AH° Standard enthalpy of formation

Calculate the standard enthalpy of formation. AH potassium bromide. 

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 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... 

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). 

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. 

The enthalpy change, AH, can be calculated for a steady-state process, using H°f, which is the enthalpy of formation of the various output and input components. Under the assumption that the inputs and outputs are at ambient conditions, the enthalpy of the components corresponds to the standard enthalpy of formation of each component. The kinetic and potential energy terms are neglected from the energy balance. It is also assumed that water enters the process as a liquid and hydrocarbon products leave the process as a liquid. All other components are in the gas phase. 

Another route to AH° is the combination of standard enthalpies of formation. 

Note that each r in this sum is positive, but, in general, each AH, may be positive or negative (in this example, each A77R is negative). As a standard enthalpy of reaction, each is obtained, at a particular T, in the usual way from standard enthalpies of formation (Am) or of combustion (A H°c ) of the species, such as are provided by Stull et al. (1969) or by JANAF (1986) ... 

Ha Hi AHr AH m AH°fi AHS, AHvaP A I see Dimensionless Groups Henry s law constant for species Pa m3 mol-1, equation 9.2-8 enthalpy of reaction for reaction as written, J standard enthalpy of reaction, J standard enthalpy of combustion of species i J mol-1 standard enthalpy of formation of species i, J mol-1 standard molar enthalpy of reaction with respect to species i, J mol-1 enthalpy of vaporization, J mol-1 standard molar enthalpy of activation (TST), J mol-1 initiator (species) inhibitor, inert species... 

We define the standard enthalpy of formation AH as the enthalpy change involved in forming 1 mol of a compound from its elements, each element existing in its standard form. Both T and p need to be specified, because both variables influence the magnitude of AH. Most books and tables cite AH at standard pressure p and at a temperature of 298 K. Table 3.1 cites a few representative values of AH. ... 

The standard enthalpy of formation AH is the enthalpy change involved in forming 1 mol of a compound or non-stable allotrope from its elements, each element being in its standard form, at s.t.p. 

Enthalpies of reaction can also be calculated from individual enthalpies of formation (or heats of formation), AHf, for the reactants and products. Because the temperature, pressure, and state of the substance will cause these enthalpies to vary, it is common to use a standard state convention. For gases, the standard state is 1 atm pressure. For a substance in an aqueous solution, the standard state is 1 molar concentration. And for a pure substance (compound or element), the standard state is the most stable form at 1 atm pressure and 25°C. A degree symbol to the right of the H indicates a standard state, AH°. The standard enthalpy of formation of a substance (AHf) is the change in enthalpy when 1 mol of the substance is formed from its elements when all substances are in their standard states. These values are then tabulated and can be used in determining A//°rxn. 

When comparing literature data for the quantities addressed in this section, it is therefore essential to check if those data are consistent, that is, if they are based on the same value for the anchor. On the other hand, note that proton affinity, basicity, and acidity values do not depend on whether we follow the electron convention, the ion convention, or the electron FD convention. This is clearly evidenced by reactions 4.25 and 4.27, which do not involve the electron as a reactant or product species. However, it is also obvious that the values of the standard enthalpies of formation of AH+ and A-, calculated from PA(A) and A acid-7/0 (AB), respectively, will vary with the convention used to derive the standard enthalpy of formation of the proton. 

When calculating aH° and AS° values for a reaction, make sure you multiply the standard enthalpy of formation and the standard entropy of each substance by its corresponding stoichiometric coefficient. 

Table l4-3 gives a few values that will be used in subsequent examples and problems. The symbol for standard enthalpies of formation is AH°f, where the superscript denotes standard and the subscript denotes formation. Look up both elemental sulfur and nitrogen to see that the standard enthalpies for elements are 0. Then find the pairs of values for IT2O and CCI4 (carbon tetrachloride) to learn that the enthalpy depends on the state of matter. 

Consulting the standard enthalpies of formation (Table l4-3), what is the enthalpy change, AH, for this reaction ... 

Equation (3.106) is the well-known starting point for thermochemical calculations as described in many elementary textbooks. Standard enthalpies of formation Af/f [Ad have been measured and tabulated (see, e.g., the NIST website http //webbook.nist.gov/ chemistry) for a vast number of chemical compounds Ah so (3.106) makes it rather easy to obtain A//lxn values under standard state conditions for virtually any chemical reaction of interest. 

You will note that Table 14-1 also contains standard enthalpies of formation for ions in aqueous solution. It is worth noting here that calorimetry was a strong argument favoring the view that all strong acids and bases exist in dilute water solution only as ions and not as molecules. No matter which combination of strong acid and base is used in a neutralization reaction, the heat value obtained is always very close to the value of AH° = - 13.36 kcal per mole of water formed. The implication of course is that, although the chemicals are different in each case, the reaction is the same it must be... 

We could calculate AH for this if we knew (for the elements) the enthalpy of formation of molecules from their atoms. Some crystalline elements (especially metals) vaporize as monatomic gases, and it is not too difficult to determine their heats of sublimation. Some elements—such as H2, 02, an( Br2—are diatomic gases that dissociate into atoms at high temperature these dissociation energies may also be determined. Table 14-1 also includes the standard enthalpies of formation of a number of atoms these are based on the normal physical form of the element at 25.0°C. For HC1 we find... 

Calculate the bond energies in the following gaseous molecules (a) CO, (b) C02, (c) S02, (d) NaCl Thc enthalpy of sublimation of NaCI is AH +54 70 kcal/mole, and the standard enthalpy of formation of NaClls, is A HI = -98 23 kcal/mole... 

E° = E°(cathode) — °(anode). standard enthalpy of combustion AHc° The change of enthalpy per mole of substance when it bums (reacts with oxygen) completely under standard conditions, standard enthalpy of formation AH° The standard reaction enthalpy per mole of compound for the compound s synthesis from its elements in their most stable form at 1 atm and the specified temperature, standard entropy of fusion ASfus° The standard entropy change per mole accompanying fusion (the conversion of a substance from the solid state to the liquid state), standard entropy of vaporization ASvap° The standard entropy change per mole accompanying vaporization (the conversion of a substance from the liquid state to the vapor state). 

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