Rate of Chemical Reaction

As any reaction proceeds, the concentration of each reactant falls while the concentration of each product rises. The rate of reaction measures how fast this happens: it can be found by measuring the extent to which a reactant's concentration falls, or a product's concentration rises, over a small time interval. Mathematically, Rate $=-\dfrac{d[\text{Reactant}]}{dT}=\dfrac{d[\text{Product}]}{dT}$, where $d[\text{reactant}]$ and $d[\text{product}]$ denote the small changes in concentration occurring during the small time interval $dT$. The negative sign on the reactant term simply reflects that its concentration is falling, so that the rate itself, however it is measured, comes out as a positive quantity. This differential definition of rate is the quantity the law of mass action (Section 12.4.2) relates directly to re …