Chapter 1 · Question 14

Differentiate between displacement and double displacement reactions with the help of a balanced chemical equation for each type.

Q14

Differentiate between displacement and double displacement reactions with the help of a balanced chemical equation for each type.

Answer Revealed

Direct Answer:

In a displacement reaction, a more reactive element displaces a less reactive element from its compound — only one element moves. Example:

\text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)}

(iron displaces copper). In a double displacement reaction, ions are exchanged between two compounds, forming two new compounds. Example:

\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}

(sodium and barium exchange their partner ions). Key difference: displacement involves one element replacing another, while double displacement involves a mutual exchange of ions between two compounds.

Simple Explanation

Displacement is like one person taking another person's seat — iron pushes copper out of copper sulphate (

\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}

). Double displacement is like two couples swapping partners — sodium stays with chloride and barium goes with sulphate (

\text{Na}_2\text{SO}_4 + \text{BaCl}_2 \rightarrow \text{BaSO}_4 + 2\text{NaCl}

). In displacement, one thing moves; in double displacement, two things swap.

Exam-Ready Structure

Displacement and double displacement reactions are distinct reaction types, and NCERT students must clearly differentiate between them: 1. Displacement reaction: (a) Definition: A reaction in which a more reactive element displaces a less reactive element from its aqueous salt solution. (b) General form:

A + BC \rightarrow AC + B

, where A is a more reactive element and B is the less reactive element in compound BC. (c) Only one displacement occurs — the more reactive element takes the place of the less reactive one. (d) A single new compound and a single new element are formed. (e) NCERT example — Iron nail in copper sulphate:

\text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)}

. Iron (more reactive) displaces copper (less reactive). Other examples:

\text{Zn(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{Cu(s)}

;

\text{Pb(s)} + \text{CuCl}_2\text{(aq)} \rightarrow \text{PbCl}_2\text{(aq)} + \text{Cu(s)}

. (f) Observations typically involve colour change and deposition of the displaced metal. 2. Double displacement reaction: (a) Definition: A reaction in which two compounds react by an exchange of ions to form two new compounds. (b) General form:

AB + CD \rightarrow AD + CB

. (c) Ions or groups of atoms of both reactants are exchanged. (d) Two new compounds are formed; often one of them is insoluble (precipitate). (e) NCERT example — Sodium sulphate with barium chloride:

\text{Na}_2\text{SO}_4\text{(aq)} + \text{BaCl}_2\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)} + 2\text{NaCl(aq)}

. Sodium sulphate and barium chloride exchange ions to form sodium chloride and insoluble barium sulphate. Other example:

\text{Pb(NO}_3)_2\text{(aq)} + 2\text{KI(aq)} \rightarrow \text{PbI}_2\text{(s)} + 2\text{KNO}_3\text{(aq)}

. (f) Often accompanied by the formation of a precipitate. 3. Key comparison: Displacement has an element plus a compound as reactants and is driven by a more reactive element replacing a less reactive one. Double displacement has two compounds as reactants and involves mutual exchange of ions or groups of atoms, commonly producing a precipitate.

Key Points

Displacement: single replacement — one element displaces another ( $A + BC \rightarrow AC + B$ )
Example of displacement: $\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}$
Double displacement: mutual ion exchange ( $AB + CD \rightarrow AD + CB$ )
Example of double displacement: $\text{Na}_2\text{SO}_4 + \text{BaCl}_2 \rightarrow \text{BaSO}_4 + 2\text{NaCl}$
In displacement, reactivity of the displacing metal is key; in double displacement, formation of a precipitate often drives the reaction

Common Mistakes

Calling a double displacement reaction a 'double replacement' and confusing it with displacement — in displacement, an ELEMENT gets replaced; in double displacement, IONS exchange
Assuming both types form a precipitate — precipitation is common in double displacement but not in displacement reactions

Differentiate between displacement and double displacement reactions with the help of a balanced chemical equation for each type.