Enthalpy of conversion

It melts without decomposition although the enthalpy of conversion to silver chloride and oxygen appears to be about —0.5 kJ/g. An explosion while grinding the salt (which had not been in contact with organic materials) has been reported [1]. A powerful oxidant. 

Some related data concerning the gas-phase reactions are shown in Table 4 [65,66], Data shown in Table 4 indicate that, in the case of primary carbonium ions (e.g., C2Hs+ cation), the enthalpy of conversion of carbenium into oxonium or ammonium ion (>220 kJ/mol) by far exceeds the strain of even the most strained rings (< 115 kJ/mol). 

Ahyd was measured in acetic acid. The discrepancy of 1.6 kcal mol 1 between results in acetic acid and the gas-phase result is larger than expected (0.5 - 0.7. kcal mol 1) nevertheless, the anomaly noted in entry (13) was found in these experiments as well. The exothermic enthalpy of conversion from the -2- isomer to the -1- isomer found by Kistiakowsky was confirmed in these experiments although the absolute values of AhyiH are different. 

The self conversion rate is an activated process and very slow, the half-life of the conversion is greater than one year at 77 K. The conversion reaction from ortho- to para-hydrogen is exothermic and the heat of conversion is also temperature dependent [9], At 300 K the heat of conversion is 270 kj kg and increases as the temperature decreases, until it reaches 519 kJ kg at 77 K. At temperatures lower than 77 K the enthalpy of conversion is 523 kJ kg and almost constant. The enthalpy... 

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 relationships between specific heat and water content and between specific heat and ash content are hnear. Given the specific heat on a dry, ash-free basis, it can be corrected to an as-received basis. The specific heat and enthalpy of coal to 1366 K (2000°F) are given in Coal Conversion Systems Technical Data Book (part lA, U.S. Dept, of Energy, 1984). 

Conversely, processes which convert carbons to sfp- carbons are more favorable for five-membered than for six-membered rings. This can be illustrated by the data for acetolysis of cyclopentyl versus cyclohexyl tosylate. The former proceeds with an enthalpy of activation about 3kcal/mol less than the latter." A molecular mechanics analysis found that the difference was largely accounted for by the relief of torsional strain in the cyclopentyl case." Notice that there is an angle-strain effect which is operating in the opposite direction, since there will be some resistance to the expansion of the bond angle at the reaction center to 120° in the cyclopentyl ring. 

An electrochemical cell is a device by means of which the enthalpy (or heat content) of a spontaneous chemical reaction is converted into electrical energy conversely, an electrolytic cell is a device in which electrical energy is used to bring about a chemical change with a consequent increase in the enthalpy of the system. Both types of cells are characterised by the fact that during their operation charge transfer takes place at one electrode in a direction that leads to the oxidation of either the electrode or of a species in solution, whilst the converse process of reduction occurs at the other electrode. 

Enthalpies of formation for a variety of compounds are listed in Table 8.3. Notice that, with a few exceptions, enthalpies of formation are negative quantities. This means that the formation of a compound from the elements is ordinarily exothermic. Conversely, when a compound decomposes to the elements, heat usually must be absorbed. 

The enthalpy of the phase conversion can be determined from Eq.(6) by plotting the log of the absorption or desorption plateau pressure, P lnleau, against the reciprocal temperature as indicated in Fig. 2. When the solubility of hydrogen in the metal (or) phase is small, then AHplM AH(, where AH( is essentially the enthalpy of forma... 

The only quantity considered here is the enthalpy of formation, A fH°, at 298.15 K. Data are given in units of kJmol-1. The conversion factor 1 thermochemical calorie = 4.1840 joules was used. 

Sublimation is the direct conversion of a solid into its vapor. Frost disappears on a cold, dry morning as the ice sublimes directly into water vapor. Solid carbon dioxide also sublimes, which is why it is called dry ice. Each winter on Mars, solid carbon dioxide is deposited as polar frost, which sublimes when the feeble summer arrives (Fig. 6.24). The enthalpy of sublimation, AHsub, is the molar enthalpy change when a solid sublimes ... 

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 ... 

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