Calculating the Standard Enthalpy of Formation

Calculate the standard enthalpy of formation. AH potassium bromide. 

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. 

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. 

To calculate the standard enthalpy of formation of the crystalline compound, our imaginary author would have used the best available values at the time, which were quoted from the widely adopted NBS Circular 500 [22] AfH° (C02, g) = -394.907 kJ mol-1 and Af77°(H20, 1) = -286.131 kJ mol-1. The final value reported in his publication was, therefore (see equation 2.7), Af//°(Ci4Hio, cr) = -6959.4 - X kJ mol-1. 

The use of equation 4.17 to calculate the standard enthalpy of formation of A+ raised some controversy in the mass spectrometry community. Holmes [66] pointed out that in electron impact experiments (i.e., when reaction 4.10 is... 

In the bomb process, reactants at the initial pressure pi and temperature 7 are converted to products at the final pressure pf and temperature Tf. The primary goal of a combustion calorimetric experiment, however, is to obtain the change of internal energy, Ac//°(7r), associated with the reaction under study, with all reactants and products in their standard states pi = pf = O.IMPa) and under isothermal conditions at a reference temperature 7r (usually 298.15 K). Once AC//°(298.15K) is known, it is possible to derive the standard enthalpy of combustion, AC77°(298.15K), and subsequently calculate the standard enthalpy of formation of the compound of interest from the known standard enthalpies of formation of the products and other reactants. 

Steele et al. [28] have measured the standard enthalpy of combustion of buckminsterfullerene Ceo with bomb calorimetry of solid samples at 298.15 K. They have found a value of 26,033 + 14kJ mol Calculate the standard enthalpy of formation of Ceo and the standard enthalpy of transition from graphite to Cgo-... 

Q Calculate the standard enthalpies of formation for the compounds (i) sodium chloride and (ii) potassium iodide. The interionic distances in the compounds are 282 and 353 pm, respectively. Compare your answers with the accepted experimental values for these quantities, which are -411 and -327.6 kJ mol1, respectively. 

Calculate the standard enthalpy of formation of PClj(s) from the enthalpy of formation of PC13(1) (see Appendix 2A) and PC13(1) + Cl2(g) - PCl5(s),... 

A 0.9222-g sample of naphthalene, C10Hg(s), a major component of moth balls, is burned in a calorimeter that has a heat capacity of 9.44 kJ-(°C) 1. The temperature of the calorimeter rose from 15.73°C to 19.66°C. Calculate the standard enthalpy of formation of naphthalene. 

Since we have a program to calculate standard Gibbs energies of formation, we can calculate the standard enthalpies of formation by use of the Gibbs Helmholtz equation. 

The enthalpies for the reactions of cis- and frans-[Pt(NH3)2X2] (X = Br or I) with 9.4% aqueous NH3 to yield [Pt(NH3)4X2] have been measured calorimetri-cally.122,123 The results have been used to calculate the standard enthalpies of formation of the solid cis- and trans- isomers the enthalpy of cis - trans isomerization has also been determined. The enthalpies of dissolution of the complexes K2[PtBr4], Rb2[PtBr4], and K[PtBr3(NH3)] have been measured the enthalpy of formation of Rb2[PtBr4] was calculated to be —224.4 kcal mol-1.124... 

ArH ° (298.15 K) to calculate the standard enthalpy of formation (1=0) of the two species of a reactant for which the species matrix (spmat) contains AfG° at zero ionic strength for the two species of the reactant. The reaction equation (equat) is of the form mannoseh+pih-x-h2oh= 1.7, where 1.7 kJ mol -1 is the heat of reaction and x is mannnoseSphosh. The species matrix (spmat) is that for mannoseSphos. The calorimetric experiment is at pH and ionic strength ionstr. The first step in the calculation is to use the information on the standard Gibbs energies of formation of the species of the reactant of interest to calculate the equilibrium mole fractions rl (base form) and r2 (acidform) of the two species of the reactant of interest. The final output is the complete species matrix for x. )... 

The only experiment, which leads to a determination of the standard enthalpy of formation of sodium selenite, appears to be that of Thomsen [1882THO]. He measured the enthalpy of the reaction between one mole of selenious acid and two moles of sodium hydroxide in solution. The data are used to calculate the standard enthalpy of formation ofNaSeOsCcr) in Table A-1. 

K. and with auxiliary data used to calculate the standard enthalpies of formation of the gallium acid selenites in Table A-102. 

At 298 K. the enthalpy change on combustion of methanol is — 727 kJ mo "l, that of hydrogen is - 286kimoI 1, and that of graphite 394kJmor1. Calculate the standard enthalpy of formation of methanol. 

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