Diamond standard enthalpy

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. 

Self-Test 6.15B You have an inspiration maybe diamonds would make a great fuel Calculate the standard enthalpy of combustion of diamonds from the information in Appendix 2A. 

Some elements exist in more than one form under standard conditions. For example, carbon can exist as either graphite or diamond, as shown in Figure 5.16. Graphite is defined as the standard state of carbon. Therefore, the standard enthalpy of formation of graphite carbon is 0 kj/mol. The standard enthalpy of formation of diamond is 1.9 kj/mol. Another example is oxygen, 02(g). Oxygen also exists in the form of ozone,... 

B) The formation of hydrogen sulfide from hydrogen gas and sulfur conforms to the definition of both standard enthalpy of reaction and standard enthalpy of formation since the heat change that is measured is for the formation of one mole of compound from the elements in their standard state. In Choice (D), diamond is not the stable aUotrope of carbon—it is graphite. 

The enthalpy of combustion of diamond is -94 50 kcal/mole The enthalpy of combustion of graphite is -94 05 kcal/mole What is the standard enthalpy and entropy of transition from diamond to graphite0... 

Values of AG°f at 25°C for some common substances are listed in Table 17.3, and additional values are given in Appendix B. Note that AG°f for an element in its most stable form at 25°C is defined to be zero. Thus, solid graphite has AG°f = 0 kj/mol, but diamond, a less stable form of solid carbon at 25°C, has AG°f = 2.9kJ/mol. As with standard enthalpies of formation, AH°f, a zero value of AG°f for elements in their most stable form establishes a thermochemical "sea level," or reference point, with respect to which the standard free energies of other substances are measured. We can t measure the absolute value of a substance s free energy (as we can the entropy), but that s not a problem because we are interested only in free-energy differences between reactants and products. 

You probably know that two or more chemical equations can be combined algebraically to give a new equation. Even before the science of thermodynamics developed in the late nineteenth century, it was observed by Germaine Hess (1802-1850) that the heats associated with chemical reactions can be combined in the same way to yield the heat of another reaction. For example, the standard enthalpy changes for the oxidation of graphite and diamond can be combined to obtain AH° for the transformation between these two forms of solid carbon, a reaction that cannot be studied experimentally. 

Calculate the standard enthalpy of formation for diamond, given that... 

The equation corresponding to the standard enthalpy of formation of diamond is ... 

The value of AH is an approximate measure of the stability of a substance relative to the elements from which it is made. The standard enthalpies of formation of graphite, diamond, water, ethyne (acetylene, C2H2), ammonia and sodium chloride are shown in Fig. 13.6. The reference states of elements define an energy baseline or sea level . Compounds such as ethyne, for which AHf is positive, and which therefore possess a greater enthalpy than their constituent elements, appear above sea level and are called endothermic compounds. Compounds such as water, ammonia and sodium chloride, for which AHf is negative and which therefore possess a lower enthalpy than their constituent elements, appear below sea level and are called exothermic compounds. 

Note that the standard enthalpy of formation of an element will depend on the form of the element. For example, the for diamond equals the enthalpy change from the stablest form of carbon (graphite) to diamond. The thermochemical equation is... 

Using data for enthalpies of formation, calculate the standard change of enthalpy when graphite reacts with O2 to form CO2. Do the same calculation but for the reaction of diamond with O2. Which of these values corresponds to the standard enthalpy of formation of CO2 ... 

Investigators must take care to read the foreword of the particular table they use so that they know which standard state has been employed, because most thermodynamic properties are calculated with respect to convenient scales. For example, the standard enthalpy of formation of a compound, AHf, is almost always quoted for a temperature of 298.15 K, and the enthalpy of formation of an element in its standard state must by definition be zero. It is therefore practically useful to look at a table and find, for an element, where a zero entry occurs. For example, the following values might appear C(graphite), A/ff = 0.000 kcalthmol" C(diamond), AHf = 0.4532 kcalthmol". It is clear that C(graphite) is the standard state adopted for carbon in the table under consideration. Entropy, on the other hand, is usually defined by taking as zero the entropy, at T = 0, of the crystalline form in which all the molecules are orientated regularly. Because many of the extant tables have used thermochemical calories, care will also have to be taken in the future to see that values taken from different tables are corrected to the same units. 

