Fugacity

The fugacity of a gas is defined by the ideal gas law, which relates the pressure and volume to the number of molecules inside a container. This article will address one concept in particular, fugacity (f). Fugacity is the measure of how easily a gas may permeate into certain substances. It is measured as partial pressure or pressure and represents an actual value rather than potential (i.e. what could be). If one were to use this definition, it would show that while some gases are more potent than others and are more readily absorbed into surfaces, they will have low fugacities due to their low solubility.

Definition of Fugacity:

Fugacity is the amount of gas that gas may absorb through a given surface (or in this case, a substance). There are two common definitions for fugacity:

1. Fugacity is the amount of gas that would be held inside a container if all the molecules were removed but nothing else.

2. Fugacity is the pressure and volume at which the partial pressure of individual molecules equals their intermolecular potential in a perfect vacuum.

These definitions are not entirely correct on their own, and even when used together, do not offer a complete picture of our understanding of fugacity.

Applications of Fugacity:

• Fugacity is used in economic geology to quantify the amount of gas that can be extracted from a deposit.

• It is also used in soil science to determine the effects of pollutants on the ground.

Significance of Fugacity:

Fugacity is important in understanding the behavior of some gases, namely methane and carbon dioxide. Fugacity relevant to our study is that it shows how easily a gas may permeate into certain substances. For example, CO2 is easier to absorb by a solid than by an open-pored porous material such as clay.

Temperature:

Temperature affects fugacity because it changes the intermolecular potential. The information presented below will explain how temperature changes fugacity, but one thing to note is that temperature affects the volatility of gases and thus their tendency to evaporate or condense in different ways.

For example, higher temperatures promote the condensation of CO2 and water vapor. At the same time, higher temperatures promote the evaporation of CH4 and N2.

Atmospheric pressure:

Moreover, atmospheric pressure affects fugacity because it can increase the ability of gas to permeate into solids. For example, atmospheric pressure increases the ability for methane to permeate through a 10-millimeter-thick quartz plate. Increased atmospheric pressure also makes gases more likely to condense in cold environments.

Fugacity Coefficient:

It is important to note that fugacity is not a constant for every substance and every gas. Fugacity coefficients are calculated by measuring how easily a gas may be absorbed into something or how easily one substance can be exchanged or converted into another. The coefficient depends on the type of material being exchanged and has values ranging from .0 – 1.0, where 0 equates to no exchange possible, and 1 equates to complete exchange (a substance can be completely converted into another).

Fugacity from a Statistical Mechanics Point of View:

Fugacity is believed to come from the theory of statistical mechanics. It is often referred to as the “ideal gas law” or the “ideal gas equation.” The equation for fugacity is:

f = RT/P Where: R= Gas constant, T= temperature, and P= Pressure

The equation can be found using these assumptions:

• All gas molecules are identical in their velocity.  

• There are no intermolecular forces between molecules. 

• The particles are very small and can be allowed to pass through each other without any hindrance.

Fugacity Coefficient:

The fugacity coefficient is a function of the temperature, pressure, and identification of the substance in which the gas is dissolved. The formula for calculating a fugacity coefficient looks like this:

dln f/f°=dG/RT=Vd/PRT

where: f= fugacity coefficient (dimensionless)

G signifies Gibbs free energy,

R signifies gas constant,

V is for the molar volume of the fluid and,

Fugacity coefficients are important for predicting how gases will be distributed within a certain material. They are used for things such as studying the thermal decomposition of organic compounds and predicting anhydrous solubility.