Standard temperature for gases

If additional significant figures are needed, 273.15 can be used in place of 273. The standard temperature for gases is 0°C, which is 273 K, a benchmark temperature used when comparing volumes of gases. No degree symbol is used with Kelvin temperatures. More information about the Kelvin temperature scale appears in the discussion of Charles law. 

Think About It With the pressure held constant, we should expect the volume to increase with increased temperature. Room temperature is higher than the standard temperature for gases (0°C), so the molar volume at room temperature (25°C) should be higher than the molar volume at 0°C— and it is. 

Student Annotation In thermochemistry we often used 25°C as the "standard" temperature—although temperature is not actually part of the definition of the standard state [Mt Section 5.6]. The standard temperature for gases is defined specifically asO°C... 

Because gas properties depend on temperature and pressure, it is useful to have a set of standard conditions of temperature and pressure that can be used for comparing different gases. The standard temperature for gases is taken to be 0 °C = 273.15 K and standard pressure, 1 bar = 100 kPa = 10 Pa. Standard conditions of temperature and pressure are usually abbreviated as STP. It is important to emphasize that STP was defined differently in the past, and some... 

You may wonder how a reaction, such as combustion of methane, can occur at 25°. The fact is that the reaction can be carried out at any desired temperature. The important thing is that the AH° value we are talking about here is the heat liberated or absorbed when you start with the reactants at 25° and finish with the products at 25°. As long as AH0 is defined this way, it does not matter at what temperature the reaction actually occurs. Standard states for gases are 1 atm partial pressure. Standard states for liquids or solids usually are the pure liquid or solid at 1 atm external pressure. 

The specific conditions used in the calculation—1 atm pressure and 0°C (273.15 K)—are said to represent standard temperature and pressure, abbreviated STP. These standard conditions are generally used when reporting measurements on gases. Note that the standard temperature for gas measurements (0°C, or 273.15 K) is different from that usually assumed for thermodynamic measurements (25°C, or 298.15 K Section 8.6). Note also that the standard pressure for gas measurements, still listed here and in most other books as 1 atm (101,325 Pa), has been redefined to be 1 bar (100,000 Pa). Thus, the new standard pressure is 0.986 923 atm, making the standard molar volume 22.711 L rather than 22.414 L. 

When pure solids or liquids, which do not form any solution, participate in reaction beside gases, the standard state for gases is chosen the same as describe in the preceding chapter, while the standard state of the condensed component is defined by their state at 1 atm pressure and at the temperature of the jafstemr Activity of liquid and solid substances varies only very slightly wit]j... 

Property values in the standard state are denoted by the degree symbol (°). For example, C°P is the standard-state heat capacity. Since the standard state for gases is the ideal-gas state, C% for gases is identical with Cj , and the data of Table 4.1 apply to the standard state for gases. All conditions for a standard state are fixed except temperature, which is always the temperature of the system. Standard-state properties are therefore functions of temperature only. 

To size a rotameter requires calculating the volumetric flow rate of a standard fluid at standard conditions. Most manufacturers calibrate rotameters using a stainless-steel float and water at a standard temperature for liquids and air at a standard temperature and pressure for gases. For other fluids, float materials, and operating conditions, the flow rate must be converted to an equivalent flow rate of water or air. To derive a formula for making this conversion, Bernoulli s equation is applied across the float shown in Figure 8.15 to give Equation 8.9. 

The standard state of a substance is a reference state that allows us to obtain relative values of such thermodynamic quantities as free energy, activity, enthalpy, and entropy. All substances are assigned unit activity in their standard state. For gases, the standard state has the properties of an ideal gas, but at one atmosphere pressure. It is thus said to be a hypothetical state. For pure liquids and solvents, the standard states are real states and are the pure substances at a specified temperature and pressure. For solutes In dilute solution, the standard state is a hypothetical state that has the properties of an infinitely dilute solute, but at unit concentration (molarity, molality, or mole fraction). The standard state of a solid is a real state and is the pure solid in its most stable crystalline form. 

Standard temperature and pressure (STP) Standard temperature 0°C (273.15 K), and standard pressure, one atmosphere, are standard conditions for gases. 

