Gas Pressure Definition and Units

A sample of gas confined to a container exerts a pressure on the walls of its container. For example, the air in the tires of your car exerts pressure on the inside walls of the tires. In fact, gases exert pressure on everything they touch. Thus, while you may add enough air to increase the pressure inside your tire to 32 pounds per square inch (psi), there is also a pressure of approximately 14.7 psi, called atmospheric pressure, acting on the outside of the tire—and on everything else, including your body. The reason you don t feel the pressure of the atmosphere pushing on the outside of your body is that an equal pressure exists inside your body so that there is no net pressure on you. 

Common pencHype pressure gauges actually measure the dHfererKe between irrtemal and external pressure. Thus, if the tire is completely flat, the reading of 0 psi means that the pressure msxte the tire is the same as that outsidb the tire. 

Figu re 11.4 A column ot air 1 cm X 1 cm from Earth s surface to the top of the atmosphere weighs approximately 1 kg. 

TABLE 11.2 1 Units iif Pressure Commonit Used in Chemistry 

Remember that when a unit is raised to a power, any conversion factor you use must also be raised to that power [M Section 1.6]. 

Pressure is defined as the force applied per unit area  

---

Frequently, particularly from the viewpoint of the technological application of a heterogeneous catalytic reaction, the conditions of experiments in a flow reactor are characterized by space velocity or contact time values. Space velocity, V, is the ratio to the volume of the catalyst bed of the volume of a gas mixture, reduced to the normal conditions (0°C, 760 Torr), passed through the reactor per hour. If the reaction involves a volume change, inlet and outlet space velocities should be distinguished. The reciprocal of V is of the dimension of time. Contact time ( conventional contact time), rc, is a value proportional to V l. It is defined as the ratio of the catalyst volume to the volume of the gas mixture passed per unit time, the gas volume being not under normal conditions but at temperature and pressure in the reactor. Usually, tc is expressed in seconds. It follows from the definitions given that... 

The macroscopic behavior of a fixed mass of a gas is completely characterized by three properties volume (V), pressure (P), and temperature (T). Volume is self-evident and needs no comment as stated in Section 2.1, we use the liter as a unit of volume. The definition of pressure and temperature require a little more care. 

The specific heat represents the amount of energy required to raise a unit mass one unit in temperature. For gases, the specific heat differs depending on whether the gas is allowed to do work by expanding against an atmosphere (constant pressure definition, c, ) or is constrained within a volume (constant volume definition, c ). Examples of units of c, and are J/g K or Btu/lbm °F. If we wish to express this on a molar basis we shall use the upper case that is, or Q having units of J/mol K or Btu/lbmol °F. 

The solubilities of gases in liquids may be expressed in various ways. In the definition of the Bunsen coefficient the solubility is expressed as the volume of the ideal gas reduced to O C and 1 atm pressure soluble in a unit volume of liquid under the gas pressure of 1 atm and at the temperature of measurement. The Ostwald solubility ki — Vgl V oiy is defined as the ratio of the volume of absorbed gas to the volume of the absorbing liquid, measured at the same temperature. The Bunsen solubility coefficient a and the Ostwald coefficient L on the assumption of ideal gas behavior are related to the simple g-moles/liter concentration scale c by the relationships which are applicable when P = 1 atm ... 

Threshold lamit Value - The term refers to toxicity by inhalation. The abbreviation used is TLV. The TLV is usually expressed in units of parts per million (ppm) - i.e., the parts of vapor (gas) per million parts of contaminated air by volume at 25 °C (77°F) and atmospheric pressure. For chemicals that form a fine mist or dust, the concentration is given in milligrams per cubic meter (mg/m ). The TLV is defined as the concentration of the chemical in air that can be breathed for five consecutive eight-hour workdays (i.e., 40 hours per week) by most people without suffering adverse health effiects. This is the definition given by the American Conference of Governmental Industrial Hygienists. 

