Problem 32 - Chemistry and Chemical Reactivity

Question

Phosphoric acid can supply one, two, or three \(\mathrm{H}_{3} \mathrm{O}^{+}\) ions in aqueous solution. Write balanced equations (like those for sulfuric acid on page 142 ) to show this successive loss of hydrogen ions.

Step-by-Step Solution

Answer

Phosphoric acid dissociates in three steps to form \( \text{H}_2\text{PO}_4^- \), \( \text{HPO}_4^{2-} \), and \( \text{PO}_4^{3-} \) ions, respectively.

Step 1: Write the Formula of Phosphoric Acid

Phosphoric acid is represented by the chemical formula \( \text{H}_3\text{PO}_4 \). This compound can release hydrogen ions \( \text{H}^+ \) in a stepwise manner.

Step 2: First Ionization

In the first ionization step, phosphoric acid loses one hydrogen ion to form dihydrogen phosphate ion:\[ \text{H}_3\text{PO}_4 (aq) \rightarrow \text{H}_2\text{PO}_4^- (aq) + \text{H}^+ (aq) \]

Step 3: Second Ionization

The dihydrogen phosphate ion formed in the first ionization step loses another hydrogen ion to form hydrogen phosphate ion:\[ \text{H}_2\text{PO}_4^- (aq) \rightarrow \text{HPO}_4^{2-} (aq) + \text{H}^+ (aq) \]

Step 4: Third Ionization

Finally, the hydrogen phosphate ion loses a third hydrogen ion to become phosphate ion:\[ \text{HPO}_4^{2-} (aq) \rightarrow \text{PO}_4^{3-} (aq) + \text{H}^+ (aq) \]

Key Concepts

Ionization Process

Every acid has a certain capability of releasing hydrogen ions when dissolved in water. This ability is referred to as the ionization process of an acid. The more ions that are released, the more ionized the acid becomes, and its strength increases. In the context of phosphoric acid (H₃PO₄), it undergoes a stepwise ionization.

This means it releases hydrogen ions in stages. Each stage sees one hydrogen ion released, starting from H₃PO₄ and eventually turning into phosphate ions. Let's break down its ionization:

• First Ionization: This step releases one H⁺ ion, creating dihydrogen phosphate (H₂PO₄⁻).
• Second Ionization: The next H⁺ ion is released, forming hydrogen phosphate (HPO₄²⁻).
• Third Ionization: Finally, the last H⁺ ion is liberated, and a phosphate (PO₄³⁻) ion is produced.

Each ionization produces an additional negative charge in the compound due to the loss of a positively charged hydrogen ion.

Phosphoric Acid

Phosphoric acid, known chemically as H₃PO₄, is a weak triprotic acid.

This means it can donate three protons (or hydrogen ions) in an aqueous solution. Triprotic acids have the ability to lose three hydrogen atoms over a series of ionization steps, making the degree of their ionization quite significant in acid-base chemistry.

• It's used commonly in fertilizers, cleaning products, and even food additives.
• The weak nature of the acid implies that it does not fully ionize in a solution like strong acids (i.e., sulfuric acid).
• The multiple ionization steps occurring in phosphoric acid are what make it a unique acid in the study of chemical reactions.

Understanding its behavior in solution is crucial for those studying chemistry because it plays a major role in various chemical processes and functions in biological systems.

Chemical Equations

Chemical equations are symbolic representations of chemical reactions. They show the reactants and products involved, as well as their physical states (such as aqueous or solid).

Balancing these equations is very important because it ensures that the same number of atoms is present on both sides of the equation. This satisfies the law of conservation of mass, where mass is neither created nor destroyed in a chemical reaction.
With phosphoric acid's ionization process, balanced chemical equations illustrate stepwise ionization:

• First equation: \[ \text{H}_3\text{PO}_4 (aq) \rightarrow \text{H}_2\text{PO}_4^- (aq) + \text{H}^+ (aq) \]
• Second equation: \[ \text{H}_2\text{PO}_4^- (aq) \rightarrow \text{HPO}_4^{2-} (aq) + \text{H}^+ (aq) \]
• Third equation: \[ \text{HPO}_4^{2-} (aq) \rightarrow \text{PO}_4^{3-} (aq) + \text{H}^+ (aq) \]

Each step of ionization is represented with its own chemical equation, allowing students to visualize the individual changes that take place at each stage of the reaction.