AP CHEM solutions
Henry’s Law describes the Solubility of gases in liquids. It states that the amount of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid, at a constant Temperature.
The Mathematical Expression of Henry’s Law
Henry’s Law is expressed mathematically as:
$ C = k_H P $
Where:
- $ C $ represents the concentration of the dissolved gas (often expressed in molarity, mol/L).
- $ k_H $ is Henry’s Law constant, a proportionality constant specific to the gas, the solvent, and the Temperature. The units of $ k_H $ depend on the units of $ C $ and $ P $ . For example, if $ C $ is in mol/L and $ P $ is in atm, then $ k_H $ has units of mol/(L·atm).
- $ P $ represents the partial pressure of the gas above the liquid (often expressed in atmospheres, atm).
Understanding Henry’s Law Constant ( $ k_H $ )
The value of $ k_H $ is crucial in understanding the Solubility of a gas. A high $ k_H $ value indicates that the gas is relatively insoluble in the solvent (it takes a high partial pressure to achieve a given concentration). Conversely, a low $ k_H $ value means the gas is relatively soluble (even a low partial pressure leads to a significant concentration in solution).
Factors Affecting Henry’s Law
While Temperature is implicitly held constant in the equation, it significantly impacts Gas Solubility and thus the applicability of Henry’s Law. Temperature Dependence of Gas Solubility The nature of both the gas and the solvent also influence the Solubility. Polar gases tend to be more soluble in polar solvents (like water), while nonpolar gases are more soluble in nonpolar solvents. intermolecular forces and Gas Solubility
Applications of Henry’s Law
Henry’s Law has numerous applications, including:
- Carbonated beverages: The high pressure of CO₂ above the liquid in a soda bottle keeps a significant amount of CO₂ dissolved. When the bottle is opened, the pressure decreases, and CO₂ escapes from the solution, resulting in fizz.
- Scuba diving: At greater depths, the partial pressure of gases (like nitrogen) increases, leading to increased Gas Solubility in the blood. This can cause decompression sickness if the diver ascends too quickly, allowing dissolved gases to form bubbles in the bloodstream.
- Environmental science: Henry’s Law is used to model the exchange of gases between the atmosphere and water bodies (e.g., oxygen and carbon dioxide).
Limitations of Henry’s Law
Henry’s Law is most accurate at low gas pressures and concentrations. At high pressures or concentrations, the interactions between dissolved gas molecules become significant, deviating from the ideal behavior assumed by the law. Moreover, the law is only applicable to gases that do not react with the solvent.