Question

Name the type of chemical reaction.
\[2HgO \to 2Hg + {O_2}\]
(A) Combination
(B) Decomposition
(C) Synthesis
(D) Displacement

Answer 1

Hint: In the above given reaction Mercuric oxide is broken down into Mercury metal and Oxygen. In order to find the above given reaction, we must know in which reaction the compound will be broken down into simpler elements.

Complete Solution :
-The equation for Combination reaction is given below:
\[A + B \to AB\]
- When a new compound can be formed in different ways, it is called Combination reaction. - - Combination reaction can occur in three ways, i.e. by combining:
 a) 2 elements,
b) 1 element and 1 binary compound (consisting of 2 elements), or
c) 2 binary compounds.
The examples of combination reactions are given below:
The rusting of iron:
\[4Fe(s) + 3{O_2}(g) \to 2F{e_2}{O_3}(s)\]
 The formation of one kind of acid rain:
\[S{O_3}(g) + {H_2}O(l) \to {H_2}S{O_4}(aq)\]

- The equation for the Decomposition reaction is given below:
\[AB \to A + B\]
When a compound can be broken down into two or more substances, it can be called the decomposition reaction. When a solid compound is heated, it will undergo a decomposition reaction. This kind of reaction will produce gas.
The following are the examples of decomposition reactions:
Heating mercury (II) oxide produces oxygen gas:
\[2HgO(s) \to 2Hg(l) + {O_2}(g)\]
- Removing the cap of the carbonated drink after continuous shaking will release carbon dioxide:
\[{H_2}C{O_3}(aq) \to {H_2}O(l) + C{O_2}(g)\]

-Synthesis reactions
Synthesis reactions are reactions that occur when two different atoms or molecules interact to form a different molecule or compound. Whenever a synthesis reaction occurs, it will release energy and the reaction is said to be exothermic. However, an endothermic reaction can also take place.
The photosynthesis in plants is one such example:
\[6C{O_2} + 6{H_2}O\xrightarrow{{light}}{C_6}{H_{12}}{O_6} + 6{O_2} + energy\]

-Displacement:
\[A + BC \to AC + B\]
Displacement reaction is a reaction in which an active displaces another metal present in the solution. In this reaction a solid metal reacts with the following:
a) a metal ion solution,
b) an acid solution, or
c) water.

Let us now consider an example for the displacement reaction:
The solid metal reacts with a metal solution that time, the metal ions from the solid will go into solution while the metal ions in solution will go to the surface of solid metal.
\[Zn(s) + CuS{O_4}(aq) \to ZnS{O_4}(aq) + Cu(s)\]
So, the correct answer is “Option B”.

Note: In Decomposition reaction, the initiation energy has to be provided into the system by the means of heat, electrolysis or any other forms. This is needed to excite the stable molecules of the reactants so that they decompose into subsequent products.