Problem 78 - Chemistry The Central Science In Si Units

Question

You have an evacuated container of fixed volume and known mass and introduce a known mass of a gas sample. Measuring the pressure at constant temperature over time, you are surprised to see it slowly dropping. You measure the mass of the gas-filled container and find that the mass is what it should be-gas plus container-and the mass does not change over time, so you do not have a leak. Suggest an explanation for your observations.

Step-by-Step Solution

Answer

The pressure drop is likely due to gas molecules getting adsorbed onto the container walls, reducing gas molecules in free state and thereby lowering the pressure.

Step 1: Understanding the Situation

The container is evacuated and then filled with a known mass of gas. The container is sealed and kept at a constant temperature. Over time, the pressure inside decreases even though the mass remains constant, indicating no leaks.

Step 2: Analyzing Possible Causes for Pressure Drop

Since there is no mass loss indicating there are no leaks, the pressure drop could be due to gas behavior inside the container. The molecules might be interacting with the container walls or some process might be causing the gas pressure to decrease without changing the mass.

Step 3: Considering Gas-Adsorption Phenomena

One plausible explanation is that the gas molecules are getting adsorbed onto the walls of the container. Adsorption is when gas molecules stick to a solid surface, reducing the free gas molecules available to exert pressure despite having the same total mass.

Step 4: Confirming Gas-Adsorption

Confirm adsorption by considering the material of the container and the chemical nature of the gas. Some materials, like activated charcoal or certain metals, are known for their adsorption capacity, especially for specific gases.

Step 5: Concluding the Explanation

The observed pressure decrease without mass change suggests that adsorption is occurring. While the mass remains constant, the number of gas molecules in free form (not adhered to the surface) decreases, lowering the pressure.

Key Concepts

Pressure Drop

The phenomenon of pressure drop inside a closed container can seem puzzling, especially when the mass remains constant, suggesting that no gas has leaked. Let's delve deeper into understanding this occurrence. Pressure in a gas arises when gas molecules collide with the walls of their container. More collisions equate to higher pressure.
When we observe a pressure drop, it implies fewer molecules are colliding with the walls as time progresses. In a situation where there are no leaks and mass remains constant, it is clear that the molecules are still present but are not behaving as expected.
This unexpected drop in pressure, with no change in mass, suggests that the behavior of the gas needs to be scrutinized. The molecules may be interacting in such a way that impacts their collision frequency with the container walls, leading to reduced pressure.

Adsorption

Adsorption is a crucial concept when explaining pressure changes in contained gases. It occurs when gas molecules adhere to the surface of a material, reducing the number of molecules floating freely in the volume of the gas. This is distinct from absorption, where molecules are taken into the bulk material.
Adsorption is significant in many chemical and physical processes. In our scenario, the gas molecules in the container are experiencing adsorption onto the inner walls. This results in fewer molecules exerting pressure in the gaseous phase.
Several factors can affect adsorption, such as the nature of the gas, the surface area of the container walls, and the temperature. Materials like activated carbon or certain metals are particularly known for their high adsorption capacities, especially for specific gas types. Adsorption provides a plausible explanation for the pressure drop without a change in mass by tying up gas molecules on the container surfaces, reducing their free presence in the system.

Gas-Container Interaction

The interaction between gas molecules and the container they're in can significantly impact the gas's behavior. This interaction is evident in the process of adsorption, where the container's material properties play a pivotal role.
Different materials have varying affinities for attracting or holding gas molecules. This affinity can lead to surface interactions that alter the number of molecules in the gas phase. When gas molecules interact with the container walls, especially if the walls have a high surface area or specific chemical characteristics, the overall gas pressure can decrease due to reduced free molecule interactions.
Therefore, understanding the material composition of the container is paramount. Materials such as metals or ceramics can show considerable gas interactions depending on the type of gas and environmental conditions. These interactions highlight why observing physical changes, like a pressure drop, requires considering the container's properties and their impact on gas behavior.