Formation standard enthalpy changes

The standard enthalpy change, Aff°, for a given thermochemical equation is equal to the sum of the standard enthalpies of formation of the product compounds minus the sum of the standard enthalpies of formation of the reactant compounds. 

However, each of the individual reactions involves the formation of a compound from its elements or the decomposition of a compound into those elements. The standard enthalpy change of a reaction that involves the formation of a compound from its elements is... 

As we mentioned, it is necessary to have information about the standard enthalpy change for a reaction as well as the standard entropies of the reactants and products to calculate the change in Gibbs function. At some temperature T, A// j can be obtained from Af/Z of each of the substances involved in the transformation. Data on the standard enthalpies of formation are tabulated in either of two ways. One method is to list Af/Z at some convenient temperature, such as 25°C, or at a series of temperatures. Tables 4.2 through 4.5 contain values of AfZ/ at 298.15 K. Values at temperatures not listed are calculated with the aid of heat capacity equations, whose coefficients are given in Table 4.8. 

Now let us consider how the standard enthalpies of formation can be used to calculate the standard enthalpy change for a chemical reaction. First, we calculate the total standard enthalpy of formation of all the products. We then calculate the total standard enthalpy of formation of all the reactants. The difference between the two totals is the... 

Just as the standard enthalpy change for a reaction can be calculated from the standard free enthalpies of formation of the reactants and products, the standard free energy... 

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). 

They thus have time to react and approach their low-pressure composition before the equilibria freeze. Both heats refer to the same standard temperature, e.g. 298°K, and can be corrected to give the standard enthalpy change on formation of the respective set of products from the explosive... 

Standard enthalpy of formation The enthalpy change that accompanies the formation of one mole of a compound at 25 °C from its elements, with all substances in their standard states at that temperature. 

Q Given that the standard enthalpy change for dissolving potassium chloride in water is - 17.0 k.l inol 1 and the standard enthalpy of formation of KCI is —436.5 kJ mol1, use the value for the enthalpy of formation of the aqueous chloride ion given in Table 2.2 to calculate a value for the enthalpy of formation of aqueous potassium ions. 

The donor number, DN [11, 13], of solvent D (Lewis base) is determined calori-metrically as the negative value of the standard enthalpy change, -AH° (in kcal mol-1), for the 1 1 adduct formation between solvent D and antimony pentachlor-ide (SbCls), both being dilute, in 1,2-dichloroethane (DCE) at 25 °C [Eq. (1.5)] ... 

Note several points about this definition. First, the "reaction" to form a substance from its constituent elements can be (and often is) hypothetical. We can t combine carbon and hydrogen in the laboratory to make methane, for instance, yet the heat of formation for methane is A H°f = —74.8kJ/mol, which corresponds to the standard enthalpy change for the hypothetical reaction... 

How can standard heats of formation be used for thermochemical calculations The standard enthalpy change for any chemical reaction is found by subtracting the sum of the heats of formation of all reactants from the sum of the heats of formation of all products, with each heat of formation multiplied by the coefficient of that substance in the balanced equation. 

An estimate of the standard enthalpy change for the formation of the methyl anion and a hydrogen cation from methane may be obtained by a calculation based on Hess s Law, as shown in Table 1. 

STANDARD ENTHALPY CHANGES (kcal.mole-1) FOR THE FORMATION OF THE METHYL... 

There is a change in enthalpy that accompanies a chemical reaction. The AHJ is the standard heat of formation, the enthalpy change that occurs when one mole of a substance is formed from its component elements, as shown below. 

In choosing a reference state, we are allowed to make a choice for each element, because elements cannot be transformed into each other by chemical means. The choice usually made for the reference state of an element is the form in which it is stable at temperature T and the standard pressure =1.0 bar. For example, at most temperatures, for 02 this would be gaseous diatomic molecules for iron, it would be the solid metal, and for bromine, it would be the diatomic in the liquid state below 59°C and in the gaseous state above 59°C. We call the standard enthalpy change of the reaction in which 1 mol of compound i is formed from its component elements in their reference states the heat of formation of compound i, A H°(T). The heat of reaction is related to heats of formation as... 

