The standard enthalpy of formation

The enthalpies of substances have no known absolute value. It is therefore necessary to choose a reference. By convention  

The relationship between the standard enthalpies of formation at temperature T and at 298.15 K can be written as follows  

The apparent standard enthalpy of formation can also be introduced  

The following values can be foimd in JANAF tables (units J mol K )  

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Calculate the standard enthalpy of formation. AH potassium bromide. 

The amonnt of energy that can be released from a given chemical reaction is determined from the energies (enthalpies of formation) of the individnal reactants and prodncts. Enthalpies are nsnally given for snbstances in their standard states, which are the stable states of pnre snbstances at atmospheric pressnre and at 25°C. The overall heat of reaction is the difference between the snms of the standard enthalpies of formation of the prodncts... 

Figure 7.7 Trends in the standard enthalpies of formation AH] for Groups 3 and 13 trihalides as illustrated by data for MF3 and MBrj.

The atom and bond concepts dominate chemistry. Dalton postulated that atoms retained their identities even when in chemical combinations with other atoms. We know that their properties are sometimes transferable from one molecule to another for example, the incremental increase in the standard enthalpy of formation of a normal hydrocarbon per CHj group is —20.6 1.3 kJmol . We also know that more often there are subtle modifications to the electron density. 

The standard enthalpies of formation of ions in aqueous solution listed at the bottom of Table 8.3 are relative values, established by taking... 

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. 

This value is the standard enthalpy of formation of glycerol dissolved in water in a hypothetical m = 1 solution that obeys Henry s law. 

Combining this result with the standard enthalpies of formation of SF6 and GeF4... 

There are millions of possible reactions, and it is impractical to list every one with its standard reaction enthalpy. However, chemists have devised an ingenious alternative. First, they report the standard enthalpies of formation of substances. Then they combine these quantities to obtain the standard enthalpy of reaction needed. Let s look at these two stages in turn. 

The standard enthalpy of formation, AH°, of a substance is the standard reaction enthalpy per mole of formula units for the formation of a substance from its elements in their most stable form, as in the reaction... 

It follows from the definition just given that the standard enthalpy of formation of an element in its most stable form is zero. For instance, the standard enthalpy of formation of C(gr) is zero because C(gr) — C(gr) is a null reaction (that is, nothing changes). We write, for instance, AHf°(C, gr) = 0. However, the enthalpy of formation of an element in a form other than its most stable one is nonzero. For example, the conversion of carbon from graphite (its most stable form) into diamond is endothermic ... 

The standard enthalpy of formation of diamond is therefore reported as AHt°(C, diamond) = + 1.9 kj-mol l. Values for a selection of other substances are listed in Table 6.5 and Appendix 2A. 

STRATEGY We expect a strongly negative value because all combustions are exothermic and this oxidation is like an incomplete combustion. First, add up the individual standard enthalpies of formation of the products, multiplying each value by the appropriate number of moles from the balanced equation. Remember that the standard enthalpy of formation of an element in its most stable form is zero. Then, calculate the total standard enthalpy of formation of the reactants in the same way and use Eq. 20 to calculate the standard reaction enthalpy. 

Standard enthalpies of formation are commonly determined from combustion data by using Eq. 20. The procedure is the same, but the standard reaction enthalpy is known and the unknown value is one of the standard enthalpies of formation. 

Self-Test 6.16A Calculate the standard enthalpy of formation of ethyne, the fuel used in oxyacetylene welding torches, from the information in Tables 6.4 and 6.5. 

Self-Test 6.19B The standard enthalpy of formation of ammonium nitrate is —365.56 kj-mol 1 at 298 K. Estimate its value at 250.°C. 

The standard enthalpy of formation of solid ammonium acetate is —616.14 kJ mol l. Calculate its standard enthalpy of combustion. 

STRATEGY We write the chemical equation for the formation of HI(g) and calculate the standard Gibbs free energy of reaction from AG° = AH° — TAS°. It is best to write the equation with a stoichiometric coefficient of 1 for the compound of interest, because then AG° = AGf°. The standard enthalpy of formation is found in Appendix 2A. The standard reaction entropy is found as shown in Example 7.9, by using the data from Table 7.3 or Appendix 2A. 

