Problem 16 - Chemistry and Chemical Reactivity

Question

Phosphoric acid can supply one, two, or three \(\mathrm{H}^{+}\) ions in aqueous solution. Write balanced equations (like those for sulfuric acid on page 186 ) to show this successive loss of hydrogen ions.

Step-by-Step Solution

Answer

Phosphoric acid ionizes in three steps: 1) \( \text{H}_3\text{PO}_4 \to \text{H}_2\text{PO}_4^- + \text{H}^+ \), 2) \( \text{H}_2\text{PO}_4^- \to \text{HPO}_4^{2-} + \text{H}^+ \), 3) \( \text{HPO}_4^{2-} \to \text{PO}_4^{3-} + \text{H}^+ \).

Step 1: Write the Formula for Phosphoric Acid

Begin by writing the chemical formula for Phosphoric acid, which is \( \text{H}_3\text{PO}_4 \). This formula indicates that phosphoric acid can potentially donate up to three hydrogen ions (\( \text{H}^+ \)).

Step 2: First Ionization

In the first ionization step, phosphoric acid loses one hydrogen ion. The balanced chemical equation for this process is: \[ \text{H}_3\text{PO}_4 \longrightarrow \text{H}_2\text{PO}_4^- + \text{H}^+ \]

Step 3: Second Ionization

During the second ionization step, the \( \text{H}_2\text{PO}_4^- \) ion further loses another hydrogen ion to form \( \text{HPO}_4^{2-} \). The balanced equation is: \[ \text{H}_2\text{PO}_4^- \longrightarrow \text{HPO}_4^{2-} + \text{H}^+ \]

Step 4: Third Ionization

In the final ionization step, the \( \text{HPO}_4^{2-} \) ion loses its last hydrogen ion to become \( \text{PO}_4^{3-} \). The equation for this step is: \[ \text{HPO}_4^{2-} \longrightarrow \text{PO}_4^{3-} + \text{H}^+ \]

Step 5: Review the Ionization Steps

Review each of the ionization steps to ensure the balance of charges and atoms. In each step, a hydrogen ion is released, reflecting the ability of phosphoric acid to lose up to three \( \text{H}^+ \) ions sequentially.

Key Concepts

Phosphoric Acid

Phosphoric acid is a common and important chemical in both organic and inorganic chemistry. Its chemical formula is \( \text{H}_3\text{PO}_4 \). This formula reveals that each molecule contains three hydrogen atoms, one phosphorus atom, and four oxygen atoms. Phosphoric acid is known for its versatility. It behaves as a triprotic acid, meaning it can donate up to three hydrogen ions, depending on the conditions. This characteristic is what makes phosphoric acid unique among acids.
In the context of ionization, phosphoric acid can donate one hydrogen ion at a time. This results in successive ionization steps where it forms different ions, like \( \text{H}_2\text{PO}_4^- \), \( \text{HPO}_4^{2-} \), and \( \text{PO}_4^{3-} \). Because of this stepwise release of hydrogen ions, phosphoric acid is used in various applications, such as food flavoring, dental products, and fertilizers. Understanding how it ionizes helps predict its behavior and functionality in different chemical environments.

Balanced Chemical Equations

A balanced chemical equation represents a chemical reaction in which the total mass of all the reactants equals the total mass of the products. In chemical reactions, atoms are neither created nor destroyed. This is why balancing equations is crucial. For phosphoric acid, it can lose up to three hydrogen ions through successive reactions.
A balanced equation must have equal numbers of each type of atom on both sides of the equation.

• For the first ionization, phosphoric acid (\( \text{H}_3\text{PO}_4 \)) loses one \( \text{H}^+ \) ion to become \( \text{H}_2\text{PO}_4^- \).
• In the second ionization step, \( \text{H}_2\text{PO}_4^- \) loses another \( \text{H}^+ \), resulting in \( \text{HPO}_4^{2-} \).
• Lastly, in the third ionization step, \( \text{HPO}_4^{2-} \) loses the final \( \text{H}^+ \), forming \( \text{PO}_4^{3-} \).

Each reaction should be checked to ensure atoms and charges are balanced. Understanding balanced chemical equations is essential, not only for clarity and accuracy but also for understanding how chemical reactions take place in real life.

Hydrogen Ions

Hydrogen ions (\( \text{H}^+ \)) play a critical role in understanding acids. They are what make a substance acidic and are central to the concept of pH. When an acid such as phosphoric acid donates a hydrogen ion, it increases the concentration of \( \text{H}^+ \) ions in a solution, making the solution more acidic.
In the ionization process of phosphoric acid:

• In the first step, one hydrogen ion is released from \( \text{H}_3\text{PO}_4 \), resulting in \( \text{H}_2\text{PO}_4^- \).
• In the subsequent ionizations, additional hydrogen ions are released from intermediate ions \( \text{H}_2\text{PO}_4^- \) and \( \text{HPO}_4^{2-} \), respectively.

These ions influence the acidity of solutions, which can be measured using pH, where a lower pH signals higher acidity due to more \( \text{H}^+ \) ions. Understanding the role of hydrogen ions aids in grasping broader concepts related to acid-base chemistry, including buffer systems and titrations.