Rate of reaction: The velocity with which the reaction occurs 

Order of reaction: The way in which the concentration of a reactant/drug affects the rate of a reaction

Zero order reaction : A chemical reaction in which the rate of the reaction does not change when the concentration of the reactant grows or increases.

First order reaction : A chemical reaction in which, as the concentration increases the rate of the reaction increases or the elimination rate increases in proportion to the increase in plasma concentration. 

Equation:

K = rate constant 

Integrating and transforming the equation : 

C = C₀.e^-kt
lnc= -kt+lnc₀

-k/2.303 = slope of the line 

C₀ = y intercept