Fe}\left(\mathrm{s}\right)$

• This reaction represents a displacement reaction.

(C) (ii) and (iv)

(ii) $\mathrm{MgO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)⟶\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)$, it is a combination reaction

• As Magnesium oxide $\left(\mathrm{MgO}\right)$ and water mix to generate Magnesium hydroxide $\left(\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\right)$, it is a combination reaction.

(iv) $\mathrm{Zn}\left(\mathrm{s}\right)+{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)⟶{\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+\mathrm{Fe}\left(\mathrm{s}\right)$

• This reaction represents a displacement reaction.

Therefore, the correct option is (D) (ii) and (iii).