If, in an infinite plane flame, the flame is regarded as stationary and a particular flow tube of gas is considered, the area of the flame enclosed by the tube does not depend on how the term flame surface or wave surface in which the area is measured is defined. The areas of all parallel surfaces are the same, whatever property (particularly temperature) is chosen to define the surface and these areas are all equal to each other and to that of the inner surface of the luminous part of the flame. The definition is more difficult in any other geometric system. Consider, for example, an experiment in which gas is supplied at the center of a sphere and flows radially outward in a laminar manner to a stationary spherical flame. The inward movement of the flame is balanced by the outward flow of gas. The experiment takes place in an infinite volume at constant pressure. The area of the surface of the wave will depend on where the surface is located. The area of the sphere for which T = 500°C will be less than that of one for which T = 1500°C. So if the burning velocity is defined as the volume of unbumed gas consumed per second divided by the surface area of the flame, the result obtained will depend on the particular surface selected. The only quantity that does remain constant in this system is the product u,fj,An where ur is the velocity of flow at the radius r, where the surface area is An and the gas density is (>,. This product equals mr, the mass flowing through the layer at r per unit time, and must be constant for all values of r. Thus, u, varies with r the distance from the center in the manner shown in Fig. 4.14. 

The most useful type of standard state is one defined in terms of a small number of molecules per unit area of adsorbent surface. In an attempt to have a definition analogous to that for three-dimensional matter—one atmosphere at any temperature—Kemball and Rideal (12) defined a standard state with an area per molecule of 22.53T A.2 where T is the absolute temperature. This corresponds to the same volume per molecule as the three-dimensional state if the thickness of the surface layer is 6A. In terms of surface pressure it corresponds to 0.0608 dynes/cm. for a perfect two-dimensional gas at all temperatures, and as such the definition may be extended to cover condensed films. 

The translational partition function is a function of both temperature and volume. However, none of the other partition functions have a volume dependence. It is thus convenient to eliminate the volume dependence of 5trans by agreeing to report values that use exclusively some volume that has been agreed upon by convention. The choices of the numerical value of V and its associated units define a standard state (or, more accurately, they contribute to an overall definition that may be considerably more detailed, as described further below). The most typical standard state used in theoretical calculations of entropies of translation is the volume occupied by one mole of ideal gas at 298 K and 1 atm pressure, namely, y° = 24.5 L. 

Other common ways of expressing abundances, particularly of solid or liquid particles, is to express them as concentrations in units of micrograms per cubic meter or nanomoles per cubic meter. For purposes of consistency, concentrations expressed in these units should be normalized to standard conditions of temperature and pressure. Because there is some confusion as to what constitutes standard conditions in atmospheric chemistry (273 K and 1.013 bar are commonly used in chemistry and physics and 293 K and 1.013 bar are used in engineering), it is important to define the standard conditions that are assumed when reporting data. This explicit definition is frequently not done. Concentrations expressed in these units can be easily converted to mixing ratios by use of the ideal gas law ... 

Henry s law is also used to find the amount of a gas that dissolves at equilibrium in a solvent at a known partial pressure of the gas. The solubility of the gas is proportional to the inverse of the Henry s law constants, as we have defined them in Eqs. (49) and (51). The reader is cautioned, however, that sometimes the Henry s law constant is defined so that solubilities are directly proportional to the Henry s law constants. The units listed for the constant indicate the definition used in any tabulation. Some Henry s law constants are given in Table 1. 

The complexities of definitions occur primarily because concentration can be expressed in so many different variables. In the above, we have assumed that it is expressed in mass per volume or moles per volume. The concentration can equally be well expressed as a mole fraction, which in the liquid phase is commonly indicated by the symbol X and in a gas phase is written as >t. In gases, one can also express concentrations as partial pressures. In some cases, especially in medicine, the concentration can be expressed in other more arcane units. For example, oxygen tension measures the amount of oxygen present in blood, but it is expressed as the partial pressure that would exist... 

The aim of this Chapter is the development of an uniform model for predicting diffusion coefficients in gases and condensed phases, including plastic materials. The starting point is a macroscopic system of identical particles (molecules or atoms) in the critical state. At and above the critical temperature, Tc, the system has a single phase which is, by definition, a gas or supercritical fluid. The critical temperature is a measure of the intensity of interactions between the particles of the system and consequently is a function of the mass and structure of a particle. The derivation of equations for self-diffusion coefficients begins with the gaseous state at pressures p below the critical pressure pc. A reference state of a hypothetical gas will be defined, for which the unit value D = 1 m2/s is obtained at p = 1 Pa and a reference temperature, Tr. Only two specific parameters, Tc, and the critical molar volume, VL, of the mono-... 

---