INDEX reaction rates

Once the initial equilibrium state of the system is known, the model can trace a reaction path. The reaction path is the course followed by the equilibrium system as it responds to changes in composition and temperature (Fig. 2.1). The measure of reaction progress is the variable , which varies from zero to one from the beginning to end of the path. The simplest way to specify mass transfer in a reaction model (Chapter 13) is to set the mass of a reactant to be added or removed over the course of the path. In other words, the reaction rate is expressed in reactant mass per unit . To model the dissolution of feldspar into a stream water, for example, the modeler would specify a mass of feldspar sufficient to saturate the water. At the point of saturation, the water is in equilibrium with the feldspar and no further reaction will occur. The results of the calculation are the fluid chemistry and masses of precipitated minerals at each point from zero to one, as indexed by . 

D. R. Stull11 developed a rating system to establish the relative potential hazards of specific chemicals the rating is called the reaction hazard index (RHI). The RHI is related to the maximum adiabatic temperature reached by the products of a decomposition reaction. It is defined as... 

Comparison of NFPA Reactivity Rating with the Reaction Hazard Index (Ref 2 ... 

Data Sources, Reaction Hazard index, and National Fire Protection Association Reactivity Rating... 

No. Formula Gaseous Compound AHp 298 kcal/mole Kef. (24) Td°K Activation Energy for Decomposition kcal/mole Ref. Reaction Hazard Index RHI NFPA Reactivity Rating... 

The hydrogen-isotope exchange reactions have already been discussed in previous sections. For various aromatic and heteroaromatic substrates, standard reaction rates in protodedeuteration and in nitration have been obtained. A plot of these shows a wide scatter. This means that there is no simple relation between these two measures for the susceptibility to electrophilic attack. No single reactivity index can be used as a measure to derive a unique order of the susceptibility of individual ring positions towards electrophilic attack (75TL1395). 

The microkinetics of the chemical reaction that takes place is described by da/dt = dbd/dt = —kaabd where kt> is the real chemical reaction rate constant and the index d refers to the dispersed phase. If the components A and B are supplied to the reactor in stoichiometric quantities and... 

We recall that c is the velocity of the molecules. The index on v means that we calculate the number of collisions necessary for reaction in the part of the zone where the reaction rate is highest and conditions are most conducive, so that i/min is the minimum value of v. Finally, tp is a dimensionless quantity of order (but less than) unity, algebraically (but not exponentially) dependent on the reaction mechanism, the activation heat, the temperatures T0 and TB, and the reagent concentrations. From the formula it is obvious first of all that u is always many times smaller than c, and less than the speed of sound. This fact will be important for the theory of detonation (Part II). 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

---