Atmosphere and Some Common Properties of Gases

Composition of the Atmosphere and Some Common Properties of Gases 12-10 Determination of Molecular Weights and Molecular Formulas of Gaseous... 

Composition of the Atmosphere and Some Common Properties of Gases... 

COMPOSITION OF THE ATMOSPHERE AND SOME COMMON PROPERTIES OF GASES... 

Recall that the mass balance equations of Eqs. (1.1a) and (1.1b) incorporate not only terms for internal chemical reactions but also terms for physical mass transport across the boundaries of the control volume. Often, useful control volume boundaries coincide with boundaries between phases, such as between air and water or between water and solid bottom sediment, as discussed for the lake control volume in Section 1.3.1. Note, however, that the terms "environmental media" and "phases" are not interchangeable. For example, chemicals in the gas phase can refer to chemicals present in gaseous form in the atmosphere or in air bubbles in surface waters or in air-filled spaces in the subsurface environment. Chemicals in the aqueous phase are chemicals dissolved in water. Chemicals in the solid phase include chemicals sorbed to solid particles suspended in air or water, chemicals sorbed to soil grains, and solid chemicals themselves. In addition, an immiscible liquid (i.e., a liquid such as oil or gasoline that does not mix freely with water) can occur as its own nonaqueous phase liquid (NAPL, pronounced "napple"). Some examples of mass transport between phases are the dissolution of oxygen from the air into a river (gas phase to aqueous phase), evaporation of solvent from an open can of paint (nonaqueous liquid phase to gas phase), and the release of gases from new synthetic carpet (solid phase to gas phase). Mass transport between phases is affected both by physics and by the properties of the chemical involved. Thus, it is important to imderstand both the types of chemical reactions that are common in the environment, and the relative affinities that various chemicals have for gas, liquid, and solid phases. 

For low pressures (a few atmospheres and lower) we can apply the ideal gas model for gases and ideal mixture models for liquids. This formulation is very common in reactor technology. In some cases at higher pressures, the pressure effect on the gas phase is important. A suitable model for these systems is to use an EOS for the gas phase, and an ideal mixture model for liquids. However, in most situations at low pressures the liquid phase is more non-ideal than the gas phases. Then we will rather apply the ideal gas law for the gas phase, and excess properties for liquid mixtures. For polar mixtures at low to moderate pressures we may apply a suitable EOS for gas phases, and excess properties for liquid mixtures. All common models for excess properties are independent of pressure, and cannot be used at higher pressures. The pressure effect on the ideal (model part of the) mixture can be taken into account by the well known Poynting factor. At very high pressures we may apply proper EOS formulations for both gas and liquid mixtures, as the EOS formulations in principle are valid for all pressures. For non-volatile electrol3d es, we have to apply a suitable EOS for gas phases and excess properties for liquid mixtures. For such liquid systems a separate term is often added in the basic model to account for the effects of ions. For very dilute solutions the Debye-Htickel law may hold. For many electrolyte systems we can apply the ideal gas law for the gas phase, as the accuracy reflected by the liquid phase models is low. 

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