Question

Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer 1

In decomposition reaction a reactant breaks down into two or more simpler products but in the combination reaction two or more simpler products combine with each other to form a new compound.

Complete step by step answer:
-First of all we will see what decomposition reactions are.
-A decomposition reaction is a reaction where one reactant breaks down into two or more products. Its general format will be as follows:
$AB \to A + B$
-Some examples of the decomposition reaction are:

1. $NaCl \to N{a^ + } + C{l^ - }$
2. Electrolysis of water to yield hydrogen and oxygen.
3. Decomposition of carbonic acids: ${H_2}C{O_3} \to {H_2}O + C{O_2}$
   -A decomposition reaction can be both endothermic and exothermic but mostly exothermic. These reactions are mostly done for the extraction of metals from their ores.
   -There are basically 3 types of decomposition reactions:
4. Thermal decomposition reactions: They are those decomposition reactions which are activated by thermal energy which means that energy is supplied for the reactants to break down into its products.
   For example: Decomposition of calcium carbonate into calcium oxide and carbon dioxide.
   $CaC{O_3} \to CaO + C{O_2}$
5. Electrolytic decomposition reaction: They are those decomposition reactions which are activated using electrical energy.
   For example: Electrolysis of water to yield hydrogen and oxygen.
   $2{H_2}O \to 2{H_2} + {O_2}$
6. Photo decomposition: They are those reactions where the reactants are broken down by absorbing energy from photons.
   For example: ${O_3} + hv \to {O_2} + {O^ \bullet }$
   -We will now talk about what combination reactions are.
   It is a type of reaction where two or more reactants combine to form a single product. They are also known as synthesis reactions. For example:
7. $CaO(s) + {H_2}O \to Ca{(OH)_2}(aq)$
8. ${H_2}(g) + C{l_2}(g) \to 2HCl(g)$
9. ${N_2} + 3{H_2} \to 2N{H_3}$
   -In the above discussion we just saw that in a decomposition reaction one reactant breaks down into two or more simpler products while in a combination reaction two or more simpler elements combine to form a single product. This is why decomposition reactions are called the opposite of combination reactions.

Note: Sometimes double decomposition reactions occur. In a double decomposition reaction two constituent reactants interchange positive and negative ions to form two new compounds. Its general form is:
$AB + CD \to CB + AD$
Most of the double decomposition reactions are precipitation reactions.
$AgN{O_3}(aq) + NaCl(aq) \to AgCl(s) + NaN{O_3}(aq)$
While, the others are acid-base neutralization reactions.
$HCl(aq) + NaOH(aq) \to NaCl(aq) + {H_2}O(l)$