Gas Phase Chemical Activity

This relationship will hold exactly for ideal gases, those that do not have any interaction between molecules. Most gases behave as nearly ideal gases as long as the pressure is not high, so this is usually a good approximation. 

The act or process by which a compound such as oxygen is molecularly mixed with a liquid (such as water) or a solid (such as a polymer) is called dissolution, and the result of the mixing is a solution. If a solution is very dilute, as is commonly found in packaging, it behaves as an ideal solution, and again the activity coefficient is approximately 1, so concentration can be substituted for activity in thermodynamic relationships. In order to describe the solubility of a compound present in a gas phase that is in contact with a solid phase, as may be the case of oxygen in air contacting a polymer, we need a relationship between the concentration in the liquid (or solid) phase and the concentration (or partial pressure) in the gas phase. In other words, we need an expression for the solubility of the substance at equilibrium, as a function of the partial pressure of the gas or vapor in the contacting gas phase. 

William Henry found in 1803 that the equilibrium vapor pressure of a solute above an ideal solution was proportional to its concentration (see Fig. 14.2)  

The Henry s law proportionality constant is called the solubility coefficient, often represented by S, and is a function of temperature. Henry s law holds exactly for ideal solutions, and is a good approximation for most real solutions, as long as they are dilute. In particular, it works well for oxygen and carbon dioxide up to about one atmosphere, and for many organic vapors at low concentrations. 

For nonideal solutions, other equations can be used to describe the solubility relationship. The choice of governing equation depends on the nature of the nonideal behavior, in other words, on the types of interactions between molecules that cause the behavior to deviate from ideality. 

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