Problem 100 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Give reasons for the following : (i) \(\mathrm{CrO}_{3}\) is an acid anhydride. (ii) The species \(\left[\mathrm{CuCl}_{4}\right]^{2-}\) exists while \(\left[\mathrm{CuI}_{4}\right]^{2-}\) does not. (iii) The colour of mercurous chloride, \(\mathrm{Hg}_{2} \mathrm{Cl}_{2}\), changes from white to black when treated with ammonia. (iv) Zinc and not copper is used for the recovery of metallic silver from complex \(\left[\mathrm{Ag}(\mathrm{CN})_{2}\right]^{-}\). Explain. (v) Most transition metal compounds are coloured. (vi) Silver bromide is used in photography.

Step-by-Step Solution

Answer

(i) It's an acid anhydride. (ii) Orbital overlap differs. (iii) Forms black mercury. (iv) Zinc is more reactive. (v) \(d\)-orbital transitions. (vi) Light-sensitive conversion.

Step 1: Understanding Acid Anhydrides

An acid anhydride is a compound that forms an acid when combined with water. \(\mathrm{CrO}_3\), or chromium(VI) oxide, is an acid anhydride because when it reacts with water, it forms chromic acid, \(\mathrm{H}_2\mathrm{CrO}_4\). This demonstrates its property as an acid anhydride.

Step 2: Analyzing Complexes' Existence

\([\mathrm{CuCl}_4]^{2-}\) exists due to the smaller size and better orbital overlap of \(\mathrm{Cu}^{2+}\) with \(\mathrm{Cl}^-\), facilitating stable complex formation. However, \(\mathrm{CuI}_2\) is unstable because \(\mathrm{Cu}\) prefers a +1 oxidation state when combined with iodide, preventing stable \([\mathrm{CuI}_4]^{2-}\) formation.

Step 3: Reaction of Mercurous Chloride with Ammonia

Mercurous chloride \(\mathrm{Hg}_2\mathrm{Cl}_2\) changes color because when treated with ammonia, it forms a black precipitate of \(\mathrm{Hg}\), or elemental mercury, through a reaction reducing \(\mathrm{Hg}_2^{2+}\) to \(\mathrm{Hg}\).

Step 4: Recovery of Silver Using Zinc

Zinc is used over copper for silver recovery from \([\mathrm{Ag}( ext{CN})_2]^{-}\) because zinc is more electropositive, meaning it can more readily donate electrons to reduce silver ions to metallic silver.

Step 5: Coloration in Transition Metal Compounds

Most transition metal compounds are colored due to electronic transitions between \(d\)-orbitals. These transitions absorb specific wavelengths of light, resulting in the observed colors.

Step 6: Silver Bromide in Photography

Silver bromide, \(\mathrm{AgBr}\), is sensitive to light, undergoing a photochemical reaction to metallic silver and bromine when exposed. This property allows it to capture images on photographic film.

Key Concepts

Acid Anhydride

An acid anhydride is a chemical compound that reacts with water to form an acid. These compounds are typically formed by removing water molecules from their corresponding acids.
In the case of chromium(VI) oxide, \(\text{CrO}_3\), it is considered an acid anhydride because when it interacts with water, it transforms into chromic acid, \(\text{H}_2\text{CrO}_4\).
This is a peculiar characteristic of acid anhydrides as it shows their potential to rehydrate and form larger acid molecules.

• For instance, \(\text{CrO}_3\) + water = \(\text{H}_2\text{CrO}_4\)
• Shows the reconstitution of an acid from its anhydride form.

Understanding these transformations helps comprehend more complex reactions involving acid and base interactions.

Complex Formation

Complex formation in chemistry involves the arrangement of metal ions surrounded by ligands, which are ions or molecules capable of donating electron pairs. Transition metals like copper have the ability to form such complexes due to their unique electronic structure.
For instance, the complex \([\text{CuCl}_4]^{2-}\) can exist because the \(\text{Cu}^{2+}\) ion can effectively overlap its orbitals with chloride ions, \(\text{Cl}^-\), stabilizing the complex formation.
Each ligand donates electrons to the metal center, forming a stable complex.

• Stable due to favorable orbital overlap and small ion size.
• \(\text{Cu}^{2+}\) has a preference for higher oxidation states.

However, iodine being larger and less efficient in orbital overlap does not form a stable \([\text{CuI}_4]^{2-}\) complex.

Reactions with Ammonia

When substances react with ammonia, they undergo significant changes due to ammonia's ability to act as a ligand and a reducing agent.
In the case of mercurous chloride \(\text{Hg}_2\text{Cl}_2\), exposure to ammonia causes a reaction where the compound changes color from white to black. This transformation occurs because ammonia reduces the \(\text{Hg}_2^{2+}\) to black elemental mercury, \(\text{Hg}\), resulting in a clear visual change.
Reactions involving ammonia often result in the dissociation of bonds and the formation of new substances.

• Ammonia can remove chloride ions through complexation.
• Acts by reducing mercury ions to metallic mercury.

This provides insights into both the complex formation and reduction reactions occurring in chemistry.

Electropositivity

Electropositivity refers to the tendency of an atom to donate electrons and form positive ions. This property is significant in displacing less electropositive metals from solutions, especially in redox reactions.
For example, in the recovery of metallic silver using zinc, zinc is preferred over copper due to its higher electropositivity.
Zinc can donate electrons more readily than copper, which makes it more effective in reducing silver ions \(([\text{Ag(CN)}_2]^{-})\) to metallic silver.

• \(\text{Zn}\) occurs on the left of copper in the reactivity series, indicating higher electropositivity.
• Ensures a more efficient reduction process.

This ability to donate electrons readily is crucial in industrial applications where metal recovery is required.

Photochemistry

Photochemistry involves chemical reactions that occur in response to light exposure. The prime example is the use of silver bromide in photography.
In the photographic process, silver bromide, \(\text{AgBr}\), undergoes a photochemical reaction when exposed to light, resulting in the formation of metallic silver and bromine.
This change is why photographic films can capture images, as the light exposure creates variations in the silver distribution that make up the picture.

• Silver halides are light-sensitive.
• Produces elemental silver upon exposure to activate an image.

Understanding the light-induced reactions in photochemistry opens the door to advancements in various photographic and imaging techniques.