Question

Phosphoric acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)\) is a triprotic acid, phosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{3}\right)\) is a diprotic acid, and hypophosphorous acid \(\left(\mathrm{H}_{3} \mathrm{PO}_{2}\right)\) is a monoprotic acid. Explain this phenomenon.

Short answer

Phosphoric acid (H₃PO₄) is a triprotic acid because it has three hydrogen atoms bonded to oxygen atoms and can donate three protons during ionization. Phosphorous acid (H₃PO₃) is a diprotic acid because it has two hydrogen atoms bonded to oxygen atoms, and can donate only two protons during ionization. Hypophosphorous acid (H₃PO₂) is a monoprotic acid because it has only one hydrogen atom bonded to an oxygen atom, allowing it to donate only one proton during ionization. The different protic behavior depends on the number of ionizable hydrogen atoms bonded to oxygen in each acid.

Step-by-step solution

Analyze the structure of phosphoric acid (H₃PO₄)

Phosphoric acid has the chemical formula H₃PO₄, and its structure is as follows: \[ \ce{H3PO4 <=> H2PO4^{-} <=> HPO4^{2-} <=> PO4^{3-}} \] In this structure, there are three hydrogen atoms bonded to the oxygen atoms, which can potentially be ionized (donated).

Analyze the structure of phosphorous acid (H₃PO₃)

Phosphorous acid has the chemical formula H₃PO₃ and consists of the following structure: \[\ce{H3PO3 <=> H2PO3^{-} <=> HPO3^{2-}}\] In phosphorous acid, there are two hydrogen atoms bonded to the oxygen atoms, which can be ionized. One hydrogen atoms is attached to the phosphorus atom and cannot be donated as it doesn't hold the acidic properties we need.

Analyze the structure of hypophosphorous acid (H₃PO₂)

Hypophosphorous acid has the chemical formula H₃PO₂ and has this structure: \[\ce{H3PO2 <=> H2PO2^{-}}\] In this acid, there is only one hydrogen atom bonded to an oxygen atom that can be ionized. The other two hydrogen atoms are bonded directly to the phosphorus atom and cannot be donated as protons.

Explain the phenomenon of different protic behaviors

Phosphoric acid (H₃PO₄) has three hydrogen atoms bonded to oxygen atoms, which means it can donate three protons when it ionizes, therefore it is a triprotic acid. Phosphorous acid (H₃PO₃) has only two hydrogen atoms bonded to oxygen atoms, so it can only donate two protons during ionization, making it a diprotic acid. ` Hypophosphorous acid (H₃PO₂) has only one hydrogen atom bonded to an oxygen atom, so it can donate only one proton during ionization, and thus it is considered a monoprotic acid. The variation in the number of ionizable hydrogen atoms bonded to oxygen in the different acids are the reason behind their different protic behavior.

Key concepts

Triprotic Acid

Triprotic acids are unique compounds capable of releasing three hydrogen ions, or protons, in a solution. A well-known example of a triprotic acid is phosphoric acid, given by the chemical formula \(H_3PO_4\). This characteristic is because, in phosphoric acid, all three hydrogen atoms are attached to oxygen atoms. In solutions, these acids can generally ionize in three separate steps:

• The first ionization, losing one hydrogen ion.
• The second ionization, releasing another hydrogen ion.
• The third ionization, releasing the final hydrogen ion.

This ability allows triprotic acids to influence the acidity of solutions significantly as they can lower the pH multiple times with each ionization step. Understanding the stepwise ionization process is crucial in topics such as buffer solutions and specific pH adjustments for chemical reactions.

Diprotic Acid

Diprotic acids, such as phosphorous acid \(H_3PO_3\), can donate two protons during ionization. Although the molecular formula might suggest the potential for three ionizations, in reality, only two hydrogen atoms are attached to oxygen atoms and thus capable of dissociating. Here's how the ionization works:

• First ionization: the first hydrogen atom dissociates from the molecule.
• Second ionization: the subsequent hydrogen ion is released.

The third hydrogen atom is bonded directly to the central phosphorus atom, forfeiting the ability to ionize as it does not possess acidic characteristics. Diprotic acids are essential in reactions that require medium-level acidity, often participating in processes like dissociation step progressions and buffer creation wherein two protons gradually elicit reactions.

Monoprotic Acid

Monoprotic acids are straightforward because they can only donate one proton per molecule. Hypophosphorous acid, \(H_3PO_2\), exemplifies this group. Despite having three hydrogen atoms in its formula, only one is bonded to oxygen and thus ionizable. Here's how monoprotic acid ionization can be understood:

• In hypophosphorous acid, one hydrogen atom, connected to oxygen, dissociates.
• The other two hydrogen atoms are attached to the phosphorus atom and are non-ionizable.

These acids are particularly important in scenarios that only necessitate a minimal acidity adjustment within a chemical reaction. Monoprotic acids offer clear and predictable behavior, which makes them frequently preferred in laboratory and industrial applications requiring controlled acidity.

Acid Ionization

Acid ionization is a process where an acid donates protons to the solution, usually water, thereby increasing the concentration of hydrogen ions \([H^+]\). The concept is critical across chemistry, influencing pH levels and reactivity. Acid ionization can be shown in a sequence of reactions, particularly for polyprotic acids:

• Primary ionization: the initial hydrogen ion is released.
• Successive ionizations: further hydrogen ions are released, specific to the acid's proticity (monoprotic, diprotic, triprotic).

For instance, phosphoric acid undergoes three stages of ionization because it contains three ionizable hydrogen atoms. Each step's strength is often different, leading to a diminishing release tendency. Understanding ionization is pertinent for fields like chemical equilibrium and analyte calibration, as it helps predict how solutions will behave under varying conditions.

Chemical Structure Analysis

Understanding the chemical structure of acids like phosphoric, phosphorous, and hypophosphorous acids is key in determining their acidic behavior. Structure plays a crucial role in whether a hydrogen atom can be ionized:

• In compounds where hydrogens bond to oxygen, hydrogen ions freely dissociate due to the polar nature of the \(O-H\) bond.
• Conversely, hydrogen atoms directly attached to central atoms, like phosphorus, remain non-ionizable.

This analysis explains why despite having similar molecular formula bases, acids can exhibit different protic behaviors. A proper grasp of chemical structure assists in predicting reactivity and acid strength, contributing significantly to complex chemical reaction predictions and the synthesis of novel compounds. Chemical structure analysis effectively acts as the blueprint for understanding acid ionization potentials and protic capabilities.