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Also contains definition of: rate of appearance
https://doi.org/10.1351/goldbook.R05156
For the general @C01033@: $a\,\text{A}+b\,\text{B}\rightarrow p\,\text{P}+q\,\text{Q}+...$ occurring under constant-volume conditions, without an appreciable build-up of reaction @I03096@, the rate of reaction $$\nu$$ is defined as: $\nu = -\frac{1}{a} \frac{\mathrm{d}\text{[A]}}{\mathrm{d}t}=- \frac{1}{b} \frac{\mathrm{d}\text{[B]}}{\mathrm{d}t}=\frac{1}{p} \frac{\mathrm{d}\text{[P]}}{\mathrm{d}t}=\frac{1}{q} \frac{\mathrm{d}\text{[Q]}}{\mathrm{d}t}$ where symbols placed inside square brackets denote amount (or @A00297@) concentrations (conventionally expressed in units of $$\text{mol dm}^{-3}$$). The symbols $$R$$ and $$r$$ are also commonly used in place of $$\nu$$. It is recommended that the unit of time should always be the second. In such a case the rate of reaction differs from the rate of increase of concentration of a product P by a constant factor (the reciprocal of its @C01124@ in the @S06021@ equation, $$p$$) and from the rate of decrease of concentration of the reactant A by $$\alpha ^{-1}$$. The quantity: $\overset{\text{.}}{\xi }=\frac{\mathrm{d}\xi }{\mathrm{d}t}$ defined by the equation: $\overset{\text{.}}{\xi } = -\frac{1}{a} \frac{\mathrm{d}n_{\text{A}}}{\mathrm{d}t}=- \frac{1}{b} \frac{\mathrm{d}n_{\text{B}}}{\mathrm{d}t}=\frac{1}{p} \frac{\mathrm{d}n_{\text{P}}}{\mathrm{d}t}=\frac{1}{q} \frac{\mathrm{d}n_{\text{Q}}}{\mathrm{d}t}$ (where $$n_{\text{A}}$$ designates the @A00297@ A, conventionally expressed in units of @M03980@) may be called the '@R05147@' and is appropriate when the use of concentrations is inconvenient, e.g. under conditions of varying volume. In a system of constant volume, the rate of reaction is equal to the @R05147@ per unit volume throughout the reaction. For a @S05970@ this definition of 'rate of reaction' (and '@E02283@', $$\xi$$) will apply only if there is no accumulation of intermediate or formation of side products. It is therefore recommended that the term 'rate of reaction' be used only in cases where it is experimentally established that these conditions apply. More generally, it is recommended that, instead, the terms '@R05148@' or 'rate of consumption' of A (i.e. $$-\frac{\text{d}\text{[A]}}{\text{d}t}$$, the rate of decrease of concentration of A) or 'rate of appearance' of P (i.e. $$\frac{\text{d}\text{[P]}}{\text{d}t}$$, the rate of increase of concentration of product P) be used, depending on the concentration change of the particular @CT01038@ that is actually observed. In some cases reference to the @C01024@ observed may be more appropriate. The symbol $$\nu$$ (without lettered subscript) should be used only for rate of reaction; $$\nu$$ with a lettered subscript (e.g. $$\nu_{\text{A}}$$) refers to a rate of appearance or @R05148@ (e.g. of the chemical species A).