Question

(a) \(\mathrm{SbCl}_{3}\) may be used as a non-aqueous solvent above its melting point. Suggest a possible self-ionization process for this solvent. (b) Explain why the reaction of NOCl with \(\mathrm{AgNO}_{3}\) in liquid \(\mathrm{N}_{2} \mathrm{O}_{4}\) can be classed as a neutralization process. Write an equation for the reaction and compare it with that of \(\mathrm{HCl}\) with \(\mathrm{Ca}(\mathrm{OH})_{2}\) in aqueous solution. (c) In water, \(\mathrm{Cr}^{3+}\) precipitates as \(\mathrm{Cr}(\mathrm{OH})_{3}\) at \(\mathrm{pH} 7,\) forms \(\left[\mathrm{Cr}\left(\mathrm{OH}_{2}\right)_{6}\right]^{3+}\) in strongly acidic solution (e.g. \(\mathrm{HClO}_{4}\) ), and \(\left[\mathrm{Cr}(\mathrm{OH})_{4}\right]^{-}\) in basic solution. Suggest what \(\mathrm{Cr}(\mathrm{III})\) species are present in liquid \(\mathrm{NH}_{3}\) as the \(\mathrm{pH}\) is varied.

Short answer

(a) Self-ionizes to \( \mathrm{SbCl}_2^+ + \mathrm{SbCl}_4^- \). (b) NOCl + AgNO3 yields AgCl, akin to HCl with Ca(OH)2. (c) Forms \( \left[ \mathrm{Cr(NH}_3)_6 \right]^{3+} \) in acidic and amido complexes in basic conditions.

Step-by-step solution

Understanding Self-ionization of SbCl3

When \ \( \mathrm{SbCl}_3 \ \) is used as a solvent, it can undergo self-ionization. A typical self-ionization process for a solvent involves the solvent molecules ionizing into cations and anions. In the case of \ \( \mathrm{SbCl}_3 \ \), the self-ionization can be represented by \ \[ 2 \mathrm{SbCl}_3 \rightarrow \mathrm{SbCl}_2^+ + \mathrm{SbCl}_4^- \].\ The \ \( \mathrm{SbCl}_2^+ \ \) cation can be thought of as a Lewis acid and \ \( \mathrm{SbCl}_4^- \ \) as a corresponding Lewis base.

Understanding Neutralization in N2O4

Neutralization reactions typically involve the reaction of an acid with a base to produce a salt and water or similar byproducts. Nitrosyl chloride (\ \( \mathrm{NOCl} \ \)) can act as an acid while silver nitrate (\ \( \mathrm{AgNO}_3 \ \)) can act as a source of base in certain non-aqueous solvents such as \ \( \mathrm{N}_2 \mathrm{O}_4 \ \). The reaction can be represented as \ \[ \mathrm{NOCl} + \mathrm{AgNO}_3 \rightarrow \mathrm{AgCl} + \mathrm{NO}_2^+ \].\ This is analogous to the neutralization of \ \( \mathrm{HCl} \ \) with \ \( \mathrm{Ca(OH)}_2 \ \) in water: \ \( 2 \mathrm{HCl} + \mathrm{Ca(OH)}_2 \rightarrow \mathrm{CaCl}_2 + 2 \mathrm{H}_2\mathrm{O} \).

Cr3+ Speciation in NH3

The species of \ \( \mathrm{Cr}^{3+} \ \) in liquid ammonia can be predicted based on the ammonia's similar behavior to water as a protic solvent. In a basic environment, analogous to the formation of \ \( \left[ \mathrm{Cr(OH)}_4 \right]^- \ \) in basic aqueous solutions, we could expect the formation of analogous Cr(III) amines. Similarly, in an acidic environment, chromium might form analogous complexes to \ \( \left[ \mathrm{Cr(OH}_2)_6 \right]^{3+} \ \). These include potential species like \ \( \left[ \mathrm{Cr(NH}_3)_6 \right]^{3+} \ \) under acidic conditions and amido complexes in basic conditions.

Key concepts

Self-Ionization

In the world of inorganic chemistry, self-ionization is an exciting concept that occurs when a solvent molecule dissociates into cations and anions without any external influence. This process is crucial for understanding the behavior of certain compounds as solvents. Let's focus on antimony trichloride (\( \mathrm{SbCl}_3 \)) as an example. When used as a non-aqueous solvent, \( \mathrm{SbCl}_3 \) undergoes self-ionization, which can be represented by the equation: \[ 2 \mathrm{SbCl}_3 \rightarrow \mathrm{SbCl}_2^+ + \mathrm{SbCl}_4^- \].Here, the \( \mathrm{SbCl}_2^+ \) acts as a cation, while \( \mathrm{SbCl}_4^- \) functions as an anion. These ions can interact with other molecules, facilitating various chemical reactions. The self-ionization of solvents like \( \mathrm{SbCl}_3 \) is pivotal in understanding how they can act in different chemical environments, influencing both their reactivity and their ability to dissolve other substances effectively.

Neutralization Reaction

Neutralization reactions are a cornerstone of acid-base chemistry, describing the process where an acid and a base react to form a salt and usually water. However, what's interesting is that these reactions can occur in non-aqueous solutions as well. For instance, in liquid \( \mathrm{N}_2\mathrm{O}_4 \), nitrosyl chloride (\( \mathrm{NOCl} \)) acts as an acid, and silver nitrate (\( \mathrm{AgNO}_3 \)) provides the base. When these react, the equation looks like this: \[ \mathrm{NOCl} + \mathrm{AgNO}_3 \rightarrow \mathrm{AgCl} + \mathrm{NO}_2^+ \].This is similar in concept to how hydrochloric acid (\( \mathrm{HCl} \)) neutralizes calcium hydroxide (\( \mathrm{Ca(OH)}_2 \)) in water: \[ 2 \mathrm{HCl} + \mathrm{Ca(OH)}_2 \rightarrow \mathrm{CaCl}_2 + 2 \mathrm{H}_2\mathrm{O} \].In these reactions, the acid donates protons (or equivalents in non-aqueous solutions), and the base accepts them. This transfer defines neutralization and is vital for creating many chemical substances safely and predictably, even outside of water.

Metal Speciation

Understanding metal speciation is crucial in predicting the forms a metal can take in various chemical environments. In the case of chromium (\( \mathrm{Cr}^{3+} \)), its behavior is significantly influenced by the acidity or basicity of its environment. For example, in water, at \( \mathrm{pH} 7 \), chromium precipitates as \( \mathrm{Cr(OH)}_3 \). In strongly acidic solutions, it forms a complex like \( \left[\mathrm{Cr(OH}_2)_6\right]^{3+} \), while in basic conditions, it turns into \( \left[\mathrm{Cr(OH)}_4\right]^{-} \).If we consider a different solvent like liquid ammonia, which behaves similarly to water but is less protic, the speciation of \( \mathrm{Cr}^{3+} \) can vary. In acidic conditions, chromium can form ammonia complexes like \( \left[\mathrm{Cr(NH}_3)_6\right]^{3+} \). Conversely, in a more basic environment, we might see the formation of amido complexes. These transformations underscore the importance of speciation in predicting reaction outcomes and the stability of metal ions in non-aqueous solutions as well.