Ammonia synthesis stoichiometric number

Guideline 6. The great majority of known elementary steps are bimolecular, the remainder being unimolecular or termolecular. Any reaction where the stoichiometric coefficients of the reactants add up to four or more must involve a multiplicity of steps. The ammonia synthesis reaction is known to occur by a number of steps rather than as... 

The equilibrium constant K is independent of pressure with standard states. The effect of the pressure is shown in Equation 6-6. Ky is usually insensitive and may either increase or decrease slightly with pressure. When (r + s) > (a + b), the stoichiometric coefficients, an increase in pressure P results in a decrease in conversion of the reactants to the products (i.e., A + B <-> R + S). Alternatively, when (r + s) < (a + b), an increase in pressure P results in an increase in the equilibrium conversion. In ammonia synthesis (N2 + 3H2 <-> 2NH3), the reaction results in a decrease in the number of moles. Therefore, an increase in pressure causes an increase in equilibrium conversion due to this factor. 

The Vi with their accompanying signs are called stoichiometric numbers. For example, when the ammonia-synthesis reaction is written... 

For instance, if we write down the ammonia synthesis reaction from the elements as N2 + 3 H2 = 2 NHs, we can particularize the elementary reactions involved in this process to the stoichiometric numbers M = 1, 2, and 3, respectively, as ... 

The determination of the stoichiometric number was made by Horiuti and Ikushiraa (102) for the hydrogen electrode process on platinum and more recently by Horiuti and Enomoto (103) for the ammonia synthesis... 

The example on the right hand side represents a mechanism for ammonia synthesis with a hanging vertex or a dead-end (step 5) with a stoichiometric number equal to zero. 

Obviously, in order to consume the intermediates formed in former steps, some steps must be carried out more than once. The stoichiometric numbers of every elementary step for ammonia synthesis reactions, as shown above, are 1, 3, 2, 2 and 2, respectively. If there is only one rate determining step and different stoichiometric number a for each elementary step, it is possible to use stoichiometric number to decide which step are RDS. Once the stoichiometric number of RDS in reactions is known, the rate equation of reverse reaction can be calculated from forward reaction rate and thermodynamic equilibrium relationships. 

In other words, the stoichiometric number determined from the transfer of a tracer is also equal to that of the overall reaction and of the rate determining step. In the example of ammonia synthesis on iron catalyst, Ords can be equal to one or two. Experimental studies have reported both values. Horiuti et foyjjcl CTrds = 2 for an iron catalyst at near equilibrium. Tanaka ... 

The rate-determining step of ammonia synthesis and decomposition on Ni powder was studied using tracer isotopes (determination of the stoichiometric number) at 743 K. Dissociative adsorption and desorption were the slowest steps [225]. Dissociative adsorption was studied in few cases. The slight extent of chemisorption of N2 at 523 K to 573 K was speculated to be atomic species on Ni powder [226]. A spectroscopic study of surface nitride on polycrystalline Ni was carried out under a N2 pressure of 200 Torr [227]. 

Stoichiometric coefficients describe the relative numbers of molecules involved in the reaction. In any actual reaction, immense numbers of molecules are involved, but the relative numbers are always related through the stoichiometric coefficients. Further, these coefficients describe both the relative numbers of molecules and the relative numbers of moles involved in the reaction. For example, the Haber reaction always involves immense numbers of molecules, but the equation describing the synthesis of ammonia tells us the following ... 

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