In chemical kinetics, the changing values are the amounts of reactants and the amounts of products. Initially, let's say we start with a mixture of N 2 and H 2 . D t are both negative. This is because the concentrations are decreasing. This will be true of any reactant.
D t is positive, because the concentration is increasing.
This will be true of any product. There is a relationship between the three rates. H 2 is disappearing three times faster than N 2 by the stoichiometry. This is because NH 3 is appearing twice as fast as N 2 is disappearing. The physical state of the reactants plays a role.