Problem 71 - Chemistry: An Atoms-Focused Approach

Question

Henry's law (Chapter 10 ) predicts that the solubility of a gas in a liquid increases with its partial pressure. Explain Henry's law in relation to Le Chatelicr's principle.

Step-by-Step Solution

Answer

Answer: Henry's law is consistent with Le Chatelier's principle. When the partial pressure of a gas increases, Henry's law predicts that the solubility of the gas in the liquid increases. This behavior aligns with Le Chatelier's principle, as the system shifts its equilibrium to counteract the stress caused by increased partial pressure, dissolving more gas molecules in the liquid and increasing its solubility.

Step 1: Define Henry's Law

Henry's law states that the solubility of a gas in a liquid is directly proportional to its partial pressure. Mathematically, it can be expressed as: C = kP Where: C = concentration of the gas in the liquid k = Henry's law constant (specific for each gas and liquid pair) P = partial pressure of the gas According to this law, as the partial pressure of a gas increases, its solubility in the liquid also increases, which can be observed in everyday life; for example, the carbonation of beverages is done by dissolving carbon dioxide under high pressure in the liquid.

Step 2: Define Le Chatelier's Principle

Le Chatelier's principle states that when a chemical system at equilibrium is subjected to an external stress (such as changes in temperature, pressure, or concentration of reactants or products), the system will adjust itself to partially counteract the stress and re-establish a new equilibrium. In other words, if a stress is applied to a system at equilibrium, the system will shift its equilibrium position to alleviate the stress. This principle helps us predict the changes in the position of equilibrium as the external conditions change.

Step 3: Connecting Henry's Law to Le Chatelier's Principle

Now, let's establish a connection between Henry's law and Le Chatelier's principle. Consider the following equilibrium between a gas and its dissolved form in a liquid: Gas (g) <=> Gas (aq) According to Henry's law, increasing the partial pressure of the gas (P) increases the concentration of the dissolved gas (C) in the liquid, and vice versa. When the partial pressure of the gas is increased, it causes a stress on the equilibrium system. In order to counteract the stress, based on Le Chatelier's principle, the system will shift its equilibrium position to aid in the dissolving of more gas molecules into the liquid. This means that the forward reaction (Gas (g) -> Gas (aq)) will be favored, and the solubility of the gas in the liquid will increase, as predicted by Henry's law. In summary, Henry's law is consistent with Le Chatelier's principle. When the partial pressure of a gas increases, Henry's law predicts that the solubility of the gas in the liquid increases. This behavior aligns with Le Chatelier's principle, as the system shifts its equilibrium to counteract the stress caused by increased partial pressure, dissolving more gas molecules in the liquid and increasing its solubility.

Key Concepts

Le Chatelier's Principle

Le Chatelier's principle is a fundamental concept in chemistry that provides insight into how a system at equilibrium responds to changes, or stresses, introduced into the system.
Whenever an equilibrium is disturbed by changing the conditions, the system will respond by shifting in a way that partially counteracts the change. This principle applies to changes in:

• Temperature
• Pressure
• Concentration of reactants or products

For example, if you increase the concentration of a reactant, the system will shift towards the products to reduce the added concentration. Similarly, if pressure is increased in gas reactions, the system tends to shift towards the side with fewer molecules to lower the pressure.
Thus, Le Chatelier's principle helps us predict how the position of equilibrium will change in response to external stresses, ensuring that the equilibrium adapts to maintain balance.

Gas Solubility

Gas solubility is an important concept when discussing how gases dissolve in liquids. It is influenced by several factors including Henry's law, which can predict the solubility behavior of gases.
Henry's law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.
Mathematically, this is expressed as:\[C = kP\]Where:

• \(C\) is the concentration of the gas in the liquid
• \(k\) is the proportionality constant or Henry's law constant, which is unique to each gas-liquid combination
• \(P\) is the partial pressure of the gas

Increasing the partial pressure "forces" more gas into solution, raising its solubility. For instance, the process of making carbonated drinks involves increasing the pressure of carbon dioxide, thus enhancing its solubility in the beverage.
This enhanced solubility is reversible; when the pressure is released, such as when the drink is opened, the gas can escape, decreasing its solubility.

Chemical Equilibrium

Chemical equilibrium occurs in a closed system when the rates of the forward and reverse chemical reactions become equal, resulting in no net change in the composition of the system.
At this point, the concentrations of the reactants and products remain constant over time, even as the reactions continue to occur:
For a typical reaction:\[A + B \rightleftharpoons C + D\]Equilibrium is reached when:\[ \text{rate of forward reaction} = \text{rate of reverse reaction} \]At equilibrium, the system's composition is stable. However, the equilibrium can shift if any of the conditions change, as dictated by Le Chatelier's principle.
Understanding equilibrium is crucial in processes like gas solubility, where the concentration of a dissolved gas balances with its gaseous state above the liquid. Henry's law helps in understanding how changes in pressure can shift this equilibrium to favor either the dissolved or gaseous state.
Therefore, learning how equilibrium works allows chemists and engineers to manipulate conditions to achieve desired outcomes in reactions and processes.