Question

(a) What happens when a strip of lead metal is placed in a solution of copper chloride? Write the balanced chemical equation for the reaction along with the colour changes observed during the reaction.

(b) What are precipitation reactions? Give one example of precipitation reaction.

Answer 1

Displacement reactions

Reactions in which a more reactive element displaces a less reactive element from its solution is called a displacement reaction. Displacement reactions are of two types- Single displacement reactions and double replacement reactions. Single displacement reactions occur when an element displaces another elements from its salt solution. In double displacement reactions, one ions of two reactants are exchanged to form two new products.

(a) The metals are arranged in the order of their reactivity in the reactivity series. From the reactivity series, Lead (Pb) is more reactive metal than Copper (Cu). Therefore, when lead metal is immersed in Copper Chloride (CuCl₂) solution, Lead replaces Copper forming Lead sulphate chloride (PbCl₂). The Copper is deposited in its elemental form.

The chemical equation for the reaction is:

$CuC{l}_2\left(aq\right)+Pb\left(s\right)\to PbC{l}_2\left(aq\right)+Cu\left(s\right)$

(Copper (Lead) (Lead (Copper

chloride) chloride)

(b) Precipitation reactions are a form of double displacement reaction in which the exchange of ions by the mixing of two aqueous solutions produces a solid precipitate. These reactions are used for qualitative analysis of ions in a given sample.

The general form of a double displacement reaction is:

$AB+CD\to AD+CB$

In precipitation reactions, one of the product will be an insoluble solid.

For example, when an aqueous solution of Silver nitrate (AgNO₃) is added to Sodium chloride (NaCl) solution, a white insoluble precipitate of Silver chloride (AgCl) is formed. Sodium nitrate (NaNO₃) is also formed which remains in the solution. The chemical equation for the reaction is:

$AgN{O}_3\left(aq\right)+NaCl\left(aq\right)\to AgCl\left(s\right)+NaN{O}_3\left(aq\right)$

(Silver (Sodium (Silver (Sodium

nitrate) chloride) chloride) nitrate)