Question

(a) “No precipitation reaction can occur without exchange of ions between the two reactants.” Justify this statement giving a balanced chemical equation for the reaction.
OR
(b) Giving one example of each, differentiate between a displacement reaction and a double displacement reaction.

Answer 1

(a). A precipitation reaction occurs when two ionic solutions are mixed, and an insoluble compound (precipitate) is formed due to the exchange of ions. For example:
\[\text{BaCl}_2 \, (\text{aq}) + \text{Na}_2\text{SO}_4 \, (\text{aq}) \rightarrow \text{BaSO}_4 \, (\text{s}) + 2\text{NaCl} \, (\text{aq}).\]
Here, barium (\(\text{Ba}^{2+}\)) and sulfate (\(\text{SO}_4^{2-}\)) ions combine to form the insoluble compound \(\text{BaSO}_4\) (precipitate).

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(b). Displacement Reaction: A more reactive element displaces a less reactive element from its compound.
Example:
\[\text{Zn (s)} + \text{CuSO}_4 \, (\text{aq}) \rightarrow \text{ZnSO}_4 \, (\text{aq}) + \text{Cu (s)}.\]
Double Displacement Reaction: Two ionic compounds exchange their ions to form new compounds. 
Example:
\[\text{AgNO}_3 \, (\text{aq}) + \text{NaCl} \, (\text{aq}) \rightarrow \text{AgCl} \, (\text{s}) + \text{NaNO}_3 \, (\text{aq}).\]