Enthalpy of reaction from enthalpies

Oalculate the enthalpies for reactions in which ethane, OeH, are the respective reactants and 002(g) and H20(/) are the products in each. Solve each by combining the known thermochemical equations using the AH values given below. Verify the result by using the general equation for finding enthalpies of reaction from enthalpies of formation. 

Calculating an Enthalpy of Reaction from Enthalpies of Formation... 

Hess s Law, enthalpy of reaction, from enthalpies of formation... 

Write the equation that can be used to calculate the enthalpy of reaction from enthalpies of formation. 

STRATEGY We write the chemical equation for the formation of HI(g) and calculate the standard Gibbs free energy of reaction from AG° = AH° — TAS°. It is best to write the equation with a stoichiometric coefficient of 1 for the compound of interest, because then AG° = AGf°. The standard enthalpy of formation is found in Appendix 2A. The standard reaction entropy is found as shown in Example 7.9, by using the data from Table 7.3 or Appendix 2A. 

This equation makes it simple to calculate enthalpies of reaction from values of AE obtained at constant volume, and vice versa. 

Titration calorimetry depends on calculation of the extent of reaction from the quantity of heat evolved. Its successful application to a given system depends on (a) the equilibrium constant and the reaction conditions being such that the reaction occurs to a moderate extent (i.e., not to completion) and (b) the enthalpy of reaction being measurably different from zero. 

Enthalpies of Reaction from the Standard Enthalpies of Formation of Species II 10.3 Calculation of Standard Transformed Entropies of Biochemical Reactions I, 10.4 Effect of Temperature... 

CALCULATION OF STANDARD TRANSFORMED ENTHALPIES OF REACTIONS FROM THE STANDARD ENTHALPIES OF FORMATION OF SPECIES... 

Since chemical thermodynamics and chemical kinetics are vast domains of science, we will select some special topics, viz. calculation of free enthalpies of reactions from group contributions and thermodynamics of free radical formation. 

CALCULATION OF THE FREE ENTHALPY OF REACTION FROM GROUP CONTRIBUTIONS... 

Calculation of the free enthalpy of reaction from group contributions, 752 Calorimetric properties, 109 Capacity... 

Next, select a heat exchanger and calculate the heat transfer area. First, calculate the required heat transfer, Q, from an energy balance. Obtain the enthalpy of reaction from Equation 7.4.19 and the standard enthalpies of reaction hsted in Table 7.1.1. 

Determine a heat of reaction from heats of other reactions using Hess s law. Determine standard enthalpies and internal energies of reaction from known standard heats of formation and heats of combustion. 

Energy Surfaces and Diagrams Transition State, AG° = -RT r Kgq, Every 1.36 kcal/mol (5.73 kJ/mol) Increase in AG° Decreases the Equilibrium Constant by a Factor of 10 at Room Temperature Driving Force AG° = AH° - 7AS° Enthalpy, Calculation of the Heat of Reaction from Bond Strengths Entropy, Disorder... 

When a reaction is composed of sub-steps, the total enthalpy change will be the sum of the changes for each step. Even if a reaction in reality contains no substeps, we may still write any number of reactions in series that lead from the same reactants to the same products and their sum will be the heat of the reaction of interest. The ability to add together these enthalpies to form ultimate products from initial reactants is known as Hess s Law. It is used to determine one heat of reaction from others ... 

We can calculate the enthalpy of reactions from values of AHf as shown in the following example. 

Thermodynamics is an extensive and far-reaching scientific discipline THAT DEALS WITH THE INTERCONVERSION OF HEAT AND OTHER FORMS OF ENERGY. THERMODYNAMICS ENABLES US TO USE INFORMATION GAINED FROM EXPERIMENTS ON A SYSTEM TO DRAW CONCLUSIONS ABOUT OTHER ASPECTS OF THE SAME SYSTEM WITHOUT FURTHER EXPERIMENTATION. For example, we saw in Chapter 6 that it is possible TO CALCULATE THE HEAT OF REACTION FROM THE STANDARD ENTHALPIES OF FORMATION OF THE REACTANT AND PRODUCT MOLECULES. 

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

The fact that it is possible in this way to calculate enthalpies of reactions from the AHf values for reactants and products is of very great convenience. Instead of having to tabulate AH" values for a large number of reactions, we can simply tabulate AHf values for the much smaller number of chemical compounds. It requires less space to tabulate AH values for, say, a thousand well-known compounds than to list the AH" values for the many millions of reactions that might occur between these compounds. 

These relations, particularly the later, are used to determine the enthalpy of reaction from the values of K obtained at various temperatures. 

For the reaction, both second and third law values of the enthalpy of reaction were determined using ion counting. The third law value was selected since the low Zrp signals resulted in second law derivations which were not meaningful [78KLE/CUB]. The enthalpy of reaction from the third law treatment was found to be (39.3 15.0) kJ-moP. On the basis of the selected enthalpy of formation values for Zrl3(g) and ZrP(g), the selected Af// value for Zrl2(g) is ... 

Table A-21 Enthalpies and entropies of reaction from Skeaflf et al. [85SKE/MAI].

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