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

Option (c) 1 and 3 are incorrect.

Step 1: Identify the Claims

Read the statements carefully and identify what each one asserts: 1. Heat of neutralization for strong acid and strong base is always less than 13.7 kJ. 2. Aniline yellow can also be used to prepare other dyes. 3. Phenolphthalein as an indicator in the titration of strong acid and weak base. 4. Methyl orange works in the range of 3.1-4.4.

Step 2: Evaluate the First Assertion

The heat of neutralization for a strong acid reacting with a strong base is typically around -57 kJ/mol, not less than 13.7 kJ/mol. This makes statement 1 incorrect.

Step 3: Check the Second Assertion

Aniline yellow is a dye that can indeed be used in the synthesis of other dyes. This statement is correct.

Step 4: Examine the Third Assertion

Phenolphthalein is not suitable for titration between strong acids and weak bases because it changes color in the pH range of 8.2 to 10, which is above the equivalence point of such titrations. Therefore, statement 3 is incorrect.

Step 5: Verify the Fourth Assertion

Methyl orange changes color over the pH range of 3.1 to 4.4, which is accurate. Thus, statement 4 is correct.

Step 6: Determine the Incorrect Pair

Statements 1 and 3 are incorrect based on the analysis above.

Key Concepts

Strong Acid and Strong Base

In chemistry, a strong acid and a strong base are substances that completely dissociate into their ions in aqueous solutions. This means that when they dissolve, they break apart completely, releasing hydrogen ions from the acid and hydroxide ions from the base. The most common example of this reaction is when hydrochloric acid ( HCl ) and sodium hydroxide ( NaOH ) combine in water to form water and salt ( NaCl ).

The heat of neutralization is the heat change that occurs when an acid reacts with a base to form water. Typically, for the reaction between a strong acid and a strong base, the heat of neutralization is around -57 ext{ kJ/mol} . This happens due to the formation of water from hydrogen ions and hydroxide ions. The process is straightforward:

• Hydrogen ions ( H^+ ) from the acid combine with hydroxide ions ( OH^- ) from the base to form water ( H_2O ).
• The energy released in this exothermic process is what we refer to as the heat of neutralization.

Understanding this concept is key in thermochemistry, which studies heat changes during chemical reactions.

Phenolphthalein Indicator

Phenolphthalein is a common acid-base indicator, frequently used in titration experiments. It is unique due to its distinct color change process. In an acidic environment, phenolphthalein is colorless. However, as the solution becomes more basic, beginning from a pH around 8.2, the indicator transitions to a pink and then a deep magenta at a pH level of around 10.

This property makes phenolphthalein a poor choice for titrations involving strong acids and weak bases. During such titrations, the pH at the equivalence point often remains below 7, nowhere near the color-change threshold of phenolphthalein. Thus, using phenolphthalein in these conditions may not accurately determine the endpoint, leading to misinterpretations.

Choosing the right indicator, such as one that changes color at a pH closer to the equivalence point of the specific reaction, is crucial for precise titrations.

Methyl Orange

Methyl orange is another well-known acid-base indicator used in titrations. This indicator is particularly useful in strong acid-weak base titrations, as it changes color over a pH range of 3.1 to 4.4. In this pH range, the methyl orange transitions from red in acidic environments to yellow in more neutral, less acidic conditions.

• Methyl orange is favored in titrations of strong acids with weak bases because such reactions have an equivalence point within its color-change range.
• Its distinct color transition provides a clear, early indication of the endpoint.

Its use underscores the importance of selecting the correct indicator to suit the specific reaction conditions, ensuring accurate titration outcomes.

Aniline Yellow

Aniline yellow is a synthetic dye that has applications extending beyond just adding color. It serves as an intermediary in the production of various other dyes, highlighting its versatility in dye chemistry. Aniline yellow is essentially used in the creation of azo dyes, which are vital in coloring textiles, leathers, and plastics.

The process of producing and utilizing aniline yellow involves converting aniline, an organic compound derived from benzene, into different dye forms. This adaptability not only showcases the significance of aniline yellow in industrial applications but also in research for developing new dyes.

Its chemistry and ability to be synthesized into other dye forms make aniline yellow an interesting study topic for students exploring organic chemistry and industrial dye processes.