Question
The main cause of basicity of oxoacids of phosphorous is:
The correct answer is
P–OH bonds
Understanding Basicity in Phosphorus Oxoacids
The basicity of an acid refers to the number of acidic hydrogen atoms that a molecule of the acid can donate in a reaction. For oxoacids of phosphorus, these acidic hydrogen atoms are specifically those attached to oxygen atoms via P–OH bonds.Structure of Phosphorus Oxoacids
Phosphorus oxoacids generally contain phosphorus atoms bonded to one or more oxygen atoms and one or more hydroxyl (OH) groups, and sometimes direct P–H bonds. The hydrogen atoms involved in the basicity are only those connected through the oxygen atom in a hydroxyl group (P–OH). Hydrogen atoms directly bonded to the phosphorus atom (P–H bonds) are typically not acidic and do not contribute to the basicity. Consider the structures of some common oxoacids of phosphorus:- Orthophosphoric acid ($\text{H}_3\text{PO}_4$): Contains one P=O double bond and three P–OH single bonds. It has no P–H bonds.
- Phosphorous acid ($\text{H}_3\text{PO}_3$): Contains one P=O double bond, two P–OH single bonds, and one P–H single bond.
- Hypophosphorous acid ($\text{H}_3\text{PO}_2$): Contains one P=O double bond, one P–OH single bond, and two P–H single bonds.
Why P–OH Bonds Determine Basicity
The hydrogen atom in a P–OH bond is bonded to an electronegative oxygen atom. This oxygen atom pulls electron density away from the hydrogen, making the P–O–H bond polar. This polarity weakens the O–H bond, allowing the hydrogen to dissociate as a proton ($\text{H}^{+}$) in aqueous solution. In contrast, the hydrogen atom in a P–H bond is bonded directly to the phosphorus atom. The electronegativity difference between phosphorus and hydrogen is small. Therefore, the P–H bond is less polar, and the hydrogen atom is not easily released as a proton. These P–H hydrogens are generally not acidic. The number of P–OH bonds in an oxoacid of phosphorus determines the number of acidic hydrogen atoms it possesses, and thus its basicity.Examples of Basicity based on P–OH Bonds
Let's look at the basicity of the common oxoacids:| Oxoacid | Formula | Number of P–OH bonds | Number of P–H bonds | Basicity |
|---|---|---|---|---|
| Orthophosphoric acid | $\text{H}_3\text{PO}_4$ | 3 | 0 | 3 (Tribasic) |
| Phosphorous acid | $\text{H}_3\text{PO}_3$ | 2 | 1 | 2 (Dibasic) |
| Hypophosphorous acid | $\text{H}_3\text{PO}_2$ | 1 | 2 | 1 (Monobasic) |
As the table shows, the basicity of these oxoacids is equal to the number of P–OH bonds they contain, not the total number of hydrogen atoms or the number of P–H bonds. Therefore, the main cause of the basicity of oxoacids of phosphorus is the presence of P–OH bonds, as these are the source of the dissociable acidic protons.
Revision Table: Phosphorus Oxoacid Basicity
| Concept | Explanation |
|---|---|
| Basicity of Acid | Number of dissociable $\text{H}^{+}$ ions. |
| Acidic Hydrogen Source | Hydrogens attached to electronegative atoms (like Oxygen) through polar bonds. |
| P–OH Bonds | Source of acidic hydrogens in Phosphorus oxoacids. |
| P–H Bonds | Generally not acidic in Phosphorus oxoacids. |
| Basicity Value | Equals the number of P–OH bonds. |
Additional Information on Phosphorus Oxoacids
- The phosphorus atom in oxoacids is typically in a tetrahedral environment, bonded to oxygen atoms.
- The oxidation state of phosphorus in these oxoacids varies: +5 in $\text{H}_3\text{PO}_4$, +3 in $\text{H}_3\text{PO}_3$, and +1 in $\text{H}_3\text{PO}_2$. However, the basicity is determined by the structure (number of P–OH bonds), not the oxidation state or the total number of hydrogens.
- The acidity of the P–OH proton can be influenced by other atoms or groups bonded to the phosphorus, but the fundamental requirement for acidity is being bonded to oxygen.
- Hydrogen bonds can occur between oxoacid molecules, but they are intermolecular forces and do not cause the dissociation of protons within a single acid molecule; thus, they do not determine basicity.
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Important Questions from p-block Elements
Second most abundant element in alloy misch metal is:
Match List-I with List-II:
List-I List-II (A) Gel (I) Hair cream (B) Foam (II) Dust (C) Emulsion (III) Cheese (D) Aerosol (IV) Whipped cream Choose the correct answer from the options given below:
Rate of a reaction changes from 2.48 × 10⁻³ mol⁻¹ sec⁻¹ to 4.96 × 10⁻³ mol⁻¹ sec⁻¹ when concentration of reactant is changed from 0.6 M to 2.4 M respectively, the order of reaction is:
Degree of dissociation, when molar conductivity of X at its concentration C is 24.14 and its limiting molar conductivity is 48.28 will be:
A divalent ion of 'V' (Atomic no. 23) in aqueous solution is:
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