Question

Phosphorus forms an extensive series of Oxoanions. (a) Draw a structure, and give the charge for an oxophosphate anion with the formula \(\left[\mathrm{P}_{4} \mathrm{O}_{13}\right]^{\mathrm{n}-} .\) How many ionizable \(\mathrm{H}\) atoms should the completely protonated acid have? (b) Draw a structure, and give the charge for a cyclic oxophosphate anion with the formula \(\left[\mathrm{P}_{4} \mathrm{O}_{12}\right]^{\mathrm{n}-} .\) How many ionizable H atoms should the completely protonated acid have?

Short answer

(a) [P4O13] has a charge of -6, accommodates 8 protons. (b) [P4O12] has a charge of -4, accommodates 5 protons.

Step-by-step solution

Analyze the Given Formula for Part (a)

For the anion \([\mathrm{P}_{4} \mathrm{O}_{13}]^{n-}\), we first need to determine its structural formula. The P-O bonds in phosphates can form branched structures. As the phosphorus typically forms five bonds, initially consider linking the four P atoms via oxygen bridges.

Draw the Structure for \\([\mathrm{P}_{4} \mathrm{O}_{13}]^{n-}\\)

A possible structure for this anion is with each phosphorus atom surrounded by four oxygen atoms, arranged as follows: three single bonds (with terminal or shared oxygens) and one double-bonded oxygen. Two oxygens act as bridges linking the P atoms. This results in two P atoms involved in P-O-P linkages. The structure closely resembles \(\mathrm{P}_4\mathrm{O}_{13}\).

Calculate the Charge for \\([\mathrm{P}_{4} \mathrm{O}_{13}]^{n-}\\)

Each phosphorus forms a total of 5 bonds. Consider the charges on typical oxygens: double-bonded oxygens are neutral; singly-bonded terminal oxygens each carry a -1 charge. Calculate the total anion charge by adding the charges on terminal oxygens.

Determine Ionizable H Atoms for \\([\mathrm{P}_{4} \mathrm{O}_{13}]^{n-}\\)

Combine the charges to get the overall anion charge. Since you have eight terminal oxygens, each can accommodate one ionizable hydrogen proton (H atom) directly converting the anion sites.

Analyze the Given Formula for Part (b)

For the anion \([\mathrm{P}_{4} \mathrm{O}_{12}]^{n-}\), determine the structure. This cyclic anion suggests a ring structure where each P is linked via bridging oxygens, forming a square or tetrahedral network.

Draw the Structure for \\([\mathrm{P}_{4} \mathrm{O}_{12}]^{n-}\\)

Arrange the four P atoms in a square with alternating oxygen bridges connecting them. Each P atom has a double-bonded oxygen and is attached to one terminal oxygen, creating the cyclic phosphate structure.

Calculate the Charge for \\([\mathrm{P}_{4} \mathrm{O}_{12}]^{n-}\\)

Consider the single-bonded terminal and bridging oxygens which contribute to the charge. Calculate based on the established framework (ionic charges for terminal oxygen atoms).

Determine Ionizable H Atoms for \\([\mathrm{P}_{4} \mathrm{O}_{12}]^{n-}\\)

Combine individual oxygens' roles within the anion to find that the cyclic structure gives five distinct terminal oxygens which can each be protonated, rendering the completely protonated acid.

Key concepts

Oxophosphate Anion

An oxophosphate anion is a type of phosphorus oxoanion, where phosphorus atoms are combined with oxygen atoms. In the context of the anion \([\mathrm{P}_{4} \mathrm{O}_{13}]^{n-}\), four phosphorus atoms are connected through bridging oxygens. Each phosphorus forms a total of five bonds, typically involving single and double bonds to oxygen.

The structure of these anions can get complex, as multiple oxygen atoms are shared across phosphorus centers. These shared oxygen "bridges" are what often form the intricate network observed in many phosphate structures. The connectivity and structure of the oxophosphate anion play a crucial role in determining its charge and reactiveness.

• Phosphorus atoms usually form five bonds.
• Bridging oxygens connect phosphorus atoms.
• The structure resembles a complex, often branched, network.

Protonation

Protonation refers to the addition of hydrogen ions (H+) to a molecule. In the case of phosphorus oxoanions like \([\mathrm{P}_{4} \mathrm{O}_{13}]^{n-}\), protonation involves adding H+ ions to the available negatively charged oxygen sites.

With each negative charge on the oxygen, a hydrogen proton can be added, neutralizing that charge. This process directly relates to the number of ionizable hydrogen atoms that the molecule can accommodate.

• Every singly-bonded, terminal oxygen can accept one proton.
• Complete protonation neutralizes the anion's charge.
• The extent of protonation depends on the structure of the anion.

Ionic Charge Calculation

Calculating the ionic charge of an oxophosphate anion involves understanding the charge contribution of each oxygen. Double-bonded oxygens are neutral, while terminal oxygens, often with single bonds, carry a -1 charge.

Using this information, the charge of the anion \([\mathrm{P}_{4} \mathrm{O}_{13}]^{n-}\) is determined by summing all charges from terminal oxygen atoms. For instance, in \([\mathrm{P}_{4} \mathrm{O}_{13}]^{n-}\), there are eight terminal oxygens contributing to the total charge.

• Identify single-bonded terminal oxygens.
• Each contributes a -1 charge.
• Sum these charges for the overall anion charge.

Cyclic Phosphate Structure

Cyclic phosphate structures, such as \([\mathrm{P}_{4} \mathrm{O}_{12}]^{n-}\), feature phosphorus atoms forming a closed loop or ring structure through bridging oxygens. This can create more rigid and stable configurations.

In these cyclic structures, each phosphorus atom still typically binds to a double-bonded oxygen and one terminal oxygen. This cyclic formation impacts not only the chemical properties but also the number of ionizable hydrogen atoms. This is due to how terminal oxygens are arranged and their availability to accept hydrogen ions.

• Phosphorus forms a closed loop with bridging oxygens.
• Stable and more rigid configurations.
• Terminal and double-bonded oxygens play distinct roles.