Problem 36 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An aqueous solution of \(X\) is added slowly to an aqueous solution of \(\mathrm{Y}\) as shown in List \(\mathrm{I}\). The variation in conductivity of these reactions is given in List II. Match list I with List II and select the correct answer using the code given below the lists: [Adv. 2013] List I List II P. \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{~N}+\mathrm{CH}_{3} \mathrm{COOH}\) 1\. Conductivity \(\mathrm{X} \quad \mathrm{Y}\) decreases and then increases Q. KI \((0.1 \mathrm{M})+\mathrm{AgNO}_{3}(0.01 \mathrm{M}) \quad 2\) Conductivity. \(\begin{array}{lll}\mathrm{X} & \mathrm{Y} & \text { decreases and then }\end{array}\) does not change much R. \(\quad \mathrm{CH}_{3} \mathrm{COOH}+\mathrm{KOH}\) 3\. Conductivity X \(\mathrm{Y} \quad\) increases and then does not change much S. \(\mathrm{NaOH}+\mathrm{HI}\) 4\. Conductivity does not X \(Y \quad\) change much and then increases Codes : \(\mathrm{P}\) \(\mathrm{Q}\) \(\mathrm{R} \quad \mathrm{S}\) \(\mathbf{Q}^{\prime}\) 4 3 1 \(\begin{array}{llll}\text { (a) } & 3 & 4 & 2 \\ \text { (b) } & 4 & 3 & 2 \\\ \text { (c) } & 2 & 3 & 2\end{array}\) 1 3 3 4 4 1 (d) 1 \(3 \quad 2\)

Step-by-Step Solution

Answer

Option (d): 1, 3, 2, 2.

Step 1: Evaluate Reaction P

The reaction given is between triethylamine \((C_2H_5)_3N\) and acetic acid \(CH_3COOH\). This is a neutralization reaction where an amine (base) reacts with an acid to form a salt and water. Initially, the solution contains ions from the acid \(CH_3COO^-\ and \ H^+ \) contributing to the conductivity. As the base \( (C_2H_5)_3N \) is added, \(CH_3COO^-\) ions are replaced by less conductive \((C_2H_5)_3NH^+\) ions, decreasing the conductivity. After neutralization, further addition results only in slightly conductive impurities, so the conductivity increases again. The correct match is '1'. Conductivity decreases and then increases.

Step 2: Evaluate Reaction Q

This reaction involves mixing potassium iodide (KJ) with silver nitrate (AgNO_3). Initially, the solution contains ions like \(K^+, I^-, Ag^+, NO_3^-\) which contribute to high conductivity. When Ag^+ ions react with I^- ions to form AgI precipitate, the number of ions decreases, leading to decreased conductivity. Beyond complete precipitation, additional K^+ ions and unreacted NO_3^- ions still present keep the conductivity relatively constant. The correct behavior is '4'. Conductivity decreases and then does not change much.

Step 3: Evaluate Reaction R

In this reaction, acetic acid \(CH_3COOH\) reacts with potassium hydroxide \(KOH\), a strong base. As KOH is added, \(CH_3COO^-\) ions are formed, which are more conductive than the unreacted acetic acid. Conductivity increases and remains relatively stable past neutralization because the solution's primary ionic species remain relatively consistent post-reaction. The correct match is '3'. Conductivity increases and then does not change much.

Step 4: Evaluate Reaction S

This reaction is between sodium hydroxide \(NaOH\) and hydroiodic acid \(HI\). Initially, ions present are \(Na^+, OH^-, H^+, I^-\). After the complete neutralization, where water and NaI form, conductivity does not change much as the number of ions remains somewhat constant. As the reaction progresses centering around excess \(NaOH\), any additional base increases the conductivity due to increased ionic species. The correct behavior is '2'. Conductivity does not change much and then increases.

Key Concepts

Neutralization Reactions

Neutralization reactions occur when an acid and a base react to form water and a salt. This process often involves the transfer of protons ( H^+ ) from the acid to the hydroxide ions ( OH^- ) provided by the base. It's a synthesis reaction leading to the decrease of free ions in the solution, particularly when the ions combine to create non-ionic compounds like water. This was exemplified in Reaction P, where triethylamine reacted with acetic acid, forming (C_2H_5)_3NH^+ , a less conductive salt. As the ions responsible for conductivity are used up, initially, the solution loses ions, decreasing its conductivity.
Once the acid and base are neutralized, the introduction of more reactants may lead to an increase in ionic concentrations, hence conductivity can increase again. This behavior underlines the dynamic nature of conductivity during neutralization, which is critical to understand in many chemical processes.

Precipitation Reactions

Precipitation reactions are reactions where two solutions containing soluble salts are mixed, resulting in the formation of an insoluble salt (precipitate). This process was illustrated in Reaction Q, where the mixing of potassium iodide and silver nitrate led to silver iodide ( AgI ) falling out of solution. Precipitates form when the product of certain ions' concentrations exceeds their solubility product, causing them to crystallize out.

During these reactions, the number of free ions in solution decreases sharply, leading to a drop in conductivity because ions, the charge carriers, are consumed to form the precipitate. Once precipitation is complete, any remaining ions stabilize the conductivity, keeping it relatively constant. Understanding this can help in predicting the behavior of ionic solutions, especially in analytical or separation processes.

Ion Conductivity

Ion conductivity in solutions refers to the solution's ability to conduct electricity through the movement of ions. In both neutralization and precipitation reactions, the nature and concentration of ions present in the solution are crucial factors. For instance, as demonstrated in Reaction R, when acetic acid was neutralized by potassium hydroxide, the solution's conductivity increased due to the formation of conductive - acetate and potassium ions. Conductivity depends largely on the concentration of ions and their mobility. For example, in a neutralization reaction, replacing highly mobile H^+ and OH^- ions with less mobile ions can cause a conductivity change. Similarly, as ions precipitate, a sudden drop in conductivity might indicate fewer ions in solution. Learning about ion conductivity provides insights into the progress and completeness of various chemical reactions, which can be applied in industrial, analytical, and experimental chemistry contexts.

Chemical Kinetics

Chemical kinetics deals with the rates of reactions and the factors affecting them, such as temperature, concentration, and catalysts. For example, in Reaction S involving sodium hydroxide and hydroiodic acid, we observe that the conductivity changed based on the instant reaction rates governed by the concentration of reactants. Initially, neutralization occurs quickly, consuming ions, but as the reaction progresses and reaches completion, changes in conductivity become less pronounced. This concept helps chemists understand how quickly a reaction reaches equilibrium, the effect of concentration changes over time, and how these impact the system's overall properties, such as conductivity. By manipulating these kinetics, chemists can control reaction pathways, making kinetics a fundamental concept in both research and industry for optimizing reactions and processes.