STOICHIOMETRY OF REACTION SCHEMES

Stoichiometry is a compound of the Greek word crrocx co, for element, with fierp-uj for count. The combination denotes accounting of the elements. More particularly here, it denotes the balancing of chemical reactions with respect to each conserved entity therein. Thus, the reaction scheme 

Suppose that a set of chemical reactions occurs at total molar rates 7 1,tot NR,tot in a given macroscopic system. (The molar rate TZj,tot is the frequency of the molecular event j in the chosen system minus that of the reverse event, divided by Avogadro s number N — 6.023 x 10 ). Then the net rate of production of species i in moles per second in the system is 

For the three-reaction scheme shown above, the resulting species production 

The two forms of rates introduced in Eq. (2.1-2) should be carefully distinguished. The net molar rate of production of chemical species i is designated by a subscripted Roman letter R, whereas the net frequency of reaction event j is designated by a subscripted calligraphic letter TZj. Some practice in writing R and TZ is advised to express each rate type unambiguously. 

It is important to distinguish homogeneous reactions (those that occur within a phase) from heterogeneous reactions (those that occur on interfaces). The presence or absence of heterogeneous reactions in a system is commonly investigated by experiments with various reactor surface/volume ratios or with various amounts or compositions of a dispersed phase. Labeling surface quantities with superscript s, one obtains the net rate of production of species i [mole/s] in a system with interfacial surface S and volume V as 

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