Problem 70 - A Complete Resource Book in Chemistry for JEE Main

Question

When sodium argentocyanide is treated with zinc dust, silver precipitates because (a) zinc forms a complex readily with cyanide (b) zinc is more electropositive than silver (c) silver is more electropositive than zinc (d) both \(\mathrm{Zn}^{2+}\) and \(\mathrm{Ag}^{+}\)ions have \(\mathrm{d}^{10}\) electronic configuration

Step-by-Step Solution

Answer

(b) zinc is more electropositive than silver

Step 1: Understanding Electropositivity

Electropositivity refers to the tendency of an element to donate electrons and form positive ions. In the periodic table, elements that are more electropositive are more likely to lose electrons and become positive ions. Since zinc (Zn) is more electropositive than silver (Ag), zinc can displace silver from a compound.

Step 2: Identifying the Displacement Reaction

In the reaction described, zinc is used to treat sodium argentocyanide. Given that zinc is more electropositive than silver, the zinc can displace silver from the argentocyanide complex. This leads to the formation of zinc cyanide and the precipitation of silver in its metallic form.

Step 3: Evaluating the Options

Among the given options: (a) says zinc forms a complex with cyanide, (b) says zinc is more electropositive than silver, (c) says silver is more electropositive than zinc, and (d) talks about electronic configurations. Since the ability of zinc to displace silver is because it is more electropositive, option (b) correctly explains the phenomenon.

Key Concepts

Electropositivity

Electropositivity is an essential concept in chemistry that explains how easily an element can donate electrons to form positive ions or cations. On the periodic table, electropositive elements are usually metals found on the left side. These elements tend to lose electrons quickly and form cations. For example, sodium (Na) is highly electropositive. It's quick to give away its one valence electron during chemical reactions.
In the displacement reaction mentioned in the exercise, zinc (Zn) showcases its electropositive nature. By being more electropositive than silver (Ag), zinc displaces silver from compounds. This property is what enables zinc to replace silver in chemical reactions, resulting in the formation of new compounds and the release of silver in its metallic form.

Displacement Reactions

Displacement reactions are a type of chemical reaction in which a more reactive element displaces a less reactive element from a compound. Such reactions are also commonly known as substitution reactions. In these reactions, the more electropositive or reactive metal takes the place of a less reactive metal.
In the context of the exercise, zinc displaces silver from sodium argentocyanide. Here's how it works:

• Zinc, being more electropositive, pushes out the silver from the argentocyanide complex.
• The silver ions gain electrons (are reduced) and become solid silver, which is seen as a precipitate.
• The displaced silver results in the formation of zinc cyanide, as the zinc takes the silver's place.

This type of reaction is crucial in understanding elemental reactivity and predicting reaction outcomes.

Complex Formation

In chemistry, complex formation involves the interaction of metal ions with ligands to form a complex. The complex ion formed is a center metal atom bonded to surrounding molecules or ions, called ligands. These ligands can include molecules like cyanide, which can form complexes with metals.
During the complex formation, the metal-ligand bond is generally a coordinate bond, where the ligand donates a pair of electrons to the metal ion. For sodium argentocyanide, silver is the central metal ion surrounded by cyanide ligands. Yet, when zinc enters the scenario, due to its electropositive nature, it dislodges the silver to form a new complex: zinc cyanide. This alteration is possible due to zinc's higher tendency to lose electrons and bond with cyanide, disrupting the silver's original complex.

Periodic Table Trends

Periodic table trends offer insights into the reactivity and properties of elements. These trends help us predict how elements will behave in chemical reactions, including their electropositivity and displacement ability.
Noticing trends in the periodic table, elements on the left, particularly alkali metals and alkaline earth metals, are highly electropositive. Their eagerness to lose electrons and form positive ions stands them apart. As for zinc and silver, both transition metals illustrate interesting behaviors. Zinc, being located more to the left of silver, shows greater electropositivity, aiding its displacement role in reactions.

• Down a group: Electropositivity typically increases as elements have more electron shells, reducing the effective nuclear charge on valence electrons.
• Across a period: Electropositivity usually decreases as elements gain more protons, strengthening their hold on valence electrons.

Being aware of these trends empowers chemists to anticipate reactions and balance equations accurately.