Appendix 2 lists the standard enthalpies of formation for a number of elements and compounds. By convention, the standard enthalpy of formation of any element in its most stable form is zero. Again, using the element oxygen as an example, we can write Af/f(O2) = 0, but A/f f(O3) 0 and A// 0) 0. Similarly, graphite is a more stable allotropic form of carbon than diamond under standard conditions and 25°C, so we have A//f (graphite) = 0 and A// (diamond) 0. 

The problem with eqn 1.22 is that we have no way of knowing the absolute enthalpies of the substances. To avoid this problem, we can imagine the reaction as taking place by an indirect route, in which the reactants are first broken down into the elements and then the products are formed from the elements (Fig. 1.24). Specifically, the standard enthalpy of formation, AfH, of a substance is the standard enthalpy (per mole of the substance) for its formation from its elements in their reference states. The reference state of an element is its most stable form under the prevailing conditions (Table 1.7). Don t confuse reference state with standard state the reference state of carbon at 25 C is graphite (not diamond) the standard state of carbon is any specified phase of the element at 1 bar. For example, the standard enthalpy of formation of liquid water (at 25 C, as always in this text) is obtained from the thermochemical equation... 

The table of thermodynamic properties [2] presents the standard enthalpies and Gibbs energies of formation for all materials as that of an ideal gas at298.15 K and 1 atm. That table does not include common solid minerals like NaCl, CaCOj, diamonds, or graphite. What difficulties might we encounter while attempting to insert these materials in that table ... 

Heating graphite at the same time as compressing it under enormous pressure will yield diamond. The energy needed to convert 1 mol of graphite to diamond is 2.4 kJmol-1. We say the enthalpy of formation AHt for the diamond is +2.4 kJ mol-1 because graphite is the standard state of carbon. 

Several comments need to be made concerning the state of aggregation of the substances. For gases, the standard state is the ideal gas at a pressure of 1 bar this definition is consistent with the standard state developed in Chapter 7. When a substance may exist in two allotropic solid states, one state must be chosen as the standard state for example, graphite is usually chosen as the standard form of carbon, rather than diamond. If the chemical reaction takes place in a solution, there is no added complication when the standard state of the components of the solution can be taken as the pure components, because the change of enthalpy on the formation of a compound in its standard state is identical whether we are concerned with the pure... 

The standard temperature selected for the energy of formation and enthalpy of formation data is 25 °C = 299.25 K. The elementary form of carbon was taken to be graphite (and not diamond, as before). The numerical values of the energies of formation (which, as known, appear both in the relevant entry and in the tables) are the optimum molar values found in the enthalpy tables of Volk, Bathelt and Kuthe Thermochemische Daten von Raketentreibstoffen, Treibladungspul-vern sowie deren Komponenten , published by the Institut fur Chem-ische Technologie (ICT), D-76327 Pfinztal-Berghausen 1972. 

Since the chosen pressure of 1 bar represents the selected standard state, we designate the standard molar enthalpy of pure i at temperature T by the symbol = 0. However, the asterisk is ordinarily omitted, it being understood that element i is in its pure state. One must be careful in the application of this rule for example, the stable configuration of carbon at room temperature and ambient pressure is graphite, not diamond. In the same vein, sulfur under these conditions is stable in the rhombic habit, and Sn, in the white rather than the grey crystalline state. As another example, Br2 at P = 1 bar and 300 K is a liquid, while as a participant in reactions at 500 K Br2 is in the gaseous state. 

Although the enthalpy of formation under standard conditions for diamond is l.OkJmoT higher than that for graphite, the first does not spontaneously transform into the latter, but rather represents a metastable modification under normal conditions. In contrast to graphite it is not at all anisotropic regarding any property. 

Reaction (c) does not form a substance from its elements. Instead, a substance decomposes to its elements, so this reaction must be reversed. Next, the element carbon is given as diamond, whereas graphite is the standard state of carbon at room temperature and I atm pressure. The equation that correctly represents the enthalpy of formation of glucose from its... 

Although outdated, this book still provides useful references to the older thermochemical literature. Tabulated are A.H and enthalpy of transition values for the elements and their compounds, with the data for carbon-containing compounds being terminated at two carbon atoms. It should be noted that the data pertain to a temperature of 18 C and to diamond, rather than graphite, as the standard state for carbon the yellow form is the reference state for phosphorus. The yellow form is thermochemically identical to the white form which is the reference state used in the NBS Thermochemical Tables [149]. The data upon which this book was based were used in preparing NBS Circular 500,.see item [131]. 

However, the enthalpy changes of formation for ozone (03(g)) and diamond (C(s, diamond)) are not zero since these are not the standard states of the elements oxygen and carbon. 

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