Problem 145 - A Complete Resource Book in Chemistry for JEE Main

Question

The rate low for the hydrolysis of thioacetamide, \(\mathrm{CH}_{3} \mathrm{CSNH}_{2}\) CC(=S)NCOCC(N)=O \(+\mathrm{H}_{2} \mathrm{~S}\) Is rate \(=\mathrm{k}\left[\mathrm{H}^{+}\right][\mathrm{TA}]\), where \(\mathrm{TA}\) is thioacetamide. In which of the following solutions, will the rate of hydrolysis of thioacetamide (TA) is least at \(25^{\circ} \mathrm{C}\) ? (a) \(0.1 \mathrm{M}\) in \(\mathrm{TA}\) and \(0.20 \mathrm{M}\) in \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) (b) \(0.1 \mathrm{M}\) in \(\mathrm{TA}\) and \(0.20 \mathrm{M}\) in \(\mathrm{HNO}_{3}\) (c) \(0.1 \mathrm{M}\) in \(\mathrm{TA}\) and \(0.20 \mathrm{M}\) in \(\mathrm{HCO}_{2} \mathrm{H}\) (d) \(0.15 \mathrm{M}\) in TA and \(0.15 \mathrm{M}\) in \(\mathrm{HCl}\)

Step-by-Step Solution

Answer

The rate of hydrolysis is least in option (a) with acetic acid.

Step 1: Understanding the Rate Equation

The rate of hydrolysis for thioacetamide is given by the equation: \( \text{rate} = k [\mathrm{H}^+][\mathrm{TA}] \). The rate is directly proportional to both the concentration of thioacetamide (\([\mathrm{TA}]\)) and the concentration of hydrogen ions (\([\mathrm{H}^+]\)). To minimize the rate, we aim to minimize the product of these concentrations.

Step 2: Analyzing Each Option

We have four options with different acid concentrations that affect \([\mathrm{H}^+]\). Since \( [\mathrm{H}^+] \) is greater in stronger acids at the same concentration, solutions with weaker acids will generally have a lower \([\mathrm{H}^+]\), reducing the rate. Therefore, consider options (a), (b), (c), and (d), and compare the strength of the acids involved.

Step 3: Determining Acid Strength

The acids in the options are:- (a) \( \mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H} \) (acetic acid)- (b) \( \mathrm{HNO}_3 \) (nitric acid)- (c) \( \mathrm{HCO}_2 \mathrm{H} \) (formic acid)- (d) \( \mathrm{HCl} \) (hydrochloric acid)Acetic acid and formic acid are weaker compared to nitric acid and hydrochloric acid. Among acetic and formic acid, acetic acid is slightly weaker. Therefore, it will have the lowest \([\mathrm{H}^+]\) at the same molarity.

Step 4: Conclusion

Given that acetic acid is the weakest acid among the choices, solution (a) with acetic acid will result in the least concentration of hydrogen ions, thus the least rate of hydrolysis.

Key Concepts

Rate Law

The rate law is an essential principle in chemical kinetics that describes how the speed of a chemical reaction is influenced by the concentration of its reactants. For the hydrolysis of thioacetamide, the rate law is expressed as \( \text{rate} = k [\mathrm{H}^+][\mathrm{TA}] \). This means the rate of reaction depends directly on the concentrations of hydrogen ions \([\mathrm{H}^+]\) and thioacetamide \([\mathrm{TA}]\). The proportional relationship implies that if either concentration increases, the reaction becomes faster.
Understanding rate laws helps chemists model reaction behaviors and predict how changes in conditions will affect the reaction speed. In practical scenarios, like determining the least rate of hydrolysis, this concept allows prediction of the slowest reacting mixture by identifying conditions that minimize the products of the reactant's concentrations.

Hydrolysis Reaction

Hydrolysis is a type of chemical reaction where water interacts with a compound, leading to its breakdown. In the case of thioacetamide hydrolysis, it is particularly influenced by the presence of acids, which provide the necessary \( \mathrm{H}^+ \) ions to drive the reaction. The general formula involves \( \text{Compound} + \mathrm{H}_2\mathrm{O} \rightarrow \text{Other Compounds} \). In the given scenario, thioacetamide reacts with water in the presence of hydrogen ions.
This reaction breaks down thioacetamide into different products, utilizing acid to catalyze the process. Knowing how hydrolysis works can help predict outcomes in various chemical environments, especially when considering the efficiency of reactions in solutions with varying acid strengths.

Acid Strength

Acid strength, a crucial concept in chemistry, refers to an acid's ability to donate protons (\( \mathrm{H}^+ \)). It is measured by its dissociation in water. Strong acids, such as \( \mathrm{HCl} \) and \( \mathrm{HNO}_3 \), dissociate completely, providing a high concentration of hydrogen ions, while weak acids like \( \mathrm{CH}_3 \mathrm{CO}_2 \mathrm{H} \) (acetic acid) dissociate less.

• Strong acids result in higher \( [\mathrm{H}^+] \), increasing the reaction rate.
• Weak acids yield lower \( [\mathrm{H}^+] \), slowing the reaction.

In the context of hydrolysis of thioacetamide, the weakest acid will produce fewer \( \mathrm{H}^+ \) ions, resulting in the minimal rate of reaction as indicated by the rate law.

Thioacetamide

Thioacetamide (TA) is an organic compound represented by the formula \( \mathrm{CH}_3 \mathrm{CSNH}_2 \). It is commonly used in chemical labs as a sulfur source for various reactions, including hydrolysis. Its structure involves a thio (sulfur-containing) group, which plays a significant role during its hydrolysis process.
During hydrolysis, thioacetamide decomposes in the presence of acids, making it essential to understand how it reacts in combination with different acids. By manipulating its concentration and the acid strength, chemists can control how quickly thioacetamide is hydrolyzed in aqueous solutions. This understanding is essential in optimizing reaction conditions and achieving desired results in chemical processes.