Question

Which of the following is a displacement reaction?
A. \[MgC{O_3} \to MgO + C{O_2}\]
B. \[2Na + {H_2}O \to 2NaOH + {H_2}\]
C. \[2{H_2} + {O_2} \to 2{H_2}O\]
D. \[2Pb{(N{O_3})_2}\xrightarrow{{Heat}}2PbO + 4N{O_2} + {O_2}\]

Answer 1

Hint: A displacement reaction is a type of reaction where an element is replaced by another element after reacting with one another. In this type of reaction, there is an interchange of a single element only; hence it should not be confused with a double displacement reaction. 

Complete step by step solution:
There are various types of chemical reactions which occur when one or more elements react with each other. They are as follows.

• • Combination reaction: In this type of reaction, two or more chemical substances react with each other to form a single chemical compound. For example: \[x + y = xy\]

• Decomposition reaction = Decomposition reaction is the complete opposite of a combination reaction as in this type of reaction the complicated chemical compounds break down and form into simpler elements. For example: \[xy \to x + y\]. In the question stated above, [MgC{O_3} \to MgO + C{O_2}\] and\[2Pb{(N{O_3})_2}\xrightarrow{{Heat}}2PbO + 4N{O_2} + {O_2}\] are examples of a decomposition reaction.

• Precipitation reaction: In this type of reaction, two solutions of salts that are soluble react with one another to form an insoluble solid compound. For example: \[x + \]Soluble salt \[y \to \] precipitated element\[ + \] soluble salt \[Z\].

• Neutralisation reaction: It is a type of reaction in which acid and base react with one another which results in the formation of a chemical salt and water.

• Combustion reaction: It is a type of reaction in which oxygen reacts with a chemical compound which in turn liberates carbon dioxide and \[{H_2}O\]. For example: \[x + {O_2} \to {H_2}O + C{O_2}\]. In the question, the reaction \[2{H_2} + {O_2} \to 2{H_2}O\ is an example of a combustion reaction. This type of reaction is exothermic which implies that there will be the liberation of heat in this reaction.

• Displacement reaction- As mentioned above, it is a type of reaction where an element is replaced by another element after reacting with one another. The best-suited example is mentioned in the question above; which is \[2Na + {H_2}O \to 2NaOH + {H_2}\]. In this, there is the displacement of sodium, which after reaction, displaces and forms sodium hydroxide.

Hence, option (B) is the correct answer.
Note: There is another type of reaction known as a double displacement reaction. In a double displacement reaction, there is a reaction between two anions of two cations in which two elements are replaced by one another.