Molar heat of formation

Equations (1) and (2) are the heats of formation of carbon dioxide and water respectively Equation (3) is the reverse of the combustion of methane and so the heat of reaction is equal to the heat of combustion but opposite in sign The molar heat of formation of a substance is the enthalpy change for formation of one mole of the substance from the elements For methane AH = —75 kJ/mol... 

Show that the ratio of the molar heat of formation of gaseous water from the elements (a chemical reaction) to the molar heat of the fusion of water (a phase change) is of the order of 50. 

The molar heat of formation of this endothermic compound (+230-250 kJ, 4.5 kJ/g) is comparable with that of buten-3-yne (vinylacety lene). While no explosive decomposition of the isocyanide has been reported, the possibility should be borne in mind [1], It is stable at — 15°C, but isomerises to acrylonitrile and polymerises at ambient temperature [2],... 

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. 

When 2.4 g of graphite are burnt completely 78.70 kJ heat is released what is the molar heat of formation of CO2 ... 

The molar heat of formation from solid arsenic and rhombic sulphur has been calculated 3 to be 28,900 calories. This value differs considerably from earlier determinations.4 From diatomic gaseous sulphur and solid arsenic the heat of formation is calculated 5 to be 51,430 calories. 

The molar heat of formation from solid arsenic and rhombic sulphur has been calculated to be 34,700 calories.9... 

B. H. Wilsdon 6 studied the energy involved in the electronic shifts during the dissociation of ammonia. J. Thomsen gave for the heat of formation of ammonia (N,3H)=ll-89 Cals. F. Haber, 10-95 Cals. and M. Berthelot, 12-20 Cals. The subject was studied by R. de Forcrand. J. Thomsen gave (N,3H,Aq.)=20-3 Cals., and M. Berthelot, 21-0 Cals. A. T. Larson and R. L. Dodge calculated the following molar heats of formation from < 3,=4-5707(log jq—log P2)(Ti-1 Tz 1) 1 calories ... 

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. 

Consequently, it would appear that the flame temperature is determined not by the specific reactants, but only by the atomic ratios and the specific atoms that are introduced. It is the atoms that determine what products will form. Only ozone and acetylene have positive molar heats of formation high enough to cause a noticeable variation (rise) in flame temperature. Ammonia has a negative heat of formation low enough to lower the final flame temperature. One can normalize for the effects of total moles of products formed by considering the heats of formation per gram (Ahf) these values are given for some fuels and oxidizers in Table 1. 

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). 

Note Data reported by dllTerent authors vary, some considerably. A high value for the molar heat of formation of AtjS) is cited by Kubaschewski and Evans < from a Russian source, which also should cast doubt on the corresponding values for AljSej and AljTe). 

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. 

Thermochemistry, When 2.000 g of magnesium is burned in fluorine to magnesium fluoride at constant pressure, 44.75 kcal are evolved. Calculate AHi, the molar heat of formation of MgF2(c). 

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

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