Standard pressure for gases

The potential of a cell depends.on the concentrations or partial pressures of the reacting substances. The standard concentrations of the dissolved substances in Table 32-2 are taken to be approximately 1 M (more accurately, unit activity, correction being made for deviation from the perfect-solution law), and the standard pressure for gases is 1 atm (corrected in very accurate work for deviation from the perfect-gas law). 

However, the standard state for gases is the ideal gas state at the standard-state pressure, for which ° = P°. Therefore... 

Since concentration variations have measurable effects on the cell voltage, a measured voltage cannot be interpreted unless the cell concentrations are specified. Because of this, chemists introduce the idea of standard-state. The standard state for gases is taken as a pressure of one atmosphere at 25°C the standard state for ions is taken as a concentration of 1 M and the standard state of pure substances is taken as the pure substances themselves as they exist at 25°C. The half-cell potential associated with a halfreaction taking place between substances in their standard states is called ° (the superscript zero means standard state). We can rewrite equation (37) to include the specifications of the standard states ... 

The standard heat of formation of a substance is the enthalpy change involved in forming 1 mole of it from its elements. The standard heat of formation is measured at 25°C (or 298 K) and one atmosphere of pressure for gases or 1 molar solutions for liquids. Tables of the heat of formation are usually given in units of kilojoules per mole. For water, the standard heat of formation is -286 kjmol The minus sign means that the reaction is exothermic and heat is given off... 

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. 

Note that the partial pressures must be in units of atmospheres (not mm Hg) because the standard-state partial pressure for gases is 1 atm. 

Standard states are usually chosen2 so that the activity reduces to the pressure for gases at low pressures, and to concentrations in dilute solution. The choices are summarized in Table 11.2.aa We note that for a gas, the standard state is the ideal gas at a pressure of 1 bar (0.1 MPa), in which case, the activity differs from the pressure (expressed in bars) by the fugacity coefficient. That is,... 

The amount of permeated substance can be expressed in mass, mole or volume units. For gases, volume is preferred, expressed as the amount permeating under conditions of standard temperature and pressure (STP), which corresponds to the standard temperature of 273.15 K and standard pressure of 1.01325 105 Pa. The corresponding dimensions for D and S are obtained at the same time from Eq. (9-4). The standard ambient temperature and pressure (SATP) are set at p°= 1 bar= 105 Pa = 0.9678 atm and T = 298.15 K. The data in handbooks however is still mostly expressed with p = 1 atm as standard pressure. For practical purposes the difference between these two conventions is insignificant compared to the variability of the materials themselves. 

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... 

STP standard temperature and pressure for gases 0°C and 1 atm pressure, stress a change in conditions imposed on a system at equilibrium, strong acid an acid that reacts completely with water to form ions. 

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 tenperature for liquids and air at a standard tenperature 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 values apply to temperature 25 C, with standard concentration for aqueous solutions 1 M and standard pressure of gases 1 atm. 

We can. see that at equilibrium the [P /[S] ratio equals hence, AG — 0. To catalog and compare AG values for various reactions, chemists have agreed upon a "standard-state where all reactants and products are considered to be maintained at steady-staie concentrations of 1 M, The standard-Slate for gases is considered to be 1 atm partial pressure. Under this condition the log [P]/[S] term (regardless of the exponents) is zero. The AG under standard-state conditions is designated AG°. 

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. 

A cell in which all reactants and products are in their thermodynamic standard states (1 M for dissolved species and 1 atm partial pressure for gases) is called a standard cell. 

Standard electrode potentials, designated refer to standard-state conditions. These standard-state conditions are one molar solutions for ions, one atmosphere pressure for gases, and all solids and liquids in their standard states at 25°C. (Remember that we refer to thermodynamic standard-state conditions, and not standard temperature and pressure as in gas law calculations.) As any of the standard cells described earlier operates, and concentrations or pressures of reactants change, the observed cell voltage drops. Similarly, cells constructed with solution concentrations different from one molar, or gas pressures different from one atmosphere, cause the corresponding potentials to deviate from standard electrode potentials. 

Standard electrochemical conditions 1 M concentration for solution species, 1 atm partial pressure for gases, and pure sohds and liquids. 

The definition of the standard state for gases has been changed to 1 bar, a slightly lower pressure than the 1 atm standard on which the data in this book are based (1 atm = 101.3 kPa = 1.013 bar). For most purposes, this makes very little difference in the standard enthalpy values. 

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... 

Section 5.7) We can define standard free energies of formation, AGJ, in a similar way. As is summarized in TABLE 19.2, standard state means 1 atm pressure for gases, the pure solid for solids, and the pure liquid for liquids. For substances in solution, the standard state is normally a concentration of 1 M. (In very accurate work it may be necessary to make certain corrections, but we need not worry about these.)... 

---