STRATEGY The reaction is endothermic and, because a gas is produced from solid reactants, occurs with an increase in entropy. Because AHc > 0 and AS° > 0, the formation of products from pure reactants becomes spontaneous at temperatures for which AH° TAS°. The temperature at which the tendency for the reaction to occur begins solves to T = AH7AS°. We use data from Appendix 2A (and remember that the standard enthalpies of formation of elements are zero). 

STRATEGY Raising the temperature of an equilibrium mixture will tend to shift its composition in the endothermic direction of the reaction. A positive reaction enthalpy indicates that the reaction is endothermic in the forward direction. A negative reaction enthalpy indicates that the reaction is endothermic in the reverse direction. To find the standard reaction enthalpy, use the standard enthalpies of formation given in Appendix 2A. 

When the partial pressure of each gaseous reagent is 1 bar and the concentration of each species in solution is 1 M, the conditions are defined to be standard. Under these conditions, the enthalpy change in a formation reaction is the standard enthalpy of formation (A... 

Our analysis of the reaction of nitrogen dioxide molecules is not unique. The same type of path can be visualized for any chemical reaction, as Figure 6-20 shows. The reaction enthalpy for any chemical reaction can be found from the standard enthalpies of formation for all the reactants and products. Multiply each standard enthalpy of formation by the appropriate stoichiometric coefficient, add the values for the products, add the values for the reactants, and subtract the sum for reactants from the sum for products. Equation summarizes this procedure ... 

Equation can also be used to calculate the standard enthalpy of formation of a substance whose formation reaction does not proceed cleanly and rapidly. The enthalpy change for some other chemical reaction involving the substance can be determined by calorimetric measurements. Then Equation can be used to calculate the unknown standard enthalpy of formation. Example shows how to do this using experimental data from a constant-volume calorimetry experiment combined with standard heats of formation. 

The enthalpy of combustion of octane (Cg Hig)is -5.5 X 10 kJ/mol. Using tabulated standard enthalpies of formation in Appendix D, determine the standard enthalpy of formation of octane. 

Again, it is convenient to follow the seven-step procedure to solve this problem. We are asked to find an enthalpy of formation. Because enthalpy is a state function, we can visualize the reaction as occurring through decomposition and formation reactions. Appendix D lists enthalpies of formation, and the experimental heat of combustion is provided. We can use Equation to relate the enthalpy of combustion to the standard enthalpy of formation for octane. 

For the thermodynamic factors Stull takes into account the decomposition temperature . This is defined as the temperature reached by the decomposition compounds of the particular substance when the latter decomposes into these constituent elements. It is therefore calculated using the standard enthalpy of formation of the compound. 

Considering that the standard enthalpies of formation of various oxides, sulfides, and halides of As, Sb, and Bi are very close to each other, the similarity between this value and the standard enthalpy for the surface formation of As(OH)3, A//f = —680 + 20 kJ/mol, was taken as an indication that, indeed, the As redox process on Pt(l 11) involves the formation of hydroxide species [Blais et al., 2001]. For comparison with A/7 for the surface formation of Bi(OH)2, the strategy followed... 

The standard enthalpy of formation AH°f of a compound is defined as the enthalpy change when one mol of the compound is formed from its constituent elements in the standard state. The enthalpy of formation of the elements is taken as zero. The standard heat of any reaction can be calculated from the heats of formation —AH of the products and reactants if these are available or can be estimated. 

Simple amides of this type are the bis(trimethylsilyl)amides M[N(SiMe3)2]2 (M = Cd and Hg) the essential thermodynamic data of which have been determined in calorimetric measurements of the heats of hydrolysis in dilute H2S04.146 Evaluation of the measured data yielded the standard enthalpies of formation AH° = —854(21)kJmoU1 and —834(9)kJmol-1 for M =Cd and Hg, respectively. Using subsidiary data, the average thermochemical bond energies E—(Cd—N) 144 and E(Hg—N) 108 kJ mol-1 were also obtained, i.e., the Cd—N bonds are considerably stronger than the Hg—N bonds. 

As a thermodynamicist working at the Lower Slobbovian Research Institute, you have been asked to determine the standard Gibbs free energy of formation and the standard enthalpy of formation of the compounds ds-butene-2 and trans-butene-2. Your boss has informed you that the standard enthalpy of formation of butene-1 is 1.172 kJ/mole while the standard Gibbs free energy of formation is 72.10 kJ/mole where the standard state is taken as the pure component at 25 °C and 101.3 kPa. 

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