Problem 3 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An open beaker of water in equilibrium with water vapour is in a sealed container. When a few grams of glucose are added to the beaker of water, the rate at which water molecules: (a) leaves the vapour increases (b) leaves the solution increases (c) leaves the solution decreases (d) leaves the vapour decreases

Step-by-Step Solution

Answer

(d) leaves the vapor decreases.

Step 1: Understand the Effect of Adding Solute

Adding a solute, such as glucose, to water decreases the water's vapor pressure because the solute molecules occupy space at the water's surface, hindering water molecules from escaping into the vapor phase. This phenomenon is known as Raoult's Law, which states that the vapor pressure of a solvent above a solution is directly proportional to the mole fraction of the solvent present.

Step 2: Analyze the Impact on Water Molecules Leaving the Solution

Because the vapor pressure of water decreases when glucose is added, fewer water molecules will have enough energy to escape from the liquid phase into the vapor phase. This means the rate at which water molecules leave the solution decreases when glucose is added.

Step 3: Evaluate the Impact on Water Molecules Leaving the Vapor

Since the addition of glucose lowers the vapor pressure, the equilibrium shifts to maintain balance. Consequently, more water molecules from the vapor phase will tend to return to the solution to re-establish equilibrium. This results in a decreased rate of water molecules leaving the vapor.

Step 4: Select the Correct Option

Considering the effects explained, the correct option is (d) leaves the vapor decreases. Adding glucose reduces the number of water molecules escaping the solution and increases the number returning from the vapor phase, decreasing the rate at which water molecules leave the vapor.

Key Concepts

Vapor Pressure Lowering

The concept of vapor pressure lowering is central to understanding how solutions behave. Vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature. When a non-volatile solute like glucose is added to a solvent like water, the vapor pressure of the water decreases. This happens because solute particles spread out and take up surface area that would otherwise be available for solvent molecules. Raoult's Law explains this effect quantitatively, stating that the vapor pressure of a solvent in a solution is proportional to the mole fraction of the solvent. In simpler terms, the more solute particles there are, the fewer solvent particles can escape into the vapor phase. With fewer water molecules transitioning to the vapor, the vapor pressure drops.
This principle is vital in applications ranging from food preservation to the creation of antifreeze agents.

Solute Effects on Boiling Point

Adding a solute, such as glucose, not only affects vapor pressure but also has a significant impact on the boiling point of a liquid. This phenomenon, known as "boiling point elevation," occurs because the solute-solvent interaction requires additional energy (heat) to allow the solvent to transition from the liquid phase to the vapor phase. As solute particles interfere with the escape of solvent molecules, the system requires a higher temperature to reach the boiling point. Therefore, solutions generally boil at a higher temperature than pure solvents. Understanding this concept is critical for practical applications, such as cooking and industrial processes, where precise control of boiling points can influence product quality and safety.
This explains why salt is often added to water during cooking: it increases the boiling point, allowing food to cook at a slightly higher temperature.

Chemical Equilibrium in Solutions

Chemical equilibrium in solutions is a delicate balance. When glucose is added to a beaker of water, the entire system experiences minor yet significant shifts. Initially, the rate at which water molecules escape into vapor decreases due to the vapor pressure lowering. According to Le Chatelier's Principle, the system will adjust in an attempt to re-establish equilibrium. As fewer water molecules leave the water surface, more are likely to return from the vapor phase. Thus, equilibrium shifts to counteract the change initiated by the solute. This process illustrates how chemical equilibrium is dynamic, constantly adjusting to changes in concentration, temperature, or pressure. Understanding this movement is crucial for predicting how solutions will react and adapt to new conditions, aiding scientists and engineers in designing more efficient chemical processes.
By appreciating this equilibrium shift, one can better understand how to manipulate reactions to achieve desired outcomes.