Heat capacity enthalpy change calculation with

Equations (9.9), (9.10), (9.11), or (9.12) in conjunction with Equation (9.2) give expressions for the molar heat capacity of a saturated phase. However, each equation contains the quantity (dS/dXi)TtPtX, which in turn contains terms such as (H — Hk) when xk is taken to be the dependent mole fraction. Evaluation of such quantities requires the knowledge of the absolute values of the enthalpies. Therefore, such terms cannot be evaluated, and the values of the molar heat capacities cannot be calculated. The necessity of knowing the absolute values of the enthalpies arises from the fact that a number of moles of some components must be added to, and the same number of moles of other components must be removed from the 1 mole of saturated phase in order to change the mole fractions of the phase. However, if the saturated phase is pure, even though it is in equilibrium with other phases that are solutions, the molar enthalpy of the phase is not a function of the mole fractions and Equations (9.9)—(9.12) reduce to Equation (9.3). 

The enthalpy change associated with freeze of water at 273K is -d.OkJmol 1. The heat capacity (Cp) for water is 75.3Jmor K I and for ice 37.6Jmol 1K 1. Calculate the... 

Suppose you wish to calculate the enthalpy change associated with a change in temperature undergone by a mixture of substances. Enthalpies and heat capacities of certain mixtures are tabulated in standard references. Lacking such data, you may use the following approximation ... 

Describe the energy changes associated with sensible heat, latent heat, and chemical reaction on both a macroscopic and a molecular level. Calculate their enthalpy changes using available data such as heat capacity, enthalpies of vaporization, fusion and sublimation, and enthalpies of formation. 

It is reasonable to expeet that models in ehemistry should be capable of giving thermodynamic quantities to chemical accuracy. In this text, the phrase thermodynamic quantities means enthalpy changes A//, internal energy changes AU, heat capacities C, and so on, for gas-phase reactions. Where necessary, the gases are assumed ideal. The calculation of equilibrium constants and transport properties is also of great interest, but I don t have the space to deal with them in this text. Also, the term chemical accuracy means that we should be able to calculate the usual thermodynamic quantities to the same accuracy that an experimentalist would measure them ( 10kJmol ). 

SAQ 3.7 Ethane burns completely in oxygen to form carbon dioxide and water with an enthalpy of AH = -1558.8 kJ mol1 at 25 °C. What is AH at 80°C First calculate the change in heat capacity Cp from the data in the following table and Equation (3.22). 

Let us return to the thermal decomposition of Fe(CO)(l,3-C4H6)2. Once the calibration constant is known, the enthalpy of the net process 9.10 can be calculated as the product of s and the area (A + B). The next step is to correct this value to 298.15 K by using heat capacity data. This exercise is, however, complicated by the cyclobutadiene polymerization. Brown et al. analyzed the reaction products by mass spectrometry and found several oligomers, in particular the dimer (C4H6)2 and the trimer (C4H6)3 [163]. With such a mixture, it is difficult to ascribe the observed enthalpy change to a well-defined chemical reaction. This is discussed in the paper by Brown and colleagues, who were nevertheless able to recommend a value for the standard enthalpy of formation of the iron-olefin... 

As we mentioned, it is necessary to have information about the standard enthalpy change for a reaction as well as the standard entropies of the reactants and products to calculate the change in Gibbs function. At some temperature T, A// j can be obtained from Af/Z of each of the substances involved in the transformation. Data on the standard enthalpies of formation are tabulated in either of two ways. One method is to list Af/Z at some convenient temperature, such as 25°C, or at a series of temperatures. Tables 4.2 through 4.5 contain values of AfZ/ at 298.15 K. Values at temperatures not listed are calculated with the aid of heat capacity equations, whose coefficients are given in Table 4.8. 

This value will be compared with a calculation using data of heat capacities and heat of solution. From Perry s Chemical Engineers Handbook (1984), the heat solution of the heptahydrate is -39.2 Btu/lb and its heat capacity is 0.36 Btu/(lb)(°F). The enthalpy change of the cooling and crystallization process is... 

Calculation of AH, the enthalpy change involved when the temperature of a substance with heat capacity,... 

The enthalpy change for this reaction at 25°C is AH = —802.2 kJ. Hence, when 1 mol of methane is converted to 1 mol of carbon dioxide gas and 2 mol of water vapor at 25°C and 1 atm, 802.2 kJ of energy is released as heat. If none of this heat escapes to the surroundings (hence the term adiabatic), all of it goes into heating the carbon dioxide and water vapor. To calculate the final temperature, the adiabatic flame temperature, one needs to know how the heat capacity of the material being heated, carbon dioxide and water vapor in this case, varies with temperature. 

The generalized correlations for HR and SR, together with ideal-gas heat capacities, allow calculation of enthalpy and entropy values of gases at any temperature and pressure by Eqs. (6.45) and (6.46). For a change from state 1 ... 

When 1.00 L of 1.00 M Ba(N03)2 at 25.0°C is mixed with 1.00 L of 1.00 M Na2S04 at 25°C in a calorimeter, the white solid BaS04 forms and the temperature of the mixture increases to 28.1°C. Assuming that the calorimeter absorbs only a negligible quantity of heat, that the specific heat capacity of the solution is 4.18 J °C-1 g-1, and that the density of the final solution is 1.0 g/mL, calculate the enthalpy change per mole of BaS04 formed. 

In a coffee cup calorimeter, 1.60 g of NH4NO3 is mixed with 75.0 g of water at an initial temperature of 25.00°C. After dissolution of the salt, the final temperature of the calorimeter contents is 23.34°C. Assuming the solution has a heat capacity of 4.18 J °C 1 g-1 and assuming no heat loss to the calorimeter, calculate the enthalpy change for the dissolution of NH4NO3 in units of kj/mol. 

Calculate each unknown specific enthalpy. For a reactant or product, start with the elemental species at 25°C and 1 atm (the references) and form 1 mol of the process species at 25°C and 1 atm (A// = A/f° from Table B.l). Then bring the species from 25°C and 1 atm to its process state, calculating AH using the appropriate heat capacities from Table B.2, specific enthalpies from Table B.8 or B.9, and latent heats from Table B.l. The specific enthalpy that goes in the inlet-outlet table is the sum of the enthalpy changes for each step in the process path. 

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