Standard states for gases. When species i is a gas at the equilibrium conditions, the standard state is usually taken to be the pure ideal gas at the equilibrium temperature T and P, = 1 bar. (Caution in older literature, the standard pressure was usually taken as 1 atm = 1.0133 bar.) Then, the standard-state fugacity becomes... 

The standard state for gases, denoted by (g), is specified as the pure gas in the ideal-gas state, at the temperature of the system and atP° = i bar. Accordingly, the fugadty is calculated using the ideal-gas... 

Standard conditions for gases n. Measured volumes of gases are quite generally recalculated to 0°C temperature and 760 mm pressure, which have been arbitrarily chosen as standard conditions. 

The reference point for all enthalpy expressions is called the standard molar enthalpy of formation (AHf) which is defined as the heat change that results when 1 mole of a compound in its standard state is formed from its elements in their standard states. The standard state of a liquid or solid substance is its most thermodynamically stable pure form at 1 bar pressure. The standard state for gases is similar, except that standard state gases are assumed to obey the ideal gas law exactly. The standard state for solutes dissolved in solution will be discussed in Chapter 10. In the notation AHf, the superscript represents standard-state conditions (1 bar), and the subscript f stands for formation. Although the standard state does not specify a temperature, we will assume, unless otherwise stated, AH° values are measured at 25°C. 

All standard states, both for pure substances and for components in mixtures and solutions, are defined for a pressure of exactly 1 atmosphere. However the temperature must be specified. (There is some movement towards metricating this to a pressure of 1 bar = 100 kPa = 0.986 924 atm. This would make a significant difference only for gases at J= 298 K, this would decrease a p by 32.6 J moT )... 

Conventional spontaneous Raman scattering is the oldest and most widely used of the Raman based spectroscopic methods. It has served as a standard teclmique for the study of molecular vibrational and rotational levels in gases, and for both intra- and inter-molecular excitations in liquids and solids. (For example, a high resolution study of the vibrons and phonons at low temperatures in crystalline benzene has just appeared [38].)... 

From this equation, the temperature dependence of is known, and vice versa (21). The ideal-gas state at a pressure of 101.3 kPa (1 atm) is often regarded as a standard state, for which the heat capacities are denoted by CP and Real gases rarely depart significantly from ideaHty at near-ambient pressures (3) therefore, and usually represent good estimates of the heat capacities of real gases at low to moderate, eg, up to several hundred kPa, pressures. Otherwise thermodynamic excess functions are used to correct for deviations from ideal behavior when such situations occur (3). 

With gases, flow rates must be available at standard temperature and pressure as well as actual temperature and pressure. The range of gas flow must be given, as well as whether the mixer is to be operated at full horsepower for all gas ranges or operated with the gas on. 

We now have the foundation for applying thermodynamics to chemical processes. We have defined the potential that moves mass in a chemical process and have developed the criteria for spontaneity and for equilibrium in terms of this chemical potential. We have defined fugacity and activity in terms of the chemical potential and have derived the equations for determining the effect of pressure and temperature on the fugacity and activity. Finally, we have introduced the concept of a standard state, have described the usual choices of standard states for pure substances (solids, liquids, or gases) and for components in solution, and have seen how these choices of standard states reduce the activity to pressure in gaseous systems in the limits of low pressure, to concentration (mole fraction or molality) in solutions in the limit of low concentration of solute, and to a value near unity for pure solids or pure liquids at pressures near ambient. 

The density of a material is a function of temperature and pressure but its value at some standard condition (for example, 293.15 K or 298.15 K at either atmospheric pressure or at the vapor pressure of the compound) often is used to characterize a compound and to ascertain its purity. Accurate density measurements as a function of temperature are important for custody transfer of materials when the volume of the material transferred at a specific temperature is known but contracts specify the mass of material transferred. Engineering applications utilize the density of a substance widely, frequently for the efficient design and safe operation of chemical plants and equipment. The density and the vapor pressure are the most often-quoted properties of a substance, and the properties most often required for prediction of other properties of the substance. In this volume, we do not report the density of gases, but rather the densities of solids as a function of temperature at atmospheric pressure and the densities of liquids either at atmospheric pressure or along the saturation line up to the critical temperature. 

---