Reaction rate in the gas phase

In order to answer the questions we have posed, we need to accomplish a number of things. First, we need to measure a reaction rate in the gas phase. Second, we need a framework for interpreting the rate constant and relating the measurement to a potential surface. Third, we need to acquire sufficient information to allow us to extract the value of a barrier height. Then, we can apply theories for interpreting these barrier heights in terms of chemical structure. 

Equation (3.54) is the simplified burning rate equation. If the reaction rates in the gas phase are known, the burning rate is given in terms of gas density (pressure), burning surface temperature, initial propellant temperature, and physical properties of the energetic material. 

If one assumes that the reaction rate in the gas phase is given by a one-step, kth-order Arrhenius-type equation and substitutes this in Eq. (3.83), one gets... 

The heat flux feedback from the gas phase to the burning surface is also determined by the chemical reaction rate in the gas phase. The reachon rate in the gas phase is altered by the addihon of catalysts. The catalysts act either on the decomposition reaction of the condensed phase or on the reaction in the gas phase of the gaseous decomposihon products. There are two types of catalysts posihve catalysts that increase the burning rate and negative catalysts that decrease the burning rate. 

Since cjijand r of the catalyzed propellant are pressure-independent in the plateau region and Qf is nearly constant, oiy should also be pressure-independent. This is a significant difference when compared with commonly observed bimolecular gas-phase reactions. Referring to Eq. (3.33), the reaction rate in the gas phase, oig, is given as a function of pressure according to... 

Fig. 11.21 shows the results of TG and DTA measurements on mixtures of AP particles and catalysts. The endothermic peak observed at 513 K is caused by the crystal structure transformation of AP from orthorhombic to cubic. A two-stage exothermic decomposition occurs in the range 573-720 K. The decomposition of the AP is seen to be drastically accelerated by the addition of catocene. The exothermic peak accompanied by mass loss occurs before the AP crystal transformation. Although the AP is sensitized by the addition of carborane, no effect is seen on the AP decomposition. The results indicate that carborane acts as a fuel component in the gas phase but does not catalyze the decomposition of AP. Thus, the critical friction energy is lowered due to the increased reaction rate in the gas phase. The results imply that the initiation of ignition by friction is caused by the ignition of the gaseous products of the AP pyrolants.PI... 

Before discussing these points in detail, it is worthwhile to consider how the diffusion equation for relative motion of two species is developed from a reduction of the diffusion equation describing the motion of both species separately. It introduces some of the complexities to the many-body problem and, at the same time, shows an interesting parallel to the theory of bimolecular reaction rates in the gas phase [475]. 

The reaction rate in the gas phase can be expressed by Equation 11.16. To solve the above equations, the parameters in the rates of reactions need to be determined experimentally. Modeling the above reaction is beyond the scope of this chapter. A simple case will be discussed in the following paragraph. 

When we turn from laws to theories, we again find that the sorts of theories that chemists name as such, and use to help describe their systems, are very different in character from the great theories of physics. Already, we have referred to the frequently pluralistic attitude of chemists to what are, ostensibly at least, rival theories. To illustrate, we cited several theories of chemical bonding. A similar picture emerges if we look at theories of reaction rates in the gas phase. 

The reaction rate increases linearly in a In [o)g] versus In p plot. The overall order of the reaction in the gas phase is given by the relationship nt=n- d, and is determined to be 1.78. This indicates that the reaction rate in the gas phase of TAGN is less pressure sensitive than that of other propellants, for example, m= 2.5 for double-base propellants1211. 

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