Problem 102 - Chemistry Matter and Change

Question

Write the chemical equation for the reaction that would occur when a base is added to a solution containing the \(\mathrm{H}_{2} \mathrm{PO}_{4}-/ \mathrm{HPO}_{4}^{2-}\) buffer system.

Step-by-Step Solution

Answer

The balanced chemical equation for the reaction between a base (\(\mathrm{B}\)) and the dihydrogen phosphate ion (\(\mathrm{H}_{2} \mathrm{PO}_{4}^-\)) in the given buffer system is: \[ \mathrm{H_{2}PO_{4}^{-}} + \mathrm{B} \rightarrow \mathrm{HPO_{4}^{2-}} + \mathrm{BH} \]

Step 1: Identify the species in the buffer system

The buffer system consists of the dihydrogen phosphate ion (\(\mathrm{H}_{2} \mathrm{PO}_{4}^-\)) and the hydrogen phosphate ion (\(\mathrm{HPO}_{4}^{2-}\)).

Step 2: Write the generic form of the proton transfer reaction

When a base reacts with an acid, it generally accepts a hydrogen ion (proton) from the acid. We can represent the base as \(\mathrm{B}\) and write a generic formula for the proton transfer reaction as: \[ \mathrm{H_{2}PO_{4}^{-}} + \mathrm{B} \rightarrow \mathrm{HPO_{4}^{2-}} + \mathrm{BH} \]

Step 3: Determine the reaction between the base and the buffer system

In this specific case, the base (\(\mathrm{B}\)) will react with the dihydrogen phosphate ion (\(\mathrm{H}_{2} \mathrm{PO}_{4}^-\)) by accepting a hydrogen ion, resulting in the formation of hydrogen phosphate ion (\(\mathrm{HPO}_{4}^{2-}\)) and the conjugate acid of the base (\(\mathrm{BH}\)). So the final balanced chemical equation is: \[ \mathrm{H_{2}PO_{4}^{-}} + \mathrm{B} \rightarrow \mathrm{HPO_{4}^{2-}} + \mathrm{BH} \]

Key Concepts

Dihydrogen Phosphate Ion

The dihydrogen phosphate ion, represented as \(\mathrm{H_{2}PO_{4}^{-}}\), is an important component in many biological systems and buffer solutions. It is derived from phosphoric acid, \(\mathrm{H_{3}PO_{4}}\), by losing one proton \(\mathrm{(H^+)}\). This ion commonly exists in solutions where it can act as an acid.

• Role in Buffer Systems: In buffer systems, dihydrogen phosphate helps maintain the pH balance by donating protons to bases, making it an effective pH stabilizer.
• Acidic Nature: The presence of the extra hydrogen allows it to behave as an acid, donating hydrogen ions to neutralize added bases.

This dual role as an acid and a base is what makes \(\mathrm{H_{2}PO_{4}^{-}}\) such a versatile ion in chemical reactions.

Hydrogen Phosphate Ion

The hydrogen phosphate ion, \(\mathrm{HPO_4^{2-}}\), plays a central role in buffering systems, often working alongside the dihydrogen phosphate ion to manage pH levels. This ion is created when \(\mathrm{H_{2}PO_4^{-}}\) loses a hydrogen ion.

• Basic Properties: In reactions, \(\mathrm{HPO_4^{2-}}\) can act as a base, accepting hydrogen ions from acids.
• Role in Neutralization: If an acid is introduced to the solution, \(\mathrm{HPO_4^{2-}}\) can accept a proton, forming \(\mathrm{H_{2}PO_4^{-}}\) again.

The interconversion between \(\mathrm{H_{2}PO_{4}^{-}}\) and \(\mathrm{HPO_4^{2-}}\) enables buffer systems to effectively resist changes in pH, crucial for maintaining stability in biological and chemical processes.

Proton Transfer Reaction

A proton transfer reaction is a fundamental process in chemistry where a proton \(\mathrm{(H^+)}\) is transferred from one atom or molecule to another. This occurs often in acid-base reactions.

• Basic Mechanism: In a solution, typically a base \(\mathrm{B}\) will accept a \(\mathrm{H^+}\) from an acid. This forms a new acid \(\mathrm{BH}\) and a new base.
• Relevance to Buffer Systems: In the phosphate buffer system, the reaction involves \(\mathrm{H_{2}PO_4^{-}}\) donating a proton to \(\mathrm{B}\), forming \(\mathrm{HPO_4^{2-}}\). This conversion helps stabilize the pH level.

Understanding proton transfer reactions is crucial for predicting the behavior of different substances in both laboratory and everyday chemical processes.