Problem 107 - A Complete Resource Book in Chemistry for JEE Main

Question

Which is incorrect here? (1) Heat of neutralization for strong acid and strong base is always less than \(13.7 \mathrm{~kJ}\). (2) Aniline yellow can also be used to prepare other dyes. (3) Phenolphthalein can be used as an indicator in the titration of strong acid and weak base (4) Methyl orange works in the range of \(3.1-4.4\). (a) 2 and 4 (b) 2 and 3 (c) 1 and 3 (d) 3 and 4

Step-by-Step Solution

Answer

The incorrect options are (c) 1 and 3.

Step 1: Review Statement 1

The heat of neutralization for a strong acid and a strong base is generally around \(-57.1 \mathrm{~kJ/mol}\). Statement 1 claims it is less than \(13.7 \mathrm{~kJ}\), which is incorrect.

Step 2: Review Statement 2

Aniline yellow can indeed be used to produce other dyes. Hence, statement 2 is correct.

Step 3: Review Statement 3

Phenolphthalein is not suitable for the titration of a strong acid and weak base because it changes color at a pH range ideal for weak acid and strong base titrations. Thus, statement 3 is incorrect.

Step 4: Review Statement 4

Methyl orange has a transition range of pH 3.1 to 4.4. Statement 4 is correct as it matches the known range for methyl orange.

Key Concepts

Heat of Neutralization

Neutralization reactions occur when an acid and a base react to form water and salt. The heat of neutralization is the heat change that happens when one mole of water is formed in such a reaction. It's important to remember that this is an exothermic process, meaning it releases heat.
For strong acids and strong bases, the heat of neutralization is fairly consistent. Typically, it's around \(-57.1 \text{ kJ/mol}\). This is because strong acids and bases fully dissociate in the solution. Thus, every hydrogen ion from the acid reacts with every hydroxide ion from the base, leading to this predictable heat release.

It's a bit different when dealing with weak acids or bases. This heat release can be less because not all of the molecules dissociate in the solution. That's why when exercising caution in exercises, remember that the heat of neutralization for strong acids and bases should be close to \(-57.1 \text{kJ/mol}\). An incorrect assertion would be any value notably less than this standard, such as the \(13.7\text{ kJ/mol}\) mentioned in the exercise.

Acid-Base Titration

Acid-base titration is a common laboratory method used to determine the concentration of an acidic or basic solution. The process involves the gradual addition of a titrant to a solution with a known volume but unknown concentration. The key is knowing when the solution has been neutralized. This is when the number of moles of hydrogen ions equals the moles of hydroxide ions in the solution.
Titrations between strong acids and strong bases are straightforward because they fully dissociate in water. However, titrations involving weak acids or bases require special care. Their partial dissociation means that the pH doesn't change as predictably. As such, choosing the right indicator becomes crucial to accurately pinpoint the endpoint.

One common mistake is using an incorrect indicator. For example, phenolphthalein is ideal for strong acid-strong base titrations due to its color change in the pH range best suited for such reactions. However, using it for strong acid-weak base titrations would be misleading, as the endpoint may not be clearly visible until a higher pH is reached.

Indicators in Chemistry

Indicators are substances used to determine the endpoint in titration by changing color at a particular pH level. The choice of indicator is pivotal because a mismatch can lead to incorrect results.
Indicators can be either natural, like litmus, or synthetic, like phenolphthalein. Each indicator has a distinct pH range where it shifts colors. For instance, phenolphthalein changes color from colorless to pink around pH 8.2 to 10.0, making it perfect for strong acid-strong base titrations.

Another well-known indicator is methyl orange, which transitions from red to yellow between pH 3.1 and 4.4. Its lower pH range makes it more suitable for strong acid-weak base titrations, not strong base-weak acid ones. Knowing these pH ranges and their corresponding color changes is essential for selecting the right indicator for a titration task.

• Phenolphthalein changes color in the range of pH 8.2-10.0.
• Methyl orange changes color in the range of pH 3.1-4.4.
• Litmus paper turns red under acidic conditions and blue under basic conditions.

Accurate endpoint detection during a titration hinges on the correct choice of indicator. As such, it's not just about having an indicator but choosing one that matches the pH transition required for the specific titration.