In the previous exercise the concentration-time plots showed that each species in the reaction has a different rate of change in its concentration. The reactants have a negative slope, because they are being consumed in the reaction.
Products have a positive slope, because they are being formed in the reaction.
A. Similarly, the concentration of species C increases three times as fast as the concentration of A decreases. Conceptually there should be a single, unambiguous rate for a reaction. The rate of reaction, r , is defined to be the slope of the concentration-time plot for a species divided by the stoichiometric coefficient of that species. Additionally, if the species is a reactant, the negative value of the slope is used, because the slope is negative and a positive rate is desired. Experiment Part 1.