Question

Which of the following is not a correct statement? (a) The aqueous solution of potash alum is acidic due to cationic hydrolysis. (b) Mohr's salt is a double salt (c) Aniline yellow is also called p-amino azobenzene. (d) Iodoform can be prepared from methanol also by haloform reaction.

Short answer

Statement (d) is incorrect.

Step-by-step solution

Understanding the Question

Identify the task, which is to determine the incorrect statement among the given options. There are four statements, each relating to chemical substances or reactions.

Reviewing Statement (a)

Statement (a) claims that the aqueous solution of potash alum is acidic due to cationic hydrolysis. Potash alum, \[ KAl(SO_4)_2 \cdot 12H_2O \]is known to be acidic in solution because of the hydrolysis of the weak acid formed by the aluminum ion. Therefore, this statement is correct.

Reviewing Statement (b)

Statement (b) states that Mohr's salt is a double salt. Mohr's salt is \[ (NH_4)_2Fe(SO_4)_2 \cdot 6H_2O \]and it is known as a double salt of ferrous ammonium sulfate. Hence, this statement is also correct.

Reviewing Statement (c)

Statement (c) claims that Aniline yellow is also called p-amino azobenzene. In reality, Aniline yellow is indeed p-amino azobenzene due to its structural composition. Hence, this statement is correct.

Reviewing Statement (d)

Statement (d) asserts that iodoform can be prepared from methanol by the haloform reaction. However, the haloform reaction requires a methyl ketone \[ (CH_3COR) \] or alcohols oxidizable to methyl ketones to occur, and methanol does not fit this condition. Thus, this statement is incorrect.

Key concepts

Cationic Hydrolysis

Cationic hydrolysis is a process where cations react with water to form a weak acid. This typically occurs with metal salts whose cations form weak bases. Potash alum, for instance, undergoes cationic hydrolysis due to its aluminum ions. When you dissolve potash alum in water, the aluminum cations hydrolyze and react with water to form

• Aluminum hydroxide
• Hydroxide ions

This process results in an acidic solution because the aluminum ion weakly holds onto the hydroxide ions, causing the solution's pH to decrease. Thus, potash alum's acidic nature in aqueous solutions is largely due to this very reaction.
Understanding the way cations and water interact helps in predicting the acidity/alkalinity of certain metal salts when dissolved.

Double Salt

Double salts are intriguing compounds that form when two different salts crystallize together in a fixed proportion. Mohr's salt, for instance, is a classic example. It is a crystalline compound obtained by combining

• Ferrous sulfate
• Ammonium sulfate

into a single crystal with six water molecules. The chemical formula for Mohr's salt is \[(NH_4)_2Fe(SO_4)_2 \cdot 6H_2O\].
What makes double salts unique is their properties in solution. When dissolved in water, double salts dissociate into all of their ion components completely. This distinguishes them from complex salts, which do not fully dissociate. Be aware that double salts revert to their individual ions when dissolved, offering a playground for understanding ion interactions and behaviors.

Haloform Reaction

The haloform reaction is a fascinating chemical process used to produce haloforms, such as iodoform. This reaction demands the presence of a methyl ketone, \[CH_3COR\], or alcohols that can oxidize to form such a ketone. Methanol, unfortunately, does not meet these criteria as it cannot oxidize to a methyl ketone. Therefore, it cannot participate effectively in the haloform reaction to produce iodoform.
During the haloform reaction, the halogen reacts with the methyl ketone to replace the hydrogen atoms with halogen atoms, eventually forming haloform. This reaction showcases the importance of recognizing suitable reactants for specific chemical processes.

Aniline Yellow

Aniline yellow, famously known as p-amino azobenzene, is a vibrant yellow dye. This compound belongs to the azo compound family, distinguished by their nitrogen-to-nitrogen double bonds (\[-N=N-\]).
In chemistry, making connections between names, structures, and properties can be remarkably helpful. In this case, the alternative name illuminates its structure, with "p-amino" indicating the amino group positioned para to the \[-N=N-\] linkage on the benzene ring.
This understanding of systematic naming in organic chemistry eases the learning process for functional groups and their influence on chemical behavior. Aniline yellow is often used in dyeing and is a reminder of how specific molecular structures can lead to distinctive and appealing colors.