Question

A beaker of water is initially saturated with dissolved air. Explain what happens when He gas at 1 atm is bubbled through the solution for a long time.

Short answer

Helium gas reduces the partial pressures of nitrogen and oxygen, causing them to leave the solution; the water eventually contains more helium.

Step-by-step solution

Understanding Initial Conditions

Initially, the beaker of water is at equilibrium with the dissolved air, meaning the concentration of gases dissolved follows Henry’s law. Air is a mixture of gases, primarily nitrogen and oxygen, and each is dissolved in the water to an extent proportional to its partial pressure in the atmosphere above the water.

Introduction of Helium Gas

When helium gas at 1 atm is bubbled through the solution, it introduces a new gas phase that does not initially contain nitrogen or oxygen. Helium is inert and does not react with the water or the dissolved gases.

Le Chatelier’s Principle Application

Applying Le Chatelier’s principle, the system will adjust to counteract the change. The introduction of helium changes the partial pressures above the water since it takes up space that air was previously occupying.

Shift in Gas Equilibrium

Due to the introduction of helium, the partial pressures of nitrogen and oxygen above the solution decrease. This causes a shift, where the dissolved gases (nitrogen and oxygen) will gradually leave the solution to re-establish equilibrium as per Henry's law.

Outcome of the Process

After helium has bubbled through for a long time, most of the nitrogen and oxygen initially dissolved in the water will have left, reaching a new equilibrium where the water now has a higher concentration of dissolved helium and fewer oxygen and nitrogen molecules.

Key concepts

Henry's law

Henry's law is a principle that describes how gases dissolve in liquids. It says that the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of that gas above the liquid.

This means if you increase the pressure of a gas above a liquid, more of that gas will dissolve in the liquid. Conversely, if the pressure decreases, less gas will dissolve.
For example:

• If a soda bottle is sealed, the carbon dioxide is under high pressure, so a lot is dissolved in the liquid.
• Open the bottle, and the pressure is released; hence, carbon dioxide escapes as bubbles.

In the context of our exercise, initially, the water has dissolved gases (like nitrogen and oxygen) as per their partial pressures from the air.
When helium is introduced at a high pressure, according to Henry's law, it too will start to dissolve in the water.

Le Chatelier's principle

Le Chatelier's principle helps predict how a system at equilibrium reacts to disturbances. It states that if an external change occurs on a system at equilibrium, the system will adjust in such a way as to counteract that change and restore equilibrium.

In our exercise, when helium gas is introduced, it changes the balance of gaseous components above the water. The system is disrupted because helium takes up space that was previously for nitrogen and oxygen.
Hence, Le Chatelier's principle predicts that the system will respond by releasing more nitrogen and oxygen out of the liquid to adjust back towards equilibrium.
What this means practically is:

• The presence of helium reduces the partial pressures of nitrogen and oxygen.
• More nitrogen and oxygen will leave the water since they now have lower partial pressures in the atmosphere above the liquid.

Partial pressure

Partial pressure refers to the pressure a single gas in a mixture would exert if it occupied the entire volume by itself.
It's an important concept for understanding how gas mixtures behave, especially when they are dissolved in liquids.

In mixtures, each gas component exerts its own pressure independently of other gases. The sum of these partial pressures equals the total pressure.
In our scenario:

• Air, made of different gases like nitrogen and oxygen, has each component contributing its partial pressure to the total pressure above the water.
• When helium is added, it occupies some of the total pressure, leading to a reduction in the partial pressures of nitrogen and oxygen.

This reduction prompts the release of nitrogen and oxygen from the water, as dictated by Henry's law and Le Chatelier's principle.

Gas equilibrium

Gas equilibrium describes a situation where the concentration of gas dissolved in a liquid remains constant over time. This typically occurs when the rates of gas dissolving into the liquid and gas escaping back into the air are equal.

When the system is at equilibrium:

• The concentration of dissolved gases reflects the partial pressures of those gases above the liquid.
• Adding a new gas, like helium, disrupts this balance.

For the exercise:

• The initial gas equilibrium consists of dissolved nitrogen and oxygen matching their partial pressures in the air.
• Bubbling helium affects this balance, causing a shift in the equilibrium as the partial pressures of nitrogen and oxygen decrease.
• The system achieves a new equilibrium state with diminished amounts of nitrogen and oxygen and increased concentration of helium within the water.