Problem 69 - Chemistry Matter and Change

Question

Identify the conjugate acid-base pairs in the reaction of \(\mathrm{H}_{3} \mathrm{PO}_{4}\) with water.

Step-by-Step Solution

Answer

The conjugate acid-base pairs in the reaction are \\(H_3PO_4/H_2PO_4^-\\) and \\(H_2O/H_3O^+\\).

Step 1: Understanding the Concept of Conjugate Acid-Base Pairs

A conjugate acid-base pair consists of an acid and a base that differ by one proton \(H^+\). The acid has an extra \(H^+\) compared to its conjugate base.

Step 2: Writing the Balanced Reaction Equation

The reaction between \(H_3PO_4\) and water can be written as \(H_3PO_4 + H_2O ightarrow H_2PO_4^- + H_3O^+\). Here, \(H_3PO_4\) donates a proton to water.

Step 3: Identifying the Acid and Base

In the reaction, \(H_3PO_4\) is the acid as it donates a proton, and \(H_2O\) is the base as it accepts a proton.

Step 4: Identifying the Conjugate Acid-Base Pairs

After \(H_3PO_4\) donates a proton, it becomes \(H_2PO_4^-\), which is its conjugate base. When \(H_2O\) accepts a proton, it becomes \(H_3O^+\), which is its conjugate acid. Therefore, the conjugate pairs are \(H_3PO_4/H_2PO_4^-\) and \(H_2O/H_3O^+\).

Key Concepts

Acid-Base Reactions

Acid-base reactions are fundamental chemical processes where an acid reacts with a base resulting in the transfer of protons. In simpler terms, an acid donates a proton (H\(^+\)) to a base. This interaction forms the basis of numerous chemical processes and everyday phenomena.

This concept is crucial in understanding various biological and chemical systems, such as metabolic pathways and industrial chemical synthesis. In these reactions, acids and bases come together to create new compounds and sometimes even release energy. In the reaction between phosphoric acid (H\(_3\)PO\(_4\)) and water (H\(_2\)O), H\(_3\)PO\(_4\) acts as an acid, donating a proton, and H\(_2\)O acts as a base, accepting the proton.

Proton Transfer

Proton transfer is the essence of acid-base chemistry. It involves the movement of a hydrogen ion (H\(^+\)) from one molecule to another. This transfer is a defining characteristic of acid-base reactions, and it can significantly alter the properties of the involved substances.

In the reaction between H\(_3\)PO\(_4\) and H\(_2\)O, a proton moves from H\(_3\)PO\(_4\) (phosphoric acid) to H\(_2\)O (water), transforming H\(_3\)PO\(_4\) into H\(_2\)PO\(_4^-\) and H\(_2\)O into H\(_3\)O\(^+\). This proton transfer changes the molecule structure and chemical behavior of the participating substances. Understanding this mechanism helps us recognize how acids and bases interact to form conjugate pairs.

Chemical Equilibrium

Chemical equilibrium occurs when the forward and reverse reactions occur at the same rate, resulting in a stable concentration of products and reactants. This dynamic balance is key to predicting the direction and extent of chemical reactions, especially in acid-base systems.

In our example of H\(_3\)PO\(_4\) and water, equilibrium is reached when the rate of proton transfer from H\(_3\)PO\(_4\) to H\(_2\)O equals the rate of proton re-uptake. At this point, the quantities of H\(_3\)PO\(_4\), H\(_2\)O, H\(_2\)PO\(_4^-\), and H\(_3\)O\(^+\) remain constant. This state of equilibrium is crucial for processes like buffer solutions, which resist drastic pH changes.

Phosphoric Acid Reactions

Phosphoric acid (H\(_3\)PO\(_4\)) is a versatile compound, playing a significant role in chemistry, industry, and biology. When H\(_3\)PO\(_4\) reacts with water, it exhibits behavior typical of acids by releasing a proton, highlighting its role as a triprotic acid.

This means phosphoric acid can donate up to three protons, undergoing successive reactions. Initially, it forms H\(_2\)PO\(_4^-\), and potentially can further react, which is applicable in various industrial processes and nutrient cycles. This ability to release multiple protons makes H\(_3\)PO\(_4\) essential in manufacturing fertilizers and food additives. Its reactions facilitate understanding of larger acid-base reaction networks in both synthetic and natural environments.