Problem 51 - Chemistry The Science in Context

Question

Identify each compound as either a weak acid or a strong acid in aqueous solution: (a) HNO \(_{3} ;\) (b) HNO \(_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH} ;(\mathrm{d}) \mathrm{H}_{2} \mathrm{SO}_{4}\)

Step-by-Step Solution

Answer

Question: Identify whether each of the following compounds is a weak or strong acid in aqueous solution: (a) HNO\(_{3}\), (b) HNO\(_{2}\), (c) CH\(_{3}\)CH\(_{2}\)CH\(_{2}\)COOH, (d) H\(_{2}\)SO\(_{4}\). Answer: (a) HNO\(_{3}\) is a strong acid, (b) HNO\(_{2}\) is a weak acid, (c) CH\(_{3}\)CH\(_{2}\)CH\(_{2}\)COOH (butanoic acid) is a weak acid, (d) H\(_{2}\)SO\(_{4}\) (sulfuric acid) is a strong acid.

Step 1: Determine if the compound is an acid

To identify if a given compound is an acid, we need to check if it is able to provide a proton (H+ ion) when dissolved in water.

Step 2: Identify strong and weak acids

The list of common strong acids includes: HCl (hydrochloric acid), HBr (hydrobromic acid), HI (hydroiodic acid), HNO\(_{3}\) (nitric acid), HClO\(_{4}\) (perchloric acid), and H\(_{2}\)SO\(_{4}\) (sulfuric acid). If a compound is not on this list, it is most likely a weak acid. Now, let's apply these steps to each given compound:

Step 3: (a) HNO\(_{3}\)

HNO\(_{3}\) is a compound that can donate a proton when dissolved in water. It is also in our list of strong acids. Therefore, it is a strong acid in aqueous solution.

Step 4: (b) HNO\(_{2}\)

HNO\(_{2}\) is a compound that can donate a proton when dissolved in water. However, it is not found in the list of strong acids. Hence, it is a weak acid in aqueous solution.

Step 5: (c) CH\(_{3}\)CH\(_{2}\)CH\(_{2}\)COOH

CH\(_{3}\)CH\(_{2}\)CH\(_{2}\)COOH (butanoic acid) is a compound that can donate a proton when dissolved in water. However, it is not in the strong acid list, so we can classify it as a weak acid in aqueous solution.

Step 6: (d) H\(_{2}\)SO\(_{4}\)

H\(_{2}\)SO\(_{4}\) (sulfuric acid) is a compound that can donate a proton when dissolved in water. It is also in our list of strong acids. Hence, H\(_{2}\)SO\(_{4}\) is considered a strong acid in aqueous solution.

Key Concepts

Strong Acids

Understanding the concept of strong acids is essential for identifying their behavior in aqueous solutions. Strong acids are characterized by their ability to completely dissociate into ions when dissolved in water. This means that they release every single proton (or hydrogen ion, H\(^+\)) they possess, resulting in an equal concentration of hydronium ions (H\(_3\)O\(^+\)) in solution.

Here are some commonly known strong acids:

• Hydrochloric acid (HCl): Widely used in industries and laboratories.
• Hydrobromic acid (HBr) and hydroiodic acid (HI): Highly reactive and used in chemical synthesis.
• Nitric acid (HNO\(_3\)): Known for its role in the manufacture of fertilizers and explosives.
• Perchloric acid (HClO\(_4\)): Used as a highly effective oxidizing agent.
• Sulfuric acid (H\(_2\)SO\(_4\)): One of the most important chemicals in industrial processes.

In our exercise, HNO\(_3\) and H\(_2\)SO\(_4\) are identified as strong acids because they are on the list above and fully dissociate in water. This complete ionization is what makes them 'strong', as they leave little to no molecules of the original acid in solution.

Weak Acids

Weak acids, in contrast to strong acids, do not completely dissociate in aqueous solutions. Instead, they partially dissociate, meaning not all molecules release their protons (H\(^+\) ions) into the solution. As a result, a significant number of the acid molecules remain intact, which leads to a lower concentration of hydronium ions than strong acids can achieve.

Identifying weak acids:

• If an acid is not listed as a strong acid, it is typically considered weak.
• Weak acids include organic acids (such as acetic acid) and some inorganic acids (like carbonic acid).

In our example, both HNO\(_2\) (nitrous acid) and CH\(_3\)CH\(_2\)CH\(_2\)COOH (butanoic acid) are referred to as weak acids. This is because they do not appear in the list of strong acids and thus, dissociate only partially when dissolved in water. The equilibrium between the ionized and non-ionized forms of these acids is what defines their nature and behavior as weak acids.

Aqueous Solution

An aqueous solution is simply a solution where water serves as the solvent. This term is commonly used when discussing compounds that dissolve in water, such as acids and bases. In these solutions, water molecules interact with the dissolved substances, affecting their ionization and other chemical properties.

Key aspects of aqueous solutions:

• The solvent: In an aqueous solution, water (H\(_2\)O) is always the solvent.
• Ionization: Acids dissociate in water, releasing protons (H\(^+\)) and forming aqueous ions.
• Water's role as a polar solvent: The polarity of water molecules helps dissolve ionic compounds and supports the dissociation process.

In our context, identifying whether an acid is strong or weak relates directly to its behavior in an aqueous solution. For example, as mentioned in our examples, acids like HNO\(_3\) and H\(_2\)SO\(_4\) are strong because they fully dissociate, whereas substances like HNO\(_2\) and CH\(_3\)CH\(_2\)CH\(_2\)COOH only partially ionize. This difference in ionization is crucial for understanding properties such as pH and reactivity.