Calculating Heats of Reaction

The failure was due to combustion of oxygen plus hydrocarbons. The combustion reaction involved was  

This is a methanation reaction. The hydrogen reacts with the carbon monoxide to produce water, methane, and heat. My job was to calculate how hot the reactor could get with 4 percent carbon monoxide in the hydrogen supply gas. To do this I looked up in Perry s Chemical Engineering Handbook, McGraw-Hill 5th ed., 1973, the heats of formation of  

Adding up the heats of formation on both sides of Eq. (44.1) and taking their difference gives a total heat of reaction of 49.2 kcal (49,200 calories). This would be the heat of reaction if there were 25 percent CO and 75 percent in the feed gas. However, the feed gas contained 4 percent CO, not 25 percent CO. Therefore, the heat of reaction was  

The 96 percent hydrogen and 4 percent CO in the feed gas would be converted to  

Dividing the heat of reaction (7900 calories) by the specific heat of the products of reaction (7.0 calories per mole per 1°C) results in a temperature rise of 1130°C or 2040°F. 

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Using this equation it is possible to calculate heats of reaction from the variation of AG with temperature. 

Table 1. Calculated Heats of Reaction at 298 K for Selected Gasification Reactions ...

Two methods of calculating heats of reaction from heats of combustion are illustrated in Example 3.9. 

The standard deviation between experimental and calculated heats of reaction are between 0.5 and 1 kcal/mol for those classes of compounds where enough experimental heats of formation are available to allow a full parameterization. For those classes of compounds where insufficient heats of formation are known to allow the determination of all parameters for 1,2- and 1,3-interactions, an estimate can be given for the bond energy terms which are the dominating parameters. Even here, therefore, a reasonable value for the reaction enthalpy is available. 

Up until now, the results of the reactivity functions have been taken as one type of evaluation, while the calculated heats of reaction (Sect. 4.1) have been considered as a second. This parallels ideas based on kinetic and thermodynamic approaches. While it is possible to interpret the EROS results in terms of these two evaluations separately, it would be preferable to consider them together in some sort of combined function. Initial studies are indicating that this idea is feasible. 

The heat of decomposition and/or reaction (in absence of ambient oxygen) is calculated (see Section 2.2.3.2). If the value of the oxygen balance is less than -240 or higher than +160 (Table 2.12) and the calculated heat of reaction/decomposition is less than 100 cal/g (420 J/g), the substance in its pure form is regarded as having a very low potential to produce a deflagration or detonation [10,11]. 

The reader should refer to the original tables for the reference material on which the thermochemical data are based. The reference state used in Chapter 1 was chosen as 298 K consequently, the thermochemical values at this temperature are identified from this listing. The logarithm of the equilibrium constant is to the base 10. The unit notation (J/K/mol) is equivalent to (JK mol ). Supplemental thermochemical data for species included in the reaction listing of Appendix C, and not given in Table A2, are listed in Table A3. These data, in combination with those of Table A2, may be used to calculate heats of reaction and reverse reaction rate constants as described in Chapter 2. References for the thermochemical data cited in Table A3 may be found in the respective references for the chemical mechanisms of Appendix C. 

Related Calculations. Use this procedure to calculate heats of reaction and standard free-energy changes for reactions that involve components listed in Tables 1.17 and 1.18. 

Related Calculations. Use this general procedure to calculate heats of reaction for other aqueous-phase reactions. Calculation of heat of reaction from standard heat of reaction is covered in Section 4 in the context of chemical-reaction equilibrium see in particular Example 4.1. 

Calculate the heat of reaction at 298 K, AH2,)fi. Calculating heat of reaction is a multistep process. One starts with standard heats of formation at 298 K, calculates the standard heat of reaction, and then adjusts for actual system temperature and pressure. 

D. Rowley and H. Steiner, Discussions Faraday Soc., 10,198 (1951). Data are from flow system and show scatter of 20 per cent. Calculated heat of reaction at 800 K is 36.3 Kcal, and there is discrepancy between it and the one calculated from difference in activation energies of forward and reverse reactions, which gives 30 Kcal. 

Table II. Calculated Heats of Reaction (kcal/mole) for Various Aluminum-Molecule Reactions...

REM NEW ENERGY BALANCE PROGRAM, WITH FILES 20 REM CALCULATES HEAT OF REACTION FROM HEATS OF FORMATION 30 REM REVISION, GWBASIC 23/11/92 40 ON ERROR GOTO 2200... 

The change in (ZlG /7 ) between two temperatures can therefore be obtained by numerical integration of heat of reaction data. The preceding equation is also useful to calculate heats of reaction in terms of free energies of reaction and their temperature variation. 

Calculate heats of reaction at other than the standard temperature. 

Heat of Reaction The heat of reaction is defined as the energy absorbed by the system when the products after reaction are restored to the same temperature as the reactants. The pressure must also be specified for a complete definition of the thermodynamic states of the products and reactants. If the same pressure is chosen for both, the heat of reaction is equal to the enthalpy change this is the customary definition of the heat of reaction. The heat of any reaction can be calculated by combining heats of formation or heats of combustion of the products and reactants. Thus the basic information necessary for calculating heats of reaction are heats of formation and... 

The enthalpies of formation of many compounds, Hf(Tg), are usually ta lated at 25 C and can readily be found in the HandbwJk of Chemistry t Physics and similar handbooks. For other substances, the heat of combust (also available in these handbooks) can be used to detemtine the enthalpy formation. The method of calculation is described in these handbooks. Fr these values of the standard heat of fonnation. //f (7 ), we can calculate heat of reaction at the reference temperature from Equation (8-24). 

Now, an exact temperature measurement of the air before and after the reaction is difficult. However, if we could insulate a portion of the surroundings, to isolate and trap the heat, we could calculate a useful quantity, the heat of the reaction. Experimental strategies for measuring temperature change and calculating heats of reactions, termed calorimetry, are discussed in Section 8.2. 

Calculating heat of reaction from enthalpy changes or bond energies (283) ... 

Herod and Harrison [175] discussed possible reaction channels for these product ions by comparing the maximum available kinetic energies, deduced from the above threshold energies, with calculated heats of reactions. For C3H4, for instance, the three possible channels are... 

Thermochemistry of AUyl Chloride Manufacture. The calculated heats of reaction of allyl chloride and by-products of reaction are shown below in Table 6-9,... 

Table 6-9. Calculated Heats of Reactions, Allyl Chloride Manufacture...

The importance of Hess s law is that it allows us to calculate heats of reaction that might he difficult or inconvenient to measure directly in a calorimeter. 

Knowledge Required (1) The meaning of standard heats of formation. (2) The methods of using heats of ftinnation to calculate heats of reaction. 

The probability that the desired reaction is accompanied by a number of undesired processes is very high. Synthesis optimization activities may improve the selectivity, but a value of 1 with respect to the desired reaction is seldom achieved. A calculated heat of reaction can account for these side reactions only if they are definitely known. Otherwise the calculated value has to be regarded as a net value which corresponds to the desired reaction only and which may significantly be wrong with respect to the process in total. 

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