Formation reaction standard enthalpy

Enthalpy of Formation. Once standard enthalpies are assigned to the elements, it is possible to determine standard enthalpies for compounds. For the reaction ... 

The enthalpy change for a chemical reaction in which all reactants and products are in their standard states and at a specified temperature is called the standard enthalpy (written AFf°) for that reaction. The standard enthalpy is the central tool in thermochemistry because it provides a systematic means for comparing the energy changes due to bond rearrangements in different reactions. Standard enthalpies can be calculated from tables of reference data. For this purpose, we need one additional concept. The standard enthalpy of formation AH° of a compound is defined to be the enthalpy change for the reaction that produces 1 mol of the compound from its elements in their stable states, all at 25°C and 1 atm pressure. For example, the standard enthalpy of formation of liquid water is the enthalpy change for the reaction... 

Gibbs energy of formation standard Gibbs energy (of reaction) standard enthalpy (of reaction) enthalpy of adsorption... 

Standard Enthalpy of Reaction Standard enthalpy of reaction is the enthalpy change when the reaction is carried out at 1 atm pressure. It can be calculated from the standard enthalpies of formation of reactants and products. Hess s law enables us to measure the standard enthalpy of formation of a compound in an indirect way. 

Enthalpy of formation see Standard enthalpy of formation. Enthalpy of reaction (A//) the change in enthalpy for a reaction at a given temperature and pressure it equals the heat of reaction at constant pressure. (6.3)... 

Calculate AH for this reaction using standard enthalpies of formation. (The standard enthalpy of formation of liquid pentane is —146.8 kJ/mol.)... 

Enthalpy of Formation The ideal gas standard enthalpy (heat) of formation (AHJoqs) of chemical compound is the increment of enthalpy associated with the reaction of forming that compound in the ideal gas state from the constituent elements in their standard states, defined as the existing phase at a temperature of 298.15 K and one atmosphere (101.3 kPa). Sources for data are Refs. 15, 23, 24, 104, 115, and 116. The most accurate, but again complicated, estimation method is that of Benson et al. " A compromise between complexity and accuracy is based on the additive atomic group-contribution scheme of Joback his original units of kcal/mol have been converted to kj/mol by the conversion 1 kcal/mol = 4.1868 kJ/moL... 

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... 

Enthalpy changes for reactions in solution can be determined using standard enthalpies of formation of aqueous ions, applying the general relation... 

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 ... 

Now let s see how to combine standard enthalpies of formation to calculate a standard reaction enthalpy. To do so, we imagine carrying out the reaction in two steps we reverse the formation of the reactants from the elements, then combine the elements to form the products. The first step is usually to calculate the reaction enthalpy for the formation of all the products from their elements. For this step, we use the enthalpies of formation of the products. Then, we calculate the reaction enthalpy for the formation of all the reactants from their elements. The difference between these two totals is the standard enthalpy of the reaction (Fig. 6.31) ... 

EXAMPLE e.ll Using standard enthalpies of formation to calculate a standard enthalpy of reaction... 

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. 

Use standard enthalpies of formation to calculate the standard enthalpy of a reaction, and vice versa (Examples 6.11 and 6.12). 

Using standard enthalpies of formation from Appendix 2A, calculate the standard reaction enthalpy for each of the following reactions ... 

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. 

Just as we can combine standard enthalpies of formation to obtain standard reaction enthalpies, we can also combine standard Gibbs free energies of formation to obtain standard Gibbs free energies of reaction ... 

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). 

What Do We Need to Know Already The concepts of chemical equilibrium are related to those of physical equilibrium (Sections 8.1-8.3). Because chemical equilibrium depends on the thermodynamics of chemical reactions, we need to know about the Gibbs free energy of reaction (Section 7.13) and standard enthalpies of formation (Section 6.18). Ghemical equilibrium calculations require a thorough knowledge of molar concentration (Section G), reaction stoichiometry (Section L), and the gas laws (Ghapter 4). 

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