Enthalpy Changes in Chlorination

Key Mechanism 4-1 Free-Radical Halogenation 136 4-4 Equilibrium Constants and Free Energy 138 4-5 Enthalpy and Entropy 140 4-6 Bond-Dissociation Enthalpies 142 4-7 Enthalpy Changes in Chlorination 143 4-8 Kinetics and the Rate Equation 145... 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

FIGURE 10.2.10. Process flowsheet for chlor-alkali mercury cell. (The numbers inside the process equipment refer to the enthalpy change in kWhrton" of chlorine.)... 

Enthalpy change in formation of sodium chloride from sodium and chlorine... 

The enthalpies for the reactions of chlorine and fluorine are shown graphically in Figure 11.2 as the relevant parts of a Born-Haber cycle. Also included on the graph are the hydration energies of the two halogen ions and hence the enthalpy changes involved in the reactions... 

Compute the enthalpy change for the destruction of ozone by atomic chlorine by subtracting the dissociation energies of O2 and CIO from the dissociation energy for ozone. What model chemistry is required for accurate modeling of each phase of this process The experimental values are given below (in kcal-moT ) ... 

In a diatomic molecule, e.g. chlorine (CI2), the bond dissociation enthalpy and the average bond enthalpy will have the same value. This is because both enthalpy changes refer to the process Cl2(g)--> 2Cl(g). 

Two isolated chlorine atoms have more freedom of motion than a single chlorine molecule. Therefore, the change in entropy is positive, and the entropy term (— TAS°) is negative. This negative (favorable) value of (—TAS°) is small, however, compared with the much larger, positive (unfavorable) value of AH° required to break the Cl—Cl bond.The chlorine molecule is much more stable than two chlorine atoms, showing that the positive enthalpy term predominates. 

Its enthalpy change must be more positive than in gas (cf. Fig. 3.4) by about 130kJ mol-1, which is the difference of the sublimation enthalpies of ZrCh and Z1CI4. With such a correction, the enthalpy of this step of chlorination of Zr atoms, which happens to be -132 kJ mol-1 in gas (see Fig. 3.4), may be about zero on the surface. Provided that Eq. 3.6 is still valid, chlorination on the surface is fast enough for the purpose. It is illustrated by the data of Table 3.3. 

Fig. 5.3 The enthalpy change of adsorption of chlorides on fused silica (in the presence of chlorinating agents) versus enthalpy change of vaporization of the compounds.

Chlorine (C12, 20,000 lb/hr) at 10 psia and 0 F is compressed to 700 psia. Estimate the change in enthalpy for the compression assuming adibatic and reversible conditions (constant entropy). 

The change in state of a system produced by a specified chemical reaction is definite. The corresponding enthalpy change is definite, since the enthalpy is a function of the state. Thus, if we transform a specified set of reactants to a specified set of products by more than one sequence of reactions, the total enthalpy change must be the same for every sequence. This rule, which is a consequence of the first law of thermodynamics, was originally known as Hess s law of constant heat summation. Suppose that we compare two different methods of synthesizing sodium chloride from sodium and chlorine. 

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