A concept related to @R05156@, particularly applicable to the progress in one direction only of component reaction steps in a complex system or to the progress in one direction of reactions in a system at dynamic equilibrium (in which there are no observable concentration changes with time). Chemical flux is a derivative with respect to time, and has the dimensions of @A00297@ per unit volume transformed per unit time. The sum of all the chemical fluxes leading to destruction of B is designated the 'total chemical flux out of B' (symbol \(\sum \varphi_{-\ce{B}}\)); the corresponding formation of B by concurrent elementary reactions is the 'total chemical flux into B or A' (symbol \(\sum \varphi_{\ce{B}}\)). For the mechanism: the total chemical flux into C is caused by the single reaction (1): \[\sum \varphi_{\ce{C}} = \varphi_{1}\] whereas the chemical flux out of C is a sum over all reactions that remove C: \[\sum\varphi_{-\ce{C}} = \varphi_{-1} + \varphi_{2}\] where \(\varphi_{-1}\) is the 'chemical flux out of C into B (and/or A)' and \(\varphi_{2}\) is the 'chemical flux out of C into E'. The @R05156@ of C is then given by: \[\frac{{\rm{d}}[\ce{C}]}{{\rm{d}}t} = \sum\varphi_{\ce{C}} - \sum\varphi_{-\ce{C}}\] In this system \(\varphi_{1}\) (or \(\sum \varphi_{-\ce{A}}\)) can be regarded as the hypothetical rate of decrease in the concentration of A due to the single (unidirectional) reaction (1) proceeding in the assumed absence of all other reactions. For a non-reversible reaction: \[\ce{A ->[1] P}\] \[-\frac{{\rm{d}}[\ce{A}]}{{\rm{d}}t} = \varphi_{1}\] If two substances A and P are in @C01023@: \[\ce{A <=> P}\] then: \[\sum\varphi_{\ce{A}} = \sum\varphi_{-\ce{A}} = \sum\varphi_{\ce{P}} = \sum\varphi_{-\ce{P}}\] and \[-\frac{{\rm{d}}[\ce{A}]}{{\rm{d}}t} = \frac{{\rm{d}}[\ce{P}]}{{\rm{d}}t} = 0\]