standard state

Reply to 1# ateanliew: There are several definitions for different standard states, and all rigorous designs should not use units such as square standards; moles or mass should be used instead.

This post was last edited by arpcd on 2011-5-7 at 14:47. The explanation in the textbook is correct. The standard state, also known as the thermodynamic standard state, has a standard definition provided by the International Union of Pure and Applied Chemistry, IUPAC. IUPAC specifies that 100 kPa is the standard pressure, denoted by p; a system under this standard pressure is said to be in its standard state. The standard states for various systems are defined as follows: pure solids and liquids, when the substance is under a standard external pressure. Pure gas (ideal gas): p = p = 100 kPa. Mixed gas: pi = p = 100 kPa (pi is the partial pressure of a particular component gas). Ideal solution: p = p = 100 kPa, c = c = 1. 0 mol/dm3 (c? standard concentration) Note: The above standard states do not specify a temperature; that is, H2 (100 kPa, 298 K) is a standard state, and H2 (100 kPa, 200 K) is also a standard state. IUPAC recommends 298.15 K as the general reference standard for temperature; if not specified, it refers to 298.15 K, while other temperatures must be specified and indicated with (T). In the past, thermodynamic data related to pure substances were published using 1\"atm\" as the standard state pressure. *The campaign to adopt the International System of Units worldwide has made “atm” an obsolete unit of pressure; in some countries, its use is now illegal. Our country officially enacted the Metrology Law in September 1985, which came into effect on July 1, 1986. The Metrology Law explicitly stipulates the abolition of the \"atm\" pressure unit in favor of \"Pascal (Pa)\". To this end, the traditional thermodynamic standard state pressure of \"1 atm\" needs to be re-evaluated from the perspective of the International System of Units. From a thermodynamic perspective, it is of course possible to use 101.325 kPa, which corresponds to 1 atm and can be expressed in SI units, as the standard state pressure, or other pressures can be used instead as the standard state pressure. Considering: 1. The new standard state pressure should be close to the actual average atmospheric pressure at sea level; 2. The new standard state should have a numerical value of 1 when expressed in the selected units (just as the old standard state pressure was 1 atm); 3. The newly established standard state pressure should minimize changes in the existing thermodynamic data tables; 4. When calculating equilibrium constants and ln(p/p0) using the new standard state pressure, it should be just as convenient as using the old value of 1 atm; 5. The new standard state pressure should be in line with SI units. Thus, among the many available new standard state pressures (including 101.325 kPa), the International Union of Pure and Applied Chemistry (IUPAC) decided in 1982 to adopt 1 bar (=100000 Pa=0.986923 atm) as the new standard state pressure (SSP, Standard State Pressure). The adoption of bar as the new SSP was proposed in 1981 by its subsidiary body, the Committee on Thermodynamics, and officially approved and published by IUPAC in 1982. Thermodynamic data tables corresponding to this new SSP have been published, and some textbooks have been revised accordingly, such as Professor Hua Tongwen’s \"Principles of General Chemistry\" from Peking University. This new change in the standard state pressure will have different effects on the thermodynamic data of pure substances. Changing the pressure from 1 atm to 1 bar essentially does not alter the standard formation enthalpy values for pure substances; that is, ΔHθ = ΔHa. The symbol “θ” denotes thermodynamic data based on the new SSP, while the symbol “a” refers to data at 1 atm. Since the pressure changes only slightly from 1 atm (1.01325 bar) to 1 bar, it has no significant effect on the enthalpy values of solids and liquids. For gases, the standard state implies ideal gas behavior, and the enthalpy of an ideal gas is independent of pressure.