Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is an acid? (a) \(\mathrm{P}(\mathrm{OH})_{3}\) (b) \(\mathrm{NaOH}\) (c) \(\mathrm{NH}_{4} \mathrm{OH}\) (d) \(\mathrm{Ca}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{P(OH)}_{3}\) is an acid.

Step 1: Understanding Acids and Bases

An acid is a substance that donates protons (H⁺ ions) when dissolved in water, while a base is a substance that accepts protons. Bases often contain the hydroxide ion (OH⁻). In this exercise, identify the substances by their chemical formulas to determine which one acts as an acid.

Step 2: Analyze the Given Substances

Look at each option: (a) \( \text{P(OH)}_{3} \) is phosphorous acid and contains hydroxide groups but acts as an acid. (b) \( \text{NaOH} \) is sodium hydroxide, a strong base. (c) \( \text{NH}_{4}\text{OH} \) is ammonium hydroxide, acting as a weak base. (d) \( \text{Ca(OH)}_{2} \) is calcium hydroxide, another base.

Step 3: Identify the Acid

From the analysis in Step 2, \( \text{P(OH)}_{3} \) (phosphorous acid) can donate protons despite having OH groups in its formula. It behaves as an acid because it can release H⁺ ions.

Key Concepts

Proton Donors

In chemistry, acids are known as proton donors. This means they have the ability to release hydrogen ions, or protons (H⁺), when they dissolve in water. The proton donor concept is fundamental because it helps define what makes a substance acidic. When these protons are released, they can interact with other molecules in solution often leading to an acidic environment.

For example, consider a general acid like hydrochloric acid (HCl). When it dissolves in water, it releases H⁺ ions into the solution:

• Releases H⁺ ions
• Increases acidity of the solution

Phosphorous acid ( ewline P(OH)₃ as one of its depictions) is an interesting example. Although it contains hydroxyl groups, it also has the capacity to donate protons, making it an acid. This shows that the presence of hydroxyle groups in a molecule doesn’t always determine the molecule’s acidic or basic nature.

Hydroxide Ion

The hydroxide ion (OH⁻) is a common feature in many bases. A base is typically defined as a substance that can accept protons, and often contains OH⁻ ions in its formula. The presence of hydroxide ions makes a solution basic or alkaline.

A classic example of a base is sodium hydroxide (NaOH), which dissociates in water to yield Na⁺ and OH⁻ ions:

• NaOH ightarrow Na⁺ + OH⁻
• Contributes to a basic solution

NaOH releases hydroxide ions into a solution, neutralizing acidity by combining with H⁺ ions to form water (H₂O). Similarly, other substances like calcium hydroxide (Ca(OH)₂) and ammonium hydroxide (NH₄OH) act as bases due to the presence of hydroxide ions.

Chemical Formula Analysis

Analyzing chemical formulas is crucial to understanding whether a compound is an acid or base. Chemical formulas give us insights into the composition and potential behavior of a substance in solution.

Let's break down a chemical formula into its components and understand what it tells us:

• Identify the elements present
• Determine the potential ions that may be released
• Understand possible molecular interactions

For instance, in the formula P(OH)₃, noting the presence of hydrogen in relation to the phosphorous and hydroxyl groups leads to the identification of phosphorous acid. Despite having OH groups, phosphorous acid can still release H⁺ ions, showcasing the importance of a detailed formula analysis to accurately categorize substances.

Phosphorous Acid

Phosphorous acid ( P(OH)₃) is a unique compound that can sometimes confuse students due to its composition. While its formula includes hydroxyl groups, which are typically associated with bases, it behaves distinctly as an acid.

This is because phosphorous acid is capable of donating protons (H⁺ ions) in solution:

• Has acidic hydrogen atoms available for donation
• Releases H⁺ ions despite having OH groups

The dual presence of these groups demonstrates an essential chemistry concept: the characteristics of compounds are not always as straightforward as their chemical formulas might imply. The presence of hydroxyl groups does not automatically equate a compound to being a base, as seen in phosphorous acid's ability to function as an acid.