Problem 9 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Two compounds I and II are eluted by column chromatography(adsorption of \(\mathrm{I}>\mathrm{II})\). Which one of the following is a correct statement? (a) II moves slower and has higher \(\mathrm{R}_{\mathrm{f}}\) value than I (b) II moves faster and has higher \(\mathrm{R}_{\mathrm{f}}\) value than \(\mathrm{I}\) (c) I moves faster and has higher \(\mathrm{R}_{\mathrm{f}}\) value than II (d) I moves slower and has higher \(\mathrm{R}_{\mathrm{f}}\) value than II

Step-by-Step Solution

Answer

Option (b) is correct; II moves faster and has a higher Rf value than I.

Step 1: Understanding Chromatography Concepts

In column chromatography, compounds separate based on their adsorption to the stationary phase and their solubility in the mobile phase. A compound that adsorbs more strongly to the stationary phase moves slower, and one with less adsorption moves faster.

Step 2: Identifying Adsorption Strength

The problem states that compound I has greater adsorption than compound II ( \(\text{adsorption of I} > \text{adsorption of II}\)). This means compound I will move slower through the column than compound II.

Step 3: Relating Movement to Rf Value

The Rf value (retention factor) is higher for compounds that move faster through the column. Therefore, the compound with less adsorption (which moves faster), II, will have a higher Rf value than compound I.

Step 4: Identifying the Correct Statement

Based on our analysis, compound II moves faster and hence will have a higher Rf value than compound I. Thus, option (b) II moves faster and has a higher \(R_{f}\) value than I is correct.

Key Concepts

Adsorption

In the world of chromatography, adsorption plays a crucial role in separating different substances. Adsorption is a process where molecules stick to the surface of another material, often used in the stationary phase of chromatography. This interaction affects how quickly or slowly a compound travels through a chromatographic system.

Consider adsorption like a relationship between the molecules in the mixture and the material they interact with. When molecules strongly adhere to the stationary phase, they tend to move more slowly, as they get 'stuck' more frequently. Conversely, those molecules with weaker adsorption to the stationary material can travel more rapidly.

In our exercise, compound I shows greater adsorption compared to compound II. This means compound I will travel at a slower rate through the chromatographic column. Understanding adsorption helps predict how different compounds will separate during the chromatography process.

Rf value

The Rf value, or retention factor, quantifies how far a component in the mixture travels relative to the solvent front in chromatography. It's a crucial metric because it provides a standardized way to describe the behavior of substances in the separation process.

Calculating the Rf value is straightforward: the distance traveled by the compound is divided by the distance traveled by the solvent front. The Rf values range between 0 and 1, where 0 means no movement from the origin, and 1 means the compound traveled as far as the solvent front.

• High Rf value: Indicates the compound traveled far during the process, suggesting weaker adsorption to the stationary phase.
• Low Rf value: Indicates stronger adsorption to the stationary phase, implying slow movement.

In our problem, compound II, with weaker adsorption and hence faster movement, has a higher Rf value than compound I.

Retention factors

Retention factors, often symbolized as Rf values, are fundamental in chromatography because they convey the extent to which a molecule remains adsorbed on a stationary phase versus being carried along by a mobile phase.

When analyzing chromatographic results, retention factors help in comparing how different compounds within a mixture behave under identical conditions. They are a simple yet powerful indicator of molecular interactions and separation efficiencies without needing sophisticated equipment.

Retention factors provide insights into a molecule's polarity and help to infer the strength of interaction with the stationary phase. In the exercise scenario, we identify compound II as having a greater retention factor than compound I, confirming that compound II moves faster with less interaction with the stationary phase.

Column chromatography

Column chromatography is a versatile technique used extensively in laboratories to separate mixtures of compounds. It involves a stationary phase packed in a column and a mobile phase that moves through it, carrying the compounds of interest.

The stationary phase is usually a solid that adsorbs substances, while the mobile phase, often a liquid, helps move these substances through the column. The compounds in the mixture are separated based on their varying interactions with these two phases.

In practice:

• Mixtures are dissolved in the mobile phase at the column's top.
• Compounds move through the column at different speeds.
• Those with greater adsorption (like our compound I) move slower.
• Those with lesser adsorption (like compound II) move faster.

The entire process requires care, as factors like flow rate, column length, and the nature of the stationary phase significantly affect the separation efficiency. In our exercise, understanding column chromatography allows us to predict and analyze the behavior of compounds I and II regarding their movement and Rf values.