6 Mole Balanoes Chap. 1 What is -r,? To summarize this point, we say that a given number of molecules (e.g., mole) of a particular chemical species have reacted or disappeared when the molecules have lost their chemical identity. The rate at which a given chemical reaction proceeds can be expressed in several ways. To illustrate, consider the reaction of chlorobenzene and chIoraI to produce the insecticide DDT (dichlorodiphenyl-trichlomthane) in the pres- ence of fuming sulfuric acid. Letting the symbol A represent chloral, B be chlorobenzcne, C be DDT, and D be H20 we obtain The numerical value of the rate of disappearance of reactant A, -r,. is a posi- tive number (e.g., -r, = 4 mol Aldm3+s). The rate of reaction, -r,, is the number of moles of A (e.g. . chloral) reacting (disappearing) per unit time per unit volume (mol/dm3.s). r~ = 4 rnol Aldm' F -rs = 8 mol Bldrn' r r, = 4 mol B/dm3 a A+ZB-+C+D The convention The symbol rj is the rate of formation (generation] of species j. If
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