Question

Discuss the following: (a) Many actinoid oxides are non-stoichiometric, but few lanthanoid oxides are. (b) The ion \(\left[\mathrm{NpO}_{6}\right]^{5-}\) can be made in aqueous solution only if the solution is strongly alkaline. (c) A solution containing Pu(IV) undergoes negligible disproportionation in the presence of an excess of \(\operatorname{molar} \mathrm{H}_{2} \mathrm{SO}_{4}\).

Short answer

(a) Actinoid oxides show more non-stoichiometry due to variable valency than lanthanoids. (b) [NpO_6]^{5-} is stable in strong alkaline solutions. (c) Excess H_2SO_4 stabilizes Pu(IV), preventing disproportionation.

Step-by-step solution

Understand Stoichiometry in Actinoid and Lanthanoid Oxides

Non-stoichiometric compounds are materials where the proportions of elements cannot be described by simple integer ratios. Many actinoid oxides show non-stoichiometry because their compounds have variable oxidation states allowing for a range of compositions. In contrast, lanthanoids have more stable oxidation states, typically +3, making stoichiometric compositions more common.

Analyze Conditions for Stability of [NpO_6]^{5-} Ion

The ion [NpO_6]^{5-} requires strong alkaline conditions to be stable in aqueous solution. In strong alkaline environments, high oxidation state actinoids are stabilized. Alkaline conditions help stabilize the anionic species, which might decompose or disproportionate under more acidic conditions.

Examine Disproportionation of Pu(IV) Under Acidic Conditions

Disproportionation is a redox reaction where a single substance is simultaneously oxidized and reduced. Pu(IV) tends to undergo this process; however, when in the presence of excess molar H_2SO_4, the acidity stabilizes Pu(IV), preventing the disproportionation. The high concentration of acid provides a consistent ionic environment that minimizes the tendency of Pu to change oxidation states.

Key concepts

Non-stoichiometric Compounds

Non-stoichiometric compounds are fascinating materials where the elements involved do not line up in simple integer ratios. In the world of chemistry, actinoid oxides like \( ext{UO}_2\) and \( ext{ThO}_2\) often exhibit non-stoichiometry. This means that the exact number of each type of atom can vary slightly, giving the material unique properties. This happens because actinoids can show a range of oxidation states. Rather like a chameleon, these compounds adapt their composition. They can incorporate a variable number of oxygen atoms without a fixed stoichiometric balance.
In contrast, lanthanoid oxides, such as \( ext{La}_2 ext{O}_3\), are usually stoichiometric. Lanthanoids typically have a stable oxidation state of +3, which means their compounds stick more faithfully to fixed ratios of their component atoms.

• Actinoids: Variable oxidation states lead to variable stoichiometry.
• Lanthanoids: Consistent +3 oxidation state results in stoichiometric compounds.

Oxidation States

Oxidation states are crucial in understanding how elements interact in compounds. They represent the hypothetical charges that atoms would have if all bonds were fully ionic. Actinoids can exhibit a range of oxidation states, often ranging from +3 to +6. This versatility in oxidation states allows actinoids to form non-stoichiometric compounds and engage in complex redox chemistry.
For example, in the ion \([ ext{NpO}_6]^{5-}\), neptunium exists in a high oxidation state. This has an important impact on its chemistry, requiring specific conditions, such as a strongly alkaline solution, to stabilize it.

• Actinoids: Complex redox chemistry due to variable oxidation states.
• High oxidation states can be stabilized in specific conditions like alkaline media.

Alkaline Conditions

Alkaline conditions refer to environments where the pH is above 7, often due to the presence of bases such as \( ext{NaOH}\). When dealing with high oxidation state actinoids, strong alkaline conditions are crucial. This is because they help stabilize highly oxidized forms, preventing them from reacting or decomposing.
For instance, the ion \( ext{NpO}_6^{5-}\) is stable in strongly alkaline aqueous solutions. In these conditions, the environment supports the maintenance of the high oxidation states by reducing the risk of disproportionation or reaction with other ions.

• Necessary for stabilizing high oxidation states.
• Prevents decomposition or unwanted reactions.

Disproportionation

Disproportionation is a special type of redox reaction where a single element undergoes both oxidation and reduction simultaneously. This might sound unusual, but it's quite common and interesting in actinoids like plutonium. For instance, Pu(IV) can disproportionate to form Pu(III) and Pu(V).
However, this process can be minimized by providing a stabilizing environment, such as a strong acidic solution. In the case of Pu(IV), adding excess molar \( ext{H}_2 ext{SO}_4\) can effectively prevent disproportionation. The acidity provides an environment that keeps the Pu(IV) ion stable and mitigates its tendency to change to other oxidation states.

• Redox process where the element is both oxidized and reduced.
• Pu(IV) stabilized with high concentrations of acids.

Stability of Ions in Solution

The stability of ions in solution is a key concept in chemistry, especially when dealing with transition metals and actinoids. The chemical environment, such as pH level or ionic strength, heavily influences ion stability. For highly oxidized actinoid ions, the solution must provide conditions that prevent the ions from undergoing unwanted reactions or decomposing.
Take the ion \([ ext{NpO}_6]^{5-}\) as an example. To remain stable in an aqueous environment, it requires a strongly alkaline pH, which helps stabilize the ion and reduce its reactive tendencies. By carefully manipulating the solution's pH and composition, scientists can control the stability and reactivity of ions in a precise manner.

• Stability is influenced by solution pH and ionic strength.
• Specific conditions prevent decomposition or unwanted reactions.