Standard molar heat of formation,

The standard (molar) heat of formation AH j. of a compound is the measured enthalpy change in the formation of one mole of the compound at a pressure of one bar and at temperature T from its constituent elements in their most stable state. 

Standard molar enthalpy of formation is often called standard molar heat of formation or, more simply, heat of formation. The superscript zero in Af/f signifies standard pressure, 1 atmosphere. Negative values for A// describe exothermic formation reactions, whereas positive values for A// describe endothermic formation reactions. 

All chemical compounds have a heat (enthalpy) of formation , AH, which equates to the heat liberated or absorbed when 1 mole of the compound is formed from its constituent elements. An element in its standard state is defined as having zero heat of formation. The standard molar heat of formation, A//f, of a compound is then the change in enthalpy, positive or negative, when 1 mole of the compound is formed at standard conditions (298.15 K and 101.325 kPa) from the elements in their most stable physical forms (gas, liquid or solid). 

Standard molar enthalpy of formation (A// ) (of a substance) The enthalpy change for the formation of one mole of a substance in a specified state from its elements in their standard states also known as standard molar heat of formation or just heat of formation. 

Standard molar heat of formation See Standard molar enthalpy of formation. 

The standard (molar) heat of formation of a compound is the measured enthalpy change in the... 

Strictly, this definition only applies to substances in equilibrium at the standard state. For the high temperatures achieved in combustion, cp should always be regarded as a function of temperature. Some values are shown in Table 2.1. Values for the molar heats of formation for several substances are listed in Table 2.2. More extensive listings have been compiled and are available in reference books. The student should recognize that the values have been determined from measurements and from the application of Equation (2.20). Some examples should show the utility and interpretation of the heat of formation. 

Besides equilibriumconstants, additional thermodynamic data were included, if available, although little emphasis was put on their completeness. The data for primary master species comprise the standard molar thermodynamic properties of formation from the elements (AfG standard molar Gibbs energy of formation AfH°m standard molar enthalpy of formation ApSm- standard molar entropy of formation), the standard molar entropy (5m), the standard molar isobaric heat capacity (Cp.m), the coefficients Afa, Afb, and Afc for the temperature-dependent molar isobaric heat capacity equation... 

The conversion of raw experimental data to a molar heat of formation AHf at a standard temperature (typically 298.15 K) for CxHyNzO , or C HyNzOa, and, so on requires elaborate corrections [76] and uses AH values for the expected oxidation products, such as C02 and H20 Computer programs for this data reduction exist. 

We burn 14.4 g of lithium in excess oxygen at constant atmospheric pressure to form Li20. Then we bring the reaction mixture back to 25°C. In this process 605 kj of heat is given off What is the standard molar enthalpy of formation of Li20 ... 

Density, vapor pressure, heat capacity, and standard molar enthalpies of formation, fusion, vaporization, and combustion of parent TP have been determined (97JCED1037). Calculated enthalpies of formation of TP and some 6-substituted derivatives have been published (00CHE714). The thermal behavior of 5-oxo TP (106) and 7-oxo TP (29) hemihydrate has been studied in a calorimeter and a thermobalance (00JST(519)165). 

EFI/PRO] Efimov, M. E., Prokopenko, L V., Tsirelnikov, V. L, Troyanov, S. L, Medvedev, V. A., Berezovskii, G. A., Paukov, L E., Thermodynamic properties of zirconium chlorides. I. The standard molar enthalpy of formation, the low-temperature heat capacity, the standard molar entropy, and the standard molar Gibbs energy of formation of zirconium trichloride, J. Chem. Thermodyn., 19, (1987), 353-358. Cited on pages 163, 164, 333,335,338. 

Robie and Hemingway [95ROB/HEM] used their accurate heat capacity measurements, combined with results from molten salt calorimetry, thermal decomposition of the Ni2Si04-olivine into its constituent oxides, and equilibrium studies, both by CO reduction and solid state electrochemical cell measurement for Reaction (V.121) [87NE1], and calculated the following standard molar enthalpy of formation of Ni2Si04-olivine (liebenbergite) A,// (298.15 K) = - (1396.5 3.0) kJ mol. ... 

When a change occurs with the evolution of heat to the surroundings at constant pressure it is referred as exothermic and the enthalpy of the system decreases AH is negative). If the change takes in heat from the surroundings it is endothermic AH is positive). Many reactions have been studied and the value of the enthalpy of reaction determined accurately. Tables of standard molar enthalpies of formation A H at 25 °C are to be found in several sources and the standard enthalpy change for another reaction may be calculated from them using Hess s Law. This... 

The standard molar enthalpy of hydration of a complete electrolyte, Ahydr fE° . is obtained from its experimental heat of solution and theoretical lattice energy. It is also the difference between the critically compiled standard molar enthalpy of formation of the infinitely dilute aqueous electrolyte (Wagman et al. 1982) and the sum of the standard molar enthalpies of formation of the ideal gaseous ions (Table 2.1), weighted according to their stoichiometric coefficients. 

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. 

Recall that the standard molar enthalpies of formation needed in Eq. 11.8.23 can be evaluated by calorimetric methods (Sec. 11.3.2). The absolute molar entropy values S° come from heat capacity data or statistical mechanical theory by methods discussed in Sec. 6.2. Thus, it is entirely feasible to use nothing but calorimetry to evaluate an equilibrium constant, a goal sought by thermod5mamicists during the first half of the 20th century. ... 

Apart from standard molar enthalpies of formation Af T (r) of substances B (see Section 8), most commonly given at T = 298.15 K and at r -> 0, and standard molar entropies iS (3T) (see Section 9) and standard molar heat capacities C, b(T), each most commonly given at T = 298.15 K, other quantities found in thermodynamic tables include values of the increments in the standard molar enthalpies, especially... 

Enthalpies of Formation of Aqueous Ions The standard molar enthalpy of formation of the infinitely dilute aqueous electrolyte, A //"(E,aq), is obtained experimentally from the standard molar enthalpy of formation of the pure electrolyte (generally for the crystalline salt but also for electrolytes such as H SO and HBr that are liquid and gaseous in their standard states), A °(E,c), and the heats of solution extrapolated to infinite dilution ... 

---