Question

What is Henry's law? Define each term in the equation, and give its units. How would you account for the law in terms of the kinetic molecular theory of gases? Give two exceptions to Henry's law.

Short answer

Henry’s Law relates gas solubility to pressure; exceptions include chemical reactions or high pressures.

Step-by-step solution

State Henry's Law

Henry's Law states that at a constant temperature, the amount of gas that dissolves in a liquid is directly proportional to the partial pressure of that gas above the liquid. The law is usually expressed as \( C = k_H P \).

Define Each Term and Units

In Henry's Law, \( C \) represents the concentration of the dissolved gas in the liquid, usually measured in mol/L. \( k_H \) is Henry's Law constant, which varies depending on the gas and liquid, with units L·atm/mol. \( P \) is the partial pressure of the gas above the liquid in atm.

Explain Using Kinetic Molecular Theory

According to the kinetic molecular theory, gas molecules move rapidly and are widely spaced. When a gas is in contact with a liquid, the molecules collide with the surface allowing some to enter the liquid phase. As partial pressure increases, more collisions occur, and consequently, more gas dissolves, leading to a higher concentration according to Henry's Law.

Provide Exceptions to Henry's Law

Two major exceptions to Henry's Law include cases of high gas pressures or when the gas reacts chemical within the solvent, deviating from the ideal behavior described by the law. For example, ammonia in water reacts chemically making it deviate from Henry's Law.

Key concepts

Kinetic Molecular Theory and Gas Solubility

The kinetic molecular theory provides a foundation for understanding the behavior of gases. This theory posits that gas molecules are in constant, rapid, and random motion. They occupy a volume that is mostly empty space, allowing them to move freely and with considerable kinetic energy.
When considering the interaction between gas and liquid under Henry's Law, this constant movement is crucial. As gaseous molecules collide with the liquid surface, some of them gradually enter the liquid phase. The more collisions there are with the surface, the more gas dissolves.
According to Henry's Law, as the partial pressure of a gas above a liquid increases, the number of collisions with the liquid increases as well. This results in a greater number of gas molecules breaking into the liquid, thus increasing the concentration of dissolved gas. This simple kinetic explanation shows us why pressure influences the solubility of gases in liquids.

Understanding Partial Pressure

Partial pressure is an essential component of Henry's Law. It refers to the pressure that a single type of gas in a mixture of gases would exert if it were alone in the entire volume. This is critical in understanding how gases dissolve in liquids.
The partial pressure of the gas above the liquid directly impacts how much of the gas will dissolve. The more considerable the partial pressure of a specific gas, the more molecules are present to collide with the liquid's surface. Thus, higher partial pressures generally increase the solubility of the gas, aligning with Henry's Law.
In a closed system, adjusting the partial pressure of the dissolved gas will alter its concentration in the liquid. This is why controlled environments, where pressure can be adjusted, are often required in various scientific and industrial processes.

Concentration of Dissolved Gas

Understanding the concentration of the dissolved gas is crucial for applying Henry's Law. This concentration is represented as "C" in the equation, expressed in units of mol/L, indicating how much gas has diffused into the liquid.
When the partial pressure of the gas is increased, the concentration of the gas molecules that dissolve in the liquid will also rise, assuming the temperature remains constant. This is because more gas particles are being forced into the liquid due to the increased number of collisions, as described by the kinetic molecular theory.
This is why Henry's Law is particularly useful in predicting the behavior of gases under various pressure scenarios, such as those found in carbonated beverages, where carbon dioxide is dissolved at high pressures.

Exceptions to Henry's Law

While Henry's Law provides a reliable model for gas solubility under many conditions, exceptions do exist. These exceptions often occur under conditions where ideal behavior assumptions break down.
One major exception is when gas is at very high pressures. Under extreme pressures, the gas may behave non-ideally and the law's proportionality between pressure and concentration no longer holds.
Another exception occurs when a gas undergoes a chemical reaction with the solvent. A common example is ammonia in water which forms ammonium hydroxide, deviating from the behavior predicted by Henry's Law. Such chemical interactions can dramatically skew the relationship between partial pressure and solubility, leading to concentrations that the simplistic model cannot predict accurately.
These exceptions highlight the importance of understanding the specific conditions and behavior of gas-solvent systems when applying Henry's Law.