Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Rate of physisorption increases with(a) decrease in temperature (b) increase in temperature (c) decrease in pressure (d) decrease in surface area

Step-by-Step Solution

Answer

The rate of physisorption increases with a decrease in temperature.

Step 1: Understanding Physisorption

Physisorption, or physical adsorption, occurs when gas molecules adhere to a surface due to weak van der Waals forces. These interactions are relatively weak compared to chemisorption and are influenced by temperature and pressure.

Step 2: Effects of Temperature on Physisorption

Physisorption is an exothermic process, meaning it releases heat. According to Le Chatelier's principle, for gases adhering to a surface in an exothermic process, decreasing the temperature will favor the adsorption, thereby increasing the rate of physisorption.

Step 3: Reviewing Pressure Influence

Physisorption occurs due to the alignment of molecules on a surface. When pressure increases, more gas molecules are available to occupy adsorption sites, thus increasing the rate of physisorption. Conversely, decreasing the pressure would decrease the rate.

Step 4: Surface Area Considerations

The rate of adsorption, including physisorption, is proportional to the available surface area. Therefore, decreasing the surface area would reduce the rate, as there are fewer sites available for the molecules to adhere.

Step 5: Choosing the Correct Answer

The correct scenario that increases the rate of physisorption involves lowering the temperature as it enhances the adsorption process. Therefore, the right answer is option (a) decrease in temperature.

Key Concepts

Temperature effects on adsorption

In the world of chemistry, temperature plays a crucial role in the process of adsorption. Particularly in physisorption, this process is exothermic, meaning it releases heat. When we apply Le Chatelier's principle, it suggests that lowering the temperature will favor the forward reaction of adsorption. This happens because the system tries to counteract the change by absorbing more gas molecules, thus enhancing adsorption.

Therefore, as the temperature decreases, the rate of adsorption on a surface increases. It's important to understand this inverse relationship between temperature and the rate of physisorption. Remember, cooler conditions mean more efficient adsorption.

Van der Waals forces

Van der Waals forces are the backbone of physisorption. They are weak electrostatic forces that emerge between molecules. Although they are weaker than covalent or ionic bonds, they play a pivotal role in adsorption.

In physisorption, these forces allow gas molecules to adhere to a solid surface. They are significant enough to cause adsorption but are easily reversible, which is why physisorption is often easily undone by increasing temperature or reducing pressure. Understanding Van der Waals forces is key to grasping how physisorption works.

Le Chatelier's Principle

Le Chatelier’s principle is a fundamental concept in chemistry used to predict the effect of a change in conditions on a chemical equilibrium. It states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

In the context of physisorption, if the process releases heat (exothermic process), a decrease in temperature will encourage more adsorption in order to produce more heat. This principle helps us predict and understand how changes in temperature can affect adsorption rates, guiding us in optimizing conditions for better adsorption.

Surface area in adsorption

The surface area of an adsorbent greatly influences the rate of adsorption. The larger the surface area, the more adsorption sites are available for the gas molecules to adhere to.

When the surface area is decreased, the number of available sites is also reduced, leading to a decreased rate of adsorption. This is a simple yet critical concept—more area means more places for molecules to stick to, ultimately enhancing the overall adsorption process. High surface area materials, like activated charcoal, are often used to maximize adsorption in various applications.

Pressure influence on adsorption

Pressure plays a direct role in physisorption by affecting the availability of gas molecules. At higher pressures, more gas molecules are present, increasing their collision frequency with the surface, and therefore enhancing the rate of adsorption.

On the flip side, decreasing the pressure results in fewer molecules available for adsorption, hence reducing the rate. This is why in practical applications requiring high rates of adsorption, maintaining sufficient pressure is critical, ensuring that the surface is constantly supplied with molecules to adsorb.