Problem 116 - Introductory Chemistry Atoms First

Question

(a) List all the weak bases phosphoric acid, \(\mathrm{H}_{3} \mathrm{PO}_{4}\), can produce via successive losses of its protons. (b) Of the bases you listed, which has no ability to serve as a weak acid? (c) Phosphorous acid, \(\mathrm{H}_{3} \mathrm{PO}_{3}\), is a diprotic acid, even though there are three \(\mathrm{H}\) atoms in the formula. Draw a Lewis dot diagram for phosphorous acid. (Hint: Only H bound directly to \(\mathrm{O}\) is acidic).

Step-by-Step Solution

Answer

The weak bases produced by the successive loss of protons from phosphoric acid are H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻. Among these, PO₄³⁻ has no ability to serve as a weak acid since it has no protons bound to it. The Lewis dot structure of H₃PO₃ is as follows: O || P - O - H | OH H₃PO₃ is a diprotic acid, as only the two hydrogen atoms bound to the oxygen atoms are acidic.

Step 1: Determine the weak bases produced by the successive loss of protons from phosphoric acid

By losing one, two, or three protons successively, phosphoric acid can form three different weak bases: 1. Loss of one proton: H₃PO₄ → H₂PO₄⁻ + H⁺ 2. Loss of two protons: H₂PO₄⁻ → HPO₄²⁻ + H⁺ 3. Loss of three protons: HPO₄²⁻ → PO₄³⁻ + H⁺ The weak bases produced are H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻.

Step 2: Determine the base with no ability to serve as a weak acid

Of the bases produced, PO₄³⁻ is the only base which cannot further lose protons, as it has no protons bound to it. Thus, PO₄³⁻ cannot serve as a weak acid.

Step 3: Draw a Lewis dot diagram for phosphorous acid (H₃PO₃)

To draw a Lewis dot diagram for H₃PO₃, we consider that there are three oxygen atoms, one phosphorus atom, and three hydrogen atoms. Remember that only H atoms bound directly to O atoms are acidic. The Lewis dot structure of H₃PO₃ is as follows: O || P - O - H | OH In H₃PO₃, two hydrogen atoms are directly bound to the oxygen atoms while one hydrogen atom is bound to the phosphorus atom. As a result, only the two hydrogen atoms bound to the oxygen atoms are acidic, making H₃PO₃ a diprotic acid.

Key Concepts

Acid-Base Equilibrium

Understanding acid-base equilibrium is crucial when studying chemistry, especially in the context of weak acids and their corresponding bases. Phosphoric acid, H₃PO₄, is a classic example of a polyprotic weak acid that can lose protons sequentially to form different weak bases. The equilibrium in an acid-base reaction describes the balance between the forward reaction (where the acid loses a proton) and the reverse reaction (where the conjugate base accepts a proton).

For phosphoric acid, this can be demonstrated as follows:

• First dissociation: H₃PO₄ ⇌ H₂PO₄⁻ + H⁺
• Second dissociation: H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺
• Third dissociation: HPO₄²⁻ ⇌ PO₄³⁻ + H⁺

In each step, a new equilibrium is established, with the anion produced in each step acting as a weak base. The PO₄³⁻ ion, lacking protons, is solely a base and cannot engage in the reverse reaction to donate a proton, hence it does not participate in acid-base equilibrium.

Lewis Dot Structure

The Lewis dot structure is a visual representation that illustrates the valence electrons of atoms within a molecule, which are important for understanding the bonding and shape of the molecule. These structures help predict the behavior of atoms in reactions, like the ability of an atom to donate or accept electrons. In the case of phosphorous acid (H₃PO₃), the structure is important for determining why it is a diprotic acid despite having three hydrogen atoms.

The drawing of the Lewis structure for H₃PO₃ reveals that not all hydrogen atoms are equivalent:

• The hydrogen atoms bound to oxygen are acidic and can be removed as protons (H⁺).
• The hydrogen atom bound to phosphorus does not contribute to acidity, as hydrogen-phosphorus bonds typically do not dissociate in water to form H⁺ ions.

This highlights why H₃PO₃ is classified as diprotic, even though the molecular formula might suggest it could be triprotic. To understand the reactivity and properties of molecules like phosphorous acid accurately, constructing and interpreting Lewis dot structures is essential.

Polyprotic Acids

Polyprotic acids are acids that are capable of donating more than one proton (H⁺) per molecule in a series of steps, each with its own equilibrium constant. The concept of polyprotic acids is significant when discussing acids like phosphoric acid (H₃PO₄), as these acids have multiple dissociation steps:

• The first proton loss turns phosphoric acid into dihydrogen phosphate (H₂PO₄⁻).
• The second one converts it into hydrogen phosphate (HPO₄²⁻).
• The third one leads to the formation of phosphate (PO₄³⁻).

Each step in the dissociation shows a different base being formed and a corresponding decrease in the acidity of the remaining anion. As it's mentioned in the exercise, H₃PO₄ yields weak bases upon losing protons. It's essential to understand that the strength of the conjugate base will typically increase as more protons are lost, with the final form (PO₄³⁻) being the weakest acid, thus not acting as an acid at all in water. In summary, polyprotic acids exhibit stepwise dissociation, which is key to their chemical behavior and interaction with bases.