Question

The reaction $Pb{{\left( N{{O}_{3}} \right)}_{2}}+2NaI\to 2NaN{{O}_{3}}+Pb{{I}_{2}}\downarrow $ is classified as
(A) Combination reaction
(B) Decomposition reaction
(C) Single replacement reaction
(D) Double replacement reaction

Answer 1

Hint: There are five main types of chemical reactions i.e. combination reaction, decomposition reaction, single replacement reaction, double replacement reaction and combustion reaction; from which four are mentioned here and need to be discussed to answer the illustration.

Complete answer:
Let us discuss the given four types of reaction in detail;
Combination reaction-
This is also known as synthesis reaction as during this reaction two or more substances combine to form a single new substance. The general form can be explained as;
$A+B\to AB$ ; where A and B combines to form AB as a final product.
Decomposition reaction-
This reaction mechanism is exactly opposite to that of combination reaction. In this reaction, a compound breaks down into two or more simpler substances. This can be better explained by generalised reaction;
$AB\to A+B$ ; where AB decomposes to give A and B. this requires input of energy in the form of light, electricity or heat.
Single replacement reaction-
This is also known as single displacement reaction. In this reaction, one element replaces a similar element in a compound. The generalised form can be explained as;
$A+BC\to AC+B$ ; where A (metal) replaces B (metal) in the compound to form a new compound with C (non-metal).
Double replacement reaction-
This is also known as double displacement reaction. In this reaction, the positive and negative ions of two ionic compounds exchange places to form new compounds. Generally, this mechanism can be explained as;
$AB+CD\to AD+CB$ ; where A and C are positively charged cations whereas, B and D are negatively charged anions.
Given reaction-
$Pb{{\left( N{{O}_{3}} \right)}_{2}}+2NaI\to 2NaN{{O}_{3}}+Pb{{I}_{2}}\downarrow $
Here, we can clearly see that the reaction is a double replacement reaction.

Therefore, option (D) is correct.

Note:
Do note that the double replacement reactions occur between the substances in aqueous solution only. In order to make this reaction possible, one of the products is usually solid precipitate, a gas or a molecular compound such as water.