Pressure expressing units

For a second-order reaction the units are liters per mole per second (L mol s ) or some other units expressing the reciprocal of concentration or pressure per unit time interval. 

The gas constant R is generally given the value 8.314 J K 1 mol 1, but in fact this numerical value only holds if each unit is the SI standard, i.e. pressure expressed in pascals, temperature in kelvin and volume in cubic metres. 

It has repeatedly been stressed that, in this case, the quantities involved may be favorably expressed in pressure-independent units such as molality or mole fractions (Asano and Le Noble, 1978). Then the last term disappears, and we can write... 

Note that in this case the rate constant will have to be expressed in pressure-dependent units. This is also true for Eq. (22). 

A quantity (symbolized by AV or A V ) derived from the pressure dependence of a reaction rate constant Ay = -RT(dlnk/dF)j where R is the molar gas constant, T is the absolute temperature, k is the reaction rate constant, and P is pressure. For this equation, the rate constants of all non-first-order reactions are expressed in pressure-independent units (e.g., molarity) at a fixed temperature and pressure. 

The minns sign in Eq. (4.2) arises dne to the fact that in order for there to be heat flow in the +y direction, the temperatnre gradient in that direction must be negative—that is, lower temperature in the direction of heat flow. If the temperature gradient is expressed in units of K/m, and the heat flux is in J/m - s, then the thermal conductivity has units of J/K m s, or W/m K. A related quantity is the thermal diffusivity, which is often represented by the lowercase Greek letter alpha, a. Thermal diffusivity is defined as k/pCp, where k is the thermal conductivity, p is the density, and Cp is the heat capacity at constant pressure per unit mass. We will see in a moment why the term diffusivity is used to describe this parameter. We will generally confine our descriptions in this chapter to thermal conductivity. 

For every strain, there arises, in an clastic substance, a corresponding stress, which represents ihe tendency of the substance tu recover its normal condition. Stress is expressed in units of force per unit area. Tensile stress, lor example, is the ratio of Ihe force of tension to ihe normal cross section of ihe rod subjected to it. Shearing stress is ihe force lending to push one layer of the material past the adjacent layer, per unit area of the layers. Pressure, expressed in like units, is the stress corresponding to volume compression, etc. 

Subatmospheric pressure usually is expressed in reference to perfect vacuum or absolute zero pressure, lake absolute zero temperature (the concept is analogous), absolute zero pressure cannot be achieved, but it does provide a convenient reference datum. Standard atmospheric pressure is 14.695 psi absolute, 30 inches of mercury absolute, or 760 mmHg of density 13.595 g/cm3 where acceleration due to gravity is g = 980.665 emir. ] mmHg. which equals 1 torr. is the most commonly used unit of absolute pressure. Derived units, the million or micrometer, representing 1/1000 of 1 mmHg or 1 torr, are also used for subtorr pressures. 

Blowdown The difference between the actual set pressure of an SRV and the actual reseating pressure, expressed as a percentage of set pressure or in pressure units. 

A patient has a systolic blood pressure of 100 mmHg. Express this pressure in units of atm, torr, and kPa. 

Standard free energy Standard free energy of a reaction gives die increase of free energy when the reactants, all in their standard states of unit pressure or unit concentration are converted at constant temperature (7) into the products, which are also in their standard states. The standard free energy is given by the expressions,... 

It does not matter what units partial pressures are expressed in. Neither does it matter, once the standard pressure p°has been decided, what units p° is expressed in. For example p° could be 1 bar, but if expressed as 100 kPa this is still the same. Taking equation (A4) as an example, with all partial pressures expressed in 1 atm (say) and p° = 1 bar, we have ... 

Mathematically stated if pn denotes the normal pressure per unit area and pt the resistance to sliding per unit area, then the angle of repose, , is simply given by the expression... 

