Problem 24 - 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 130 ) to show this successive loss of hydrogen ions.

Step-by-Step Solution

Answer

Phosphoric acid dissociates as follows: \(H_3PO_4 \rightarrow H_2PO_4^- \rightarrow HPO_4^{2-} \rightarrow PO_4^{3-}\).

Step 1: Write the first dissociation equation

Phosphoric acid (\(H_3PO_4\)) first loses one hydrogen ion to form dihydrogen phosphate (\(H_2PO_4^-\)) and a hydronium ion (\(H_3O^+\)). The balanced equation for the first dissociation is: \[ H_3PO_4 (aq) + H_2O (l) \rightarrow H_2PO_4^- (aq) + H_3O^+ (aq) \]

Step 2: Write the second dissociation equation

Dihydrogen phosphate (\(H_2PO_4^-\)) then loses another hydrogen ion to form hydrogen phosphate (\(HPO_4^{2-}\)) and another hydronium ion (\(H_3O^+\)). The balanced equation for the second dissociation is: \[ H_2PO_4^- (aq) + H_2O (l) \rightarrow HPO_4^{2-} (aq) + H_3O^+ (aq) \]

Step 3: Write the third dissociation equation

Finally, hydrogen phosphate (\(HPO_4^{2-}\)) loses its third hydrogen ion to form phosphate (\(PO_4^{3-}\)) and another hydronium ion (\(H_3O^+\)). The balanced equation for the third dissociation is: \[ HPO_4^{2-} (aq) + H_2O (l) \rightarrow PO_4^{3-} (aq) + H_3O^+ (aq) \]

Key Concepts

Phosphoric Acid

Phosphoric acid is a commonly known tri-protic acid, meaning it can donate three hydrogen ions ( H^+ ) in sequential steps. It is a vital component that plays an essential role in both industry and biology.

• In aqueous solutions, phosphoric acid exists as a molecule that can dissociate to release hydrogen ions.
• The process involves losing its hydrogen ions one by one, reacting with water each time, leading to the formation of different anion species.

Understanding its behavior in water is crucial as it impacts the acidity and pH levels of the solution. This dissociation process makes phosphoric acid useful for adjusting pH values and is widely used in fertilizers, detergents, and food flavoring.

Hydronium Ion

The hydronium ion ( H_3O^+ ) is a crucial player in acid-base chemistry.

• When an acid dissociates in water, it donates a proton ( H^+ ) to a water molecule ( H_2O ), forming the hydronium ion.
• It is a key indicator of acidity, with higher concentrations of hydronium ions translating to a lower pH.

In the context of phosphoric acid, each dissociation step forms a hydronium ion, contributing to the increasing acidity of the solution. Understanding hydronium ions helps explain how acids function in solutions and their effect on chemical reactions, which is fundamental in fields ranging from biochemistry to industrial applications.

Chemical Equations

Chemical equations offer a shorthand representation of chemical reactions. They show how molecules rearrange to form new substances.

• In the context of phosphoric acid, each dissociation step is depicted with a balanced chemical equation.
• These equations highlight the transformation from one chemical species to another, such as from phosphoric acid to dihydrogen phosphate, and so on.

Balanced chemical equations are crucial for ensuring the conservation of mass and charge. In these equations, the number of atoms of each element and the charge are the same on both sides of the equation. This principle is fundamental for understanding stoichiometry and predicting the amounts of reactants and products in a reaction.