Question

Which choice is an example of a synthesis reaction?
A.\[2K{\text{ }} + {\text{ }}{F_2} \to {\text{ }}2KF\]
B.\[2Al{\text{ }} + {\text{ }}6HCl{\text{ }} \to {\text{ }}2AlC{l_3}{\text{ }} + {\text{ }}3{H_2}\]
C.\[{C_3}{H_8}{\text{ }} + {\text{ }}5{O_2} \to {\text{ }}3C{O_2}{\text{ }} + {\text{ }}4{H_2}O\]
D.\[NaCl{\text{ }} + {\text{ }}AgN{O_3} \to {\text{ }}AgCl{\text{ }} + {\text{ }}NaN{O_3}\]

Answer 1

Hint: One such example of synthesis reaction is- when sodium and chlorine reacts to form sodium chloride compound. One can define it as a direct reaction.

Complete step-by-step solution:
There are five basic types of reactions: synthesis, decomposition, combustion, neutralization and displacement reactions.
 In a chemical reaction, when two or more reactants combine to form a single product or compound, we call it a combination reaction. In such a reaction, we are synthesising a compound by reaction of two or more reactants, therefore it is a synthesis reaction too.
\[A{\text{ }} + {\text{ }}B{\text{ }} \to {\text{ }}AB\]
Out of the four options available, we see that in the first option, potassium combines with fluorine to form potassium fluoride. So, it is a synthesis reaction.
Decomposition reaction is the opposite of synthesis reaction. In this, a single reactant splits up into two or more simpler products by the means of thermal, electrolytic and photolytic methods. For example,
\[AB \to A + B\]
\[NaCl \to N{a^ + } + C{l^ - }\]
In a combustion reaction, a substance reacts with oxygen and releases heat and light. It is also called burning as oxygen is a necessary element in it. For example, option (C),
\[{C_3}{H_8}{\text{ }} + {\text{ }}5{O_2} \to {\text{ }}3C{O_2}{\text{ }} + {\text{ }}4{H_2}O\]
Here, propane reacts with oxygen and produces carbon dioxide and water.
When acid reacts with base in a quantitative manner and gets neutralised or vice versa, it is called neutralisation reaction. For example,
\[HCl + NaOH \to NaCl + {H_2}O\]
Displacement reactions are of two types: single displacement reaction and double displacement reaction. As we look at option (B),
\[2Al{\text{ }} + {\text{ }}6HCl{\text{ }} \to {\text{ }}2AlC{l_3}{\text{ }} + {\text{ }}3{H_2}\]
 chlorine gets displaced from HCl to Al i.e., an element moves out of a compound or get swapped. Whereas in option (D), \[NaCl{\text{ }} + {\text{ }}AgN{O_3} \to {\text{ }}AgCl{\text{ }} + {\text{ }}NaN{O_3}\]
a part of two ionic compounds each got exchanged and produced two new compounds.
Therefore, option (B) is a single displacement reaction and option (D) is an example of double displacement reaction. Also, option (C) is a combustion reaction.

Hence, the correct option is (A)

Note: If energies are provided for each reaction, we can identify synthesis reactions easily because they are exothermic in nature. The reactants may be elements or compounds but the product will always be a compound.