The Standard Heat of Formation

The standard heat of formation for crystalline H3PO2 is —608.8 kJ/mol (—145.5 kcal/mol) (39). The acid can be prepared by the oxidation of phosphine by iodine and water. 

Caleulating the heat of reaetion is a multi-step proeess. Beginning with the standard heats of formation at 298 K, first ealeulate the standard heat of reaetion, and then ealeulate AH for the aetual system temperature and pressure. The heat of reaetion at 298 K, AHjgg is usually refeiTed to as the standard heat of reaetion. This ean be readily ealeulated from the standard heats of formation of the reaetion eom-ponents. The standard heat of reaetion is expressed as ... 

Using the standard heats of formation of the reaetion eomponents, ealeulate the standard heat of reaetion and the heat of reaetion at system temperature of 350°C and 1 atm. The reaetion is ... 

The reverse reaction to give the gaseous species AlX(g) at high temperature accounts for the enhanced volatility of AIF3 when heated in the presence of A1 metal, and the ready volatilization of A1 metal in the presence of AICI3. Using calculations of the type outlined on p. 82 the standard heats of formation of the crystalline monohalides AIX and their heats of disproportionation have been estimated as ... 

Some comparative figures for the standard heats of formation —AH are in Table 16.8. Accordingly, Se03 can not be made by direct oxidation of Se or Se02 and is even hard to make by the dehydration of H2Se04 with P2O5 a better... 

The standard heats of formation AH of gaseous HX diminish rapidly with increase in molecular weight and HI is endothermic. The very small (and positive) value for the standard free energy of formation AGj of HI indicates that (under equilibrium conditions) this species is substantially dissociated at room temperature and pressure. However, dissociation is slow in the absence of a catalyst. The bond dissociation energies of HX show a similar trend from the very large value of 574kJmol for HF to little more than half this (295kJmol ) for HI. 

The standard heat of formation ( AH ) of a chemical compound is the standard heat of reaction corresponding to the chemical combination of its constituent elements to form one mole of the compound, each existing in its standard state at 1 atm and 25°C. It has units of cal/g-mole. 

Find the standard heat of formation of benzene (() given the following heats of combustion data (in kcal/g-mole) at 1 atm and 25°C ... 

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... 

Heats of reaction can be calculated from the heats of formation of the reactants and products, as described in Chapter 3, Section 3.11. Values of the standard heats of formation for the more common chemicals are given in various handbooks see also Appendix C. A useful source of data on heats of formation, and combustion, is the critical review of the literature by Domalski (1972). 

Calculate the standard heat of formation for HC2H302(1) using the following ... 

It is, of course, not necessary to have an extensive list of heats of reaction to determine the heat absorbed or evolved in every possible chemical reaction. A more convenient and logical procedure is to list the standard heats of formation of chemical substances. The standard heat of formation is the enthalpy of a substance in its standard state referred to its elements in their standard states at the same temperature. From this definition it is obvious that heats of formation of the elements in their standard states are zero. 

Determine the adiabatic decomposition temperature and the product composition a, b, and c. Take the standard heat of formation of liquid hydrazine as 50.07 kJ/mol. The hydrazine enters the system at 298 K. 

The standard heats of formation of NbOPj, TaOFj, Nb02F, and Ta02p have been calculated as —1510, —1514, —1138, and —1205 kJ mol" , respectively. ... 

If the reaction is a standard state reaction where the starting materials in their standard states react to give products in their standard states, and the standard heats of formation of the elements are assumed... 

Further use of tabulated data (such as those in Table 2.3) enables an estimate to be made of the temperature of the reaction of black powder. Using equation (2.18), the standard enthalpy change may be calculated from the standard heats of formation of the reactants and products as in equation (2.20). 

The standard heat of formation of phenyl benzoate as the gaseous species at 25°C. has been determined as AHt°(PhC02Ph) = — 35 1 kcal. per mole. Seven distinct bond dissociation energies are immediately related to this reference basis. Values for these, together with values for the heats of formation of related free radicals, are discussed, and a provisional set is presented. They include the following estimates (kcal. per mole) D(PhC02—Ph) = 94 D(Ph—C02Ph) = 96, D( C02—Ph) = 62 D(PhCO—OPh) = 64. Errors are likely to be around 5 kcal. per mole. 

However, the absolute heat contents of individual substances are generally not available. The arbitrary assumption is therefore made that H° is zero for the chemical elements in their most stable form at 25 °C and 1 bar. The heat contents of chemical compounds are then defined as the standard heat of formation AH from the elements at 25 °C and 1 bar. For the formation of liquid water,... 

Here, v is the muzzle velocity of the projectile in m s-1, m and M are the mass of the propellant and projectile in grams, respectively, and Q is the amount of chemical energy released by the combustion of the propellants in J g-1. The value for Q (also known as the Heat of Deflagration ) can be calculated from the standard heats of formation as shown in Equation 8.2. 

The enthalpy change involved in the formation of one mole of a substance from the elements is called the heat of formation of the substance. The standard heat of formation is the heat of formation when all substances in the reaction are in their standard states. 

FIGURE 8.11 The standard reaction enthalpy, AH°, for the generalized reaction A —> B is the difference between the standard heats of formation of products and reactants. 

Because enthalpy is a state function, AH is the same no matter what path is taken between reactants and products. Thus, the sum of the enthalpy changes for the individual steps in a reaction is equal to the overall enthalpy change for the entire reaction, a relationship known as Hess s law. Using this law, it is possible to calculate overall enthalpy changes for individual steps that can t be measured directly. Hess s law also makes it possible to calculate the enthalpy change of any reaction if the standard heats of formation (AH°f) are known for the reactants and products. The standard heat of formation is the enthalpy change for the hypothetical formation of 1 mol of a substance in its standard state from the most stable forms of the constituent elements in their standard states (1 atm pressure and a specified temperature, usually 25°C). 

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