We have combined this result with data from the NBS tables to derive a value of 177 kJ/mol for the standard enthalpy of formation of NO+. A value for AfH° of 78 kJ/mol for aqueous NO can be derived from A(H° of gaseous NO and its heat of hydration (328). These data lead to a value of — 99 kJ/mol for the standard enthalpy change of the NO+/NO half-cell. 

Note These are values in Keal/mole for the standard enthalpy change at 25°C, 1 atm. The values for the diatomic species are accurate to within 0.5 Kcal. For all other species the expected reliability is 2 Kcal and may be as large as +4 Kcal. Where heats of formation of radicals are known, the method of group contributions may be used to deduce values for other radicals related by the additions of known groups. Thus... 

Using the standard enthalpies of formation listed in Table 9.4, calculate the standard enthalpy change for the overall reaction that occurs when ammonia is burned in air to form nitrogen dioxide and water. This is the first step in the manufacture of nitric acid. 

Compared to two molecules of -l,3,5-hexatriene, the pentacyclic ladderane core of 1 is unstable, which is indicated by the difference of the standard enthalpy change of formation (AA//) of 19.6 kcal/mol. On the other hand, 1 is kinetically inert to decomposition into trienes. The strain energy of 1 is estimated at ca. 75 kcal/mol, which is approximately three times that of cyclobutane (26.5 kcal/mol )." ... 

The enthalpy change in forming 1 mol of a substance from elements in their standard states is called the standard enthalpy of formation of the substance, AHf. Many values of AHf are listed in Table 2. Note that the values of the standard enthalpies of formation for elements are 0. From a list of standard enthalpies of formation, the enthalpy change of any reaction for which data is available can be calculated. For example, the following reaction can be considered to take place in four steps. 

The most important contributor to the strength of hardened Portland cement is tricalcium silicate, (Ca0)3-Si02, for which the measured standard enthalpy of formation is — 2929.2 kj moUT Calculate the standard enthalpy change for the production of 1.00 mol tricalcium silicate from quartz and lime. 

The enthalpy change of the reaction between Na2Se03(cr) and a lead nitrate solution with formation of crystalline lead selenite was measured in a calorimeter. The data have been used to calculate the standard enthalpy change of the reaction Na2Se03(cr) + Pb PbSe03(cr) + 2Na in Table A-26. 

The standard enthalpy change for this reaction is -594.1 kJ. (Because the reactants and product are in their standard states, that is, at 1 atm, the enthalpy change is also the standard enthalpy of formation for LiF.) Keeping in mind that the sum of enthalpy... 

In this chapter, a mathematical expression of the First Law will be discussed. The two functions internal energy, U, and enthalpy, H, will figure prominently. In addition, tabulations of standard enthalpy changes of formation, A//0 for a number of compounds, will be given. From such tables it is possible to derive the amount of thermal energy associated with any reaction, as long as all reactants and products are listed. 

It will be clear from what has been said that there are multitudes of possible reactions which might concern us, and therefore multitudes of All values. New compounds are being synthesized daily, and each will undergo many possible reactions. Clearly, it is almost impossible to determine AH for eveiy possible change. Luckily, it is also unnecessary. It is sufficient, as we shall see, to know the standard enthalpy change of formation (sometimes abbreviated to enthalpy of formation or called standard heat of formation), for each compound with which we are dealing. The formation reaction is that in which the compound is formed from its elements in their standard states, that is, their most stable form under chosen standard conditions of temperature and pressure (s.t.p.). The standard pressure and temperature recommended by IUPAC since 1982 are / °=105Pa... 

The standard enthalpy change of formation for crystalline Na20 is found to be -418.0 kJ/mol. This necessarily implies the following exact equation ... 

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