Enthalpy change, of formation

Stability criteria, 449-454 Standard Gibbs-energy change of formation, 510, 512-513 of reaction, 504-510 effect of temperature on, 507-508 Standard heat (enthalpy change) of formation, 118-123 of reaction, 116-133, 505, 507-508 effect of temperature on, 123-127, 508 Standard state, 117-118, 399-400, 504-505, 515-518... 

Compared to two molecules of -l,3,5-hexatriene, the pentacyclic ladderane core of 1 is unstable, which is indicated by the difference of the standard enthalpy change of formation (AA//) of 19.6 kcal/mol. On the other hand, 1 is kinetically inert to decomposition into trienes. The strain energy of 1 is estimated at ca. 75 kcal/mol, which is approximately three times that of cyclobutane (26.5 kcal/mol )." ... 

For dissolved species the standard state is defined as an ideal solution with a concentration of 1 M (this is obtained in practice by extrapolating the dilute solution behavior up to this concentration). A special comment is in order on the standard enthalpies of formation of ions. When a strong electrolyte dissolves in water, both positive and negative ions form it is impossible to produce one without the other. It is therefore also impossible to measure the enthalpy change of formation of ions of only one charge. Only the sum of the enthalpies of formation of the positive and negative ions is accessible to calorimetric experiments. Therefore, chemists have agreed that AH° of H (aq) is set to zero. 

In this chapter, a mathematical expression of the First Law will be discussed. The two functions internal energy, U, and enthalpy, H, will figure prominently. In addition, tabulations of standard enthalpy changes of formation, A//0 for a number of compounds, will be given. From such tables it is possible to derive the amount of thermal energy associated with any reaction, as long as all reactants and products are listed. 

It will be clear from what has been said that there are multitudes of possible reactions which might concern us, and therefore multitudes of All values. New compounds are being synthesized daily, and each will undergo many possible reactions. Clearly, it is almost impossible to determine AH for eveiy possible change. Luckily, it is also unnecessary. It is sufficient, as we shall see, to know the standard enthalpy change of formation (sometimes abbreviated to enthalpy of formation or called standard heat of formation), for each compound with which we are dealing. The formation reaction is that in which the compound is formed from its elements in their standard states, that is, their most stable form under chosen standard conditions of temperature and pressure (s.t.p.). The standard pressure and temperature recommended by IUPAC since 1982 are / °=105Pa... 

The standard enthalpy change of formation for crystalline Na20 is found to be -418.0 kJ/mol. This necessarily implies the following exact equation ... 

Compare the value of-134.5 kJ/mol for the standard enthalpy change of formation of gaseous 2-methyl propane (C4Hi0) with that calculated from bond energy data (Table 3.6 and Appendix III). 

Given the standard enthalpy change of formation of MgO = — 602 kjmol" and of AljOj = — 1700kJmoP. calculate the standard enthalpy change for the reaction... 

If we heat hydrogen, oxygen and graphite together, so little methanoic acid is produced that it is impossible to directly measure its enthalpy change of formation. For this reason, the enthalpy change of formation is calculated indirectly, using Hess s law. 

II) Use the data to calculate the standard enthalpy change of formation (AHf) of calcium oxide (CaO). 

The enthalpy change of formation, AH, of a substance is the heat change (at constant pressure) on production of one mole of the pure substance from its elements in their standard states under standard thermodynamic conditions (298 K and 1 atm pressure). 

The following balanced equations do not represent enthalpy changes of formation ... 

Enthalpy changes of formation are often difficult to measure in practice due to competing side reactions and slow rates of reaction. For example, methane and potassium manganate(vii) cannot be prepared from their elements via the following thermochemical equations ... 

The IB Chemistry data booklet tabulates enthalpies of formation for selected organic compounds on page 12. Note that enthalpy changes of formation for elements (in their standard states) are zero since the thermochemical equation representing the formation of an element is a null... 

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. 

Enthalpies of formation are usually calculated indirectly from other enthalpy changes of reaction including bond enthalpies. Enthalpy changes of formation are commonly used to calculate enthalpy changes of reaction, using Hess s law. 

Enthalpy changes of formation are usually negative, that is, the corresponding reactions are exothermic. However, some compounds have positive enthalpies of formation, for example benzene and nitrogen monoxide ... 

The enthalpy change of any reaction can be determined by calculation, from the enthalpy changes of formation of all the substances in the chemical equation, using Hess s law. 

AT/ [El2(g)] = -286kJmoM Draw an energy cycle and use Eless s law to produce an equation for the enthalpy change of formation. [4]... 

As expected, because of the decrease in the amount of gas (from 4.5 moles to 2 moles), there is an increase in the order of the system, hence the entropy change is negative. Note that, in contrast to standard enthalpy changes of formation, the absolute entropy values of elements, such as oxygen in this example, are not zero (under standard thermodynamic conditions). 

Actual values for standard-state molar enthalpies are not available because an arbitrary constant can be added to each internal energy without any physical effect. We use the standard-state enthalpy change of formation to calculate AH° for chemical reactions. The standard-state enthalpy change of formation of substance i is denoted... 

Find the standard-state enthalpy change of the reaction of Eq. (2.7-6) at 298.15 K, using values of enthalpy changes of formation from Table A.8. 

The standard-state enthalpy change of combustion of methane at 298.15 K equals —890.36kJmoF, with hquid water as one of the products. Find the enthalpy change of formation of methane at 298.15 K using the enthalpy changes of formation of H2O and CO2. 

Because gaseous O2 is the most stable form of oxygen at 298.15 K, Af//°(02) — 0. Using values of the enthalpy changes of formation of the other substances,... 

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