Question

Classify each acid as strong or weak. (a) \(\mathrm{HCHO}_{2}\) (b) \(\mathrm{H}_{2} \mathrm{SO}_{4}\) (c) \(\mathrm{HNO}_{3}\) (d) \(\mathrm{H}_{2} \mathrm{CO}_{3}\)

Short answer

(a) Weak, (b) Strong, (c) Strong, (d) Weak.

Step-by-step solution

Understand Acid Strength

An acid's strength is determined by its ability to ionize in water. A strong acid completely ionizes in solution, while a weak acid only partially ionizes. When an acid ionizes in water, it donates protons (hydrogen ions) to the water, forming hydronium ions.

Identify Common Strong Acids

There are seven common strong acids: hydrochloric acid (HCl), hydrobromic acid (HBr), hydroiodic acid (HI), sulfuric acid (H2SO4), nitric acid (HNO3), perchloric acid (HClO4), and chloric acid (HClO3). If the acid is one of these, it can be classified as strong.

Classify Each Acid

Using the list of common strong acids, classify each given acid. If the acid is not on the list, it is most likely a weak acid.

Classify \(\mathrm{HCHO}_{2}\)

Formic acid (HCHO2) is not one of the common strong acids, so it is classified as a weak acid.

Classify \(\mathrm{H}_{2}\mathrm{SO}_{4}\)

Sulfuric acid (H2SO4) is one of the common strong acids, so it is classified as a strong acid.

Classify \(\mathrm{HNO}_{3}\)

Nitric acid (HNO3) is one of the common strong acids, so it is classified as a strong acid.

Classify \(\mathrm{H}_{2}\mathrm{CO}_{3}\)

Carbonic acid (H2CO3) is not one of the common strong acids, so it is classified as a weak acid.

Key concepts

Strong vs. Weak Acids

Understanding the difference between strong and weak acids is crucial for students studying chemistry. The key factor that determines the strength of an acid is its ability to ionize in water. Strong acids are characterized by their complete ionization in aqueous solutions, which means they donate all their available protons (hydrogen ions, H⁺) to water molecules, producing a high concentration of hydronium ions (H₃O⁺). This results in a solution with a low pH value, indicating a high level of acidity.

On the other hand, weak acids only partially ionize in water, meaning that only a fraction of their protons are donated to water, leading to a lower concentration of hydronium ions. Weak acids establish an equilibrium between the un-ionized acid and the ions produced, which ultimately means that the pH of their solutions is higher compared to strong acids.

To put it into practice, when a student encounters an acid in their homework, they should question the degree to which the acid ionizes. For instance, in the provided exercise, formic acid (HCHO₂) does not ionize completely and thus is identified as a weak acid, whereas sulfuric acid (H₂SO₄) fully ionizes and is recognized as a strong acid.

Acid Ionization

Acid ionization refers to the process by which an acid molecule donates a proton to water, resulting in the formation of hydronium ions. This process is quantitatively represented by the acid dissociation constant (Ka), which provides a measure of the acid's strength. The greater the value of Ka, the stronger the acid, and vice versa.

The ionization process can be depicted in a chemical equation, where a generic acid HA ionizes in water (H₂O) to produce H₃O⁺ and the conjugate base A^-:\[\begin{equation}\text{HA} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{A}^-\end{equation}\]In this reaction, the complete transfer of protons in the case of strong acids shifts the equilibrium far to the right, leading to negligible amounts of un-ionized acid in solution. For weak acids, equilibrium is established with significant quantities of both the acid and its ions present. This crucial distinction is why strong acids are better conductors of electricity and have greater reactive properties compared to weak acids. For example, in the exercise provided, nitric acid (HNO₃) represented a case of complete ionization, highlighting it as a strong acid.

Common Strong Acids

There exists a list of acids that are universally accepted as strong acids due to their high degree of ionization. Students will often be required to memorize this list, as it is frequently referenced in chemical problems and exercises. The seven common strong acids include:

• Hydrochloric acid (HCl)
• Hydrobromic acid (HBr)
• Hydroiodic acid (HI)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)
• Perchloric acid (HClO₄)
• Chloric acid (HClO₃)

Identifying strong acids in exercises is therefore simplified by checking if the given acid appears on this list. A prime example of this is sulfuric acid (H₂SO₄), which is recognized as a strong acid in the exercise, as it is one of the seven. In contrast, carbonic acid (H₂CO₃), also featured in the exercise, is not listed and thus is a weak acid. This straightforward classification is an essential skill for students to develop, enabling them to predict the behavior of acids in different chemical reactions and solutions.