Problem 6 - Chemistry and Chemical Reactivity

Question

Write balanced equations showing how the HPO \(_{4}^{2-}\) ion of sodium hydrogen phosphate, \(\mathrm{Na}_{2} \mathrm{HPO}_{4},\) can be a Bronsted acid or a Bronsted base.

Step-by-Step Solution

Answer

HPO₄²⁻ acts as an acid: HPO₄²⁻ → PO₄³⁻ + H⁺, and as a base: HPO₄²⁻ + H⁺ → H₂PO₄⁻.

Step 1: Understanding the Problem

We need to understand that sodium hydrogen phosphate, \({Na}_2{HPO}_4\), contains the \(HPO_4^{2-}\) ion, which can behave as a Bronsted acid or a Bronsted base. A Bronsted acid donates a proton (\(H^+\)), while a Bronsted base accepts a proton.

Step 2: Identifying as Bronsted Acid

When \(HPO_4^{2-}\) acts as a Bronsted acid, it donates a proton to form \(PO_4^{3-}\). The balanced equation for this reaction is: \[\begin{align*} HPO_4^{2-} & \rightarrow PO_4^{3-} + H^+ \end{align*}\]This shows the loss of a proton, resulting in a phosphate ion with a \(3-\) charge.

Step 3: Identifying as Bronsted Base

When \(HPO_4^{2-}\) acts as a Bronsted base, it accepts a proton to form \(H_2PO_4^{-}\). The balanced equation for this reaction is: \[\begin{align*} HPO_4^{2-} + H^+ & \rightarrow H_2PO_4^{-} \end{align*}\]This illustrates the gain of a proton, resulting in a dihydrogen phosphate ion with a \(1-\) charge.

Key Concepts

Sodium Hydrogen Phosphate

Sodium hydrogen phosphate is a chemical compound represented by the formula \( \mathrm{Na}_2 \mathrm{HPO}_4 \). It contains the \( \mathrm{HPO}_4^{2-} \) ion, which plays a crucial role in the Bronsted acid-base theory.
This theory explains how substances can either donate or accept protons \((H^+)\) during chemical reactions. In the context of the exercise, sodium hydrogen phosphate can behave as both a Bronsted acid and a Bronsted base.
This dual capability is due to the presence of the \( \mathrm{HPO}_4^{2-} \) ion. Understanding this ion's behavior helps in predicting how it interacts with other chemical species.

Proton Donation

Proton donation is a key aspect of the Bronsted acid-base theory. It refers to the ability of a substance to donate a proton \((H^+)\) to another substance.
When \( \mathrm{HPO}_4^{2-} \) acts as a Bronsted acid, it undergoes proton donation.The reaction is represented by the equation:\[ \mathrm{HPO}_4^{2-} \rightarrow \mathrm{PO}_4^{3-} + \mathrm{H}^+ \]- This equation shows that the \( \mathrm{HPO}_4^{2-} \) ion releases a proton, forming the \( \mathrm{PO}_4^{3-} \) ion.
- As a result, the charge on the ion changes from \(2-\) to \(3-\), indicating the loss of a proton. Proton donation is important in chemical reactions as it affects the behavior and characteristics of substances involved.

Proton Acceptance

Proton acceptance is the opposite function in the Bronsted acid-base interaction. \( \mathrm{HPO}_4^{2-} \) can also act as a Bronsted base by accepting a proton.
This process is crucial for understanding how equilibrium in reactions is balanced.The reaction for this process is:\[ \mathrm{HPO}_4^{2-} + \mathrm{H}^+ \rightarrow \mathrm{H}_2\mathrm{PO}_4^- \]- Here, \( \mathrm{HPO}_4^{2-} \) accepts a proton and becomes \( \mathrm{H}_2\mathrm{PO}_4^- \).
- The addition of a proton changes the charge from \(2-\) to \(1-\), showing the ion's transition into a new form.Understanding proton acceptance is vital to forecast how substances will react when interacting with others.

Balanced Chemical Equations

Balanced chemical equations ensure that the same number of each type of atom appears on both sides of the equation, maintaining mass conservation. In the context of sodium hydrogen phosphate, two balanced equations can illustrate its dual role.
The first equation shows \( \mathrm{HPO}_4^{2-} \) behaving as an acid:- \( \mathrm{HPO}_4^{2-} \rightarrow \mathrm{PO}_4^{3-} + \mathrm{H}^+ \)The second equation shows it behaving as a base:- \( \mathrm{HPO}_4^{2-} + \mathrm{H}^+ \rightarrow \mathrm{H}_2\mathrm{PO}_4^- \)Each balanced equation maintains both mass and charge, illustrating how chemical equations are not only about atoms but also account for the electrical neutrality of the system.The skill of balancing chemical equations is essential for predicting and understanding the outcomes of chemical reactions.