Formation reaction standard heat

The first step is to calculate K. The method involving the standard heat of reaction and the standard entropy change is usually used because these data are available for most substances. In the tables giving standard heats of formation (or standard heats of combustion) and absolute ejn-tropies, the standard state is usually specified as unit fugacity for all ga es and is usually stated as th pure gas at 1 atm pressure. The calculation must be based upon the standard state specified by these tables and is not arbitrary, as many people are led to believe. If the standard state for each component is that of unit fugacity, the equilibrium equation becomes... 

The standard heat of a reaction is therefore obtained by taking the difference between the standard heat of formation of the products and that of the reactants. If the standard heat of reaction or formation is negative exothermic), as is the case with most reactions, then energy is liberated as a result of the reaction. Energy is absorbed if is positive endothermic). Standard heat of formation and standard heat (enthalpy) of combustion data at 25°C are available in the literature. Both of these heat (or enthalpy) effects find extensive application in thermodynamic calculations. ... 

The standard Gibbs-energy change of reaction AG° is used in the calculation of equilibrium compositions. The standard heat of reaclion AH° is used in the calculation of the heat effects of chemical reaction, and the standard heat-capacity change of reaction is used for extrapolating AH° and AG° with T. Numerical values for AH° and AG° are computed from tabulated formation data, and AC° is determined from empirical expressions for the T dependence of the C° (see, e.g., Eq. [4-142]). 

The reverse reaction to give the gaseous species AlX(g) at high temperature accounts for the enhanced volatility of AIF3 when heated in the presence of A1 metal, and the ready volatilization of A1 metal in the presence of AICI3. Using calculations of the type outlined on p. 82 the standard heats of formation of the crystalline monohalides AIX and their heats of disproportionation have been estimated as ... 

The standard heat of formation ( AH ) of a chemical compound is the standard heat of reaction corresponding to the chemical combination of its constituent elements to form one mole of the compound, each existing in its standard state at 1 atm and 25°C. It has units of cal/g-mole. 

The standard heat of combustion (ziH") of a chemical substance (usually an organic compound) is the same as the standard heat of reaction for complete oxidation of 1 mole of the substance in pure oxygen to yield COj(g) and HjO(f) as products. A reference state of 25°C and 1 atm is assumed in quoting standard heats of combustion in cal/g-mole. The value of AH" is always negative because combustion is an exothermic reaction. Note that the standard heats of combustion for carbon and hydrogen are the same as the heats of formation for CO,(g) and HjO(f), respectively. 

Applications. (1) Heats of formation data of reactants and products can be used to calculate the standard heat of a chemical reaction by applying Hess s law. Thus,... 

It does not matter that there is no known catalyst that can accomplish the reaction in Equation (7.21) directly. Heats of reaction, including heats of formation, depend on conditions before and after the reaction but not on the specific reaction path. Thus, one might imagine a very complicated chemistry that starts at standard conditions, goes through an arbitrary trajectory of temperature and pressure, returns to standard conditions, and has Equation (7.21) as its overall effect. A77. =-1-147,360 J/mol of styrene formed is the net heat effect associated with this overall reaction. 

Equation can also be used to calculate the standard enthalpy of formation of a substance whose formation reaction does not proceed cleanly and rapidly. The enthalpy change for some other chemical reaction involving the substance can be determined by calorimetric measurements. Then Equation can be used to calculate the unknown standard enthalpy of formation. Example shows how to do this using experimental data from a constant-volume calorimetry experiment combined with standard heats of formation. 

Again, it is convenient to follow the seven-step procedure to solve this problem. We are asked to find an enthalpy of formation. Because enthalpy is a state function, we can visualize the reaction as occurring through decomposition and formation reactions. Appendix D lists enthalpies of formation, and the experimental heat of combustion is provided. We can use Equation to relate the enthalpy of combustion to the standard enthalpy of formation for octane. 

C06-0080. Using standard heats of formation, determine A H for the foiiowing reactions ... 

The standard heat of formation of a substance is the enthalpy change involved in forming 1 mole of it from its elements. The standard heat of formation is measured at 25°C (or 298 K) and one atmosphere of pressure for gases or 1 molar solutions for liquids. Tables of the heat of formation are usually given in units of kilojoules per mole. For water, the standard heat of formation is -286 kjmol The minus sign means that the reaction is exothermic and heat is given off... 

The standard heat of reaction can be calculated from the heats of formation as... 

For this reaction the standard heat of reaction at 298 K, AH298, equals -393510 J. Now let the following two reactions, which also finally result in the formation of C02, be considered... 

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. 

The relationship between standard heats of reaction and formation is given by equation 3.26 and illustrated by Examples 3.8 and 3.9... 

Calculate the standard heat of the following reaction, given the enthalpies of formation ... 

Heats of reaction can be calculated from the heats of formation of the reactants and products, as described in Chapter 3, Section 3.11. Values of the standard heats of formation for the more common chemicals are given in various handbooks see also Appendix C. A useful source of data on heats of formation, and combustion, is the critical review of the literature by Domalski (1972). 

The reaction favours the formation of ozone with a significant equilibrium constant. Appendix C also lists the enthalpies of formation and the standard enthalpy of the reaction ArH° can be calculated. The answer for the enthalpy calculation is ArH° = —106.47 kJ mol, showing this to be an exothermic reaction, liberating heat. The entropy change at 298 K can also be calculated because ArG° = ArH° — T ArS°, so ArS° = 25.4 Jmol-1 K-1, indicating an increase in the entropy of the reaction as it proceeds by creating one molecule from two. 

The ArG° for the reaction, under standard conditions, is obtained by looking up the tabulated standard heats of formation and then using Equation 8.16 to give ... 

The standard molar enthalpy of formation, A// , is the amount of heat absorbed when 1 mole of the substance is produced from its elements in their standard states. At 25°C, A// of liquid water is -285.8 kJ/mol and A// of water vapor is -241.8 kJ/mol. This means that more heat is released when liquid water is formed from its elements, then when gaseous water is formed from its elements. So, the formation reaction of liquid water is... 

Many, but not all, endothermic compounds have been involved in violent decompositions, reactions or explosions, and in general, compounds with significantly positive values of standard heat of formation may be considered suspect on stability grounds. Notable exceptions are benzene and toluene (AH°f +82.2, 50.0 kJ/mol 1.04, 0.54 kJ/g, respectively), where there is the resonance stabilising effect of aromaticity. Values of thermodynamic constants for elements and compounds are tabulated conveniently [1], but it should be noted that endothermicity may change to exothermicity with increase in temperature [2], There is a more extended account of the implications of endothermic compounds and energy release in the context of fire and explosion hazards [3], Many examples of endothermic compounds will be found in the groups ... 

It is, of course, not necessary to have an extensive list of heats of reaction to determine the heat absorbed or evolved in every possible chemical reaction. A more convenient and logical procedure is to list the standard heats of formation of chemical substances. The standard heat of formation is the enthalpy of a substance in its standard state referred to its elements in their standard states at the same temperature. From this definition it is obvious that heats of formation of the elements in their standard states are zero. 

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