Thermochemical equations processes

To see where a thermochemical equation comes from, consider the process by which ammonium nitrate dissolves in water ... 

Thermochemical equations like (2) and (3) are used to express, accurately, the conditions to which reported calorimetric results refer. The symbol for the process... 

Sample The complete combustion of butane gas, C4H10, in oxygen gas, Oz, produces C02 and H20. It is a highly exothermic process releasing 2845 kj of heat per mole of butane. Write the balanced thermochemical equation, using all whole-number coefficients. Also, determine the enthalpy change in burning 50.0 g of butane gas. 

The nature of the reaction or process described by a thermochemical equation is often written as a subscript of Mi. For example, the highly exothermic combustion (comb) of glucose (C6Hi20g) occurs in the body as food is metabolized to produce energy for activities such as the one shown in Figure 16-9. The thermochemical equation for the combustion of glucose is... 

Write thermochemical equations for chemical reactions and other processes. 

A final assumption of thermochemical equations is that when you reverse the direction of a chemical reaction, you only change the sign of the enthalpy. For example, the synthesis of ammonia is an exothermic process that releases 91.8 k.T of heat. If you reverse the direction to show the decomposition of ammonia, the reaction becomes endothermic ... 

In order to define the thermochemical properties of a process, it is first necessary to write a thermochemical equation that defines the actual change taking place, both in terms of the formulas of the substances involved and their physical states (temperature, pressure, and whether solid, liquid, or gaseous. 

EXAMPLE 3 In the thermochemical equation for the synthetic methanol process (all substances are in the gaseous state)... 

A thermochemical equation is a balanced equation that includes the heat of reaction (A//rxn). Keep in mind that the A//,xn value shown refers to the amounts (moles) of substances and their states of matter in that specific equation. The enthalpy change of any process has two aspects ... 

Important examples of negative electron affinities, i.e., those pertaining to energy-absorbing processes, are those for the formation of the oxide and sulfide ions from the oxygen and sulfur atoms. The relevant thermochemical equations are given in Tabic 2-3. 

To calculate the attraction energy of the atom A by the active center of the catalyst K, i.e, the bond energy Qak> we should examine the process expressed by the thermochemical equation... 

Thermochemical equations can be rearranged and added to give enthalpy changes for reactions not included in the data tables. The basis for calculating enthalpies of reaction is known as Hess s law The overall enthalpy change in a reaction is equal to the sum of enthalpy changes for the individual steps in the process. The energy difference between reactants and products is independent of the route taken to get from one to the other. In fact, measured enthalpies of reaction can be combined to calculate enthalpies of reaction that are difficult or impossible to actually measure. 

A thermochemical equation is a reaction equation for a chemical reaction or phase transition with specification of the reaction enthalpy AH for an isobaric process at a given temperature T. In thermochemical equations the following notation is used... 

Ekjuations of the forms (a), (b), and (c) are called thermochemical equations. Generally, these kinds of equations can be added, subtracted, reversed and multiplied by constants. Thus, it is possible to obtain information of changes of enthalpy for processes for which no thermodynamic data are available, or of processes for which experimental investigations are difficult. 

This reaction is very exothermic (A// —180 to —200kJ mol-1) and, therefore, seems to be very probable from the thermochemical point of estimation. The pre-exponential factor is expected to be low due to the concentration of the energy on three bonds at the moment of TS formation (see Chapter 3). To demonstrate that this reaction is responsible for the oxidative destruction of polymers, PP and PE were oxidized in chlorobenzene with an initiator and analyzed for the rates of oxidation, destruction (viscosimetrically), and double bond formation (by the reaction with ozone) [131]. It was found that (i) polymer degradation and formation of double bonds occur concurrently with oxidation (ii) the rates of all three processes are proportional to v 1/2, (iii) independent of p02, and (iv) vs = vdbf in PE and vs = 1.6vdbf in PP (vdbf is the rate of double bond formation). Thus, the rates of destruction and formation of double bonds, as well as the kinetic parameters of these reactions, are close, which corroborates with the proposed mechanism of polymer destruction. Therefore, the rate of peroxyl macromolecules degradation obeys the kinetic equation ... 

The second comment is that we have chosen the most prevalent elementary processes (unimolecular and bimolecular reactions) to illustrate how to relate thermochemical with kinetic data. Different molecularities will naturally change many equations just presented, but the basic relations 3.8 and 3.9 will not be affected. 

The expectation that the k rate constants correlate with thermochemical bond-energy data in this radical process has indeed found quantitative support through the determination of the activation parameters, on running the H-abstraction experiments by BTNO from selected substrates at various temperatures. From the Arrhenius equation (logfe = log A — Ei /RT), log A and were obtained (Table 7). 

To develop the governing equations for thermochemical modeling, consider the material volume element in Figure 8.5. Performing an energy balance over this volume while neglecting convective processes yields... 

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