The thermodynamic criterion for the equilibria CaCO, ) = Ca0(,) + C02 (,) is AG ° = -RT n Kp, where AG° is the change in Gibbs free energy of the reactants and products in their standard state, R is the gas constant, and Kp is the equilibrium constant. For this equilibria, A p = pco, for pressure in units of atmospheres. Values for AG are tabulated in the form AG° = a+ bT combining these expressions yields an exponential relationship between the partial pressure of CO2 and temperature for the above equilibria. Complete derivations and discussion of these equations may be found in physical chemistry textbooks such as references [13] and [14]. 

The piessure of the sumdard atmosphere is equal to thc piessure produced by a column of meicury 700 inm high. The density of mercury is 18.6g/cm b What is the standard atmospheric pressure expressed in the units g/cm- What is it in units Ibs/sq. in. ... 

Convert a pressure expressed as a head of a fluid to the equivalent pressure expressed as a force per unit area, and vice versa. 

The conversion table on the inside front cover of this book lists values of a pressure expressed in several common force/area units and as heads of mercury and water. The use of this table for pressure unit conversion is illustrated by the conversion of 20.0 psi to cm Hg ... 

The pressure gauge is reading psig, not psia. From Eq. (1.25) the absolute pressure is the sum of the gauge pressure and the atmospheric (barometric) pressure expressed in the same units. We will change the atmospheric pressure to psia. 

Thermod5mamic equilibrium relations are used to define the partitioning of gases between the vapor and hquid phases. The amormt of gas in the vapor phase is most often expressed as pressure (in units of atmospheres, bars or Pascals). One atmosphere is equal to 1013.25 millibars pressure (mbar) and 101.325 kilopascals (kPa 1 bar = 10 Pa). In a mixture of gases the partial pressure, Pi, of an individual gas, i, is its fraction of the total gas pressure. The total pressure of gases in the atmosphere, Patm. is equal to the sum of the partial pressures of the individual gases... 

The SI unit of P is the pascal (Pa). However, millimeters of mercury (also called torr) has continued to remain popular (see Box 27-2 for conversion factors). Use of SI units does have a practical advantage in that 1 atm almost equals 100 kPa (1 atm = 101.325 kPa). Partial pressures expressed in kilopascals are therefore very close estimates of percentages of the gases in the mixture at 1 atm. Pressure, P (or p), may mean either total pressure, as in the expression P(Amb) for the mixture of gases in ambient air, or partial pressure in blood, as in P02(aB). Numerical value and units of R differ depending on the units used for P, so that ... 

To decide which value of R to use when you convert between and K, you can reason as follows. involves molar concentrations, for which the units are mol/L involves pressures expressed in atm. Thus the appropriate value of R to use for these conversions... 

The rate constant in the above equation is expressed in terms of pressure independent units (mole fraction). If the rate constant is expressed in terms of pressure... 

If the equilibrium constant is expressed in terms of pressure dependent units (such as concentration), the relevant equation is ... 

Because O2 is a gas, it is convenient to express [O2] in terms of its partial pressure in units of Torr (or mm Hg multiply by 0.133 to obtain kilopascals). Therefore,... 

Table 7.2 Changes of chemical potential resulting from non-unit pressures expressed as (ju-p°), for an ideal gas at 298 K...

Fig. 3-4. Meridonal cross section of ozone in the atmosphere. Left frame Expressed as partial pressure in units of nbar. Right frame Expressed as mixing ratio in units of ppmv. [From Diitsch (1980), with permission ]...

Expression. Both sedimentation and filtration are suitable separation techniques when the mixture of liquid and solids is sufficiently mobile to allow pumping, or similar method of motion, of the fluid to a barrier which retains the solid but not the liquid. If such movement is not possible then separation can be accomplished by compressing the mixture under conditions which permit the liquid to escape while retaining the solid between the compressing surfaces. This technique is called expression. Design of expression equipment is varied. Batch systems usually operate by the application of hydraulic pressure in units such as the box press, pot press, curb press and cage press. Continuous expression utilises equipment such as screw presses, roller mills, and belt presses [23]. 

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