So, when a reaction has a rate constant that obeys the Arrhenius equation, a plot of ln k versus T gives a straight line, whose gradient and intercept can be used to determine E a and A. This procedure has become so common in experimental chemical kinetics that practitioners have taken to using it to define the activation energy for a reaction. That is, the activation energy is defined to be (− R) times the slope of a plot of ln k vs. (1/ T): E a ≡ ≡ − − R [ ∂ ∂ ln ⁡ ⁡ k ∂ ∂ ( 1 / T ) ] P .