Displacement and Double Displacement Reactions

Displacement Reactions in Aqueous Solutions

Displacement reactions are a fundamental class of chemical reactions where one element takes the place of another element within a compound. As demonstrated in Activity 1.8, this process can be observed by placing an iron nail into a solution of copper sulphate. In this specific scenario, iron acts as the more reactive element, successfully displacing the copper from its sulphate compound. The chemical equation representing this transformation is written as:

$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$

In this reaction, iron sulphate $(FeSO_4)$ is formed in the aqueous phase while solid copper $(Cu)$ is precipitated. The definition provided for this process states that a displacement reaction is one in which an element has displaced or removed another element from its solution. The visual evidence of this reaction is distinct: the iron nail, which is initially a metallic grey, becomes brownish in color due to the deposition of copper. Concurrently, the characteristic blue color of the copper sulphate solution fades as the concentration of copper sulphate decreases and iron sulphate begins to form.

Reactivity Comparison involving Zinc and Lead

The principles of displacement are further illustrated through additional examples involving different metals. When solid zinc is added to copper sulphate, the zinc displaces the copper to form zinc sulphate, as shown in the following equation:

$Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$

Similarly, when solid lead is introduced to a solution of copper chloride, lead chloride is produced alongside solid copper. The chemical equation for this reaction is:

$Pb(s) + CuCl_2(aq) \rightarrow PbCl_2(aq) + Cu(s)$

These reactions demonstrate a clear hierarchy of chemical reactivity. Based on these observations, it is concluded that zinc and lead are more reactive elements than copper. Because of their higher reactivity, they are capable of displacing copper from its various compounds, such as copper sulphate or copper chloride.

Double Displacement Reactions and Precipitation

Double displacement reactions involve the exchange of ions between two reactants to form new compounds. This is explored in Activity 1.10, where approximately $3\,cm^3$ of sodium sulphate solution is mixed with approximately $3\,cm^3$ of barium chloride solution in different test tubes. Upon mixing these two solutions, a white substance is formed that is insoluble in water. This insoluble solid is formally known as a precipitate.

A precipitation reaction is defined as any chemical reaction that results in the production of a precipitate. The chemical interaction between sodium sulphate and barium chloride is expressed by the following equation:

$Na_2SO_4(aq) + BaCl_2(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$

In this process, the barium ions $(Ba^{2+})$ from the barium chloride react with the sulphate ions $(SO_4^{2-})$ from the sodium sulphate to form solid barium sulphate $(BaSO_4)$, which appears as the white precipitate. The remaining ions form sodium chloride $(NaCl)$, which remains dissolved in the aqueous solution.