This paper addresses the control of a continuous stirred tank reactor (CSTR) by feedback linearization and a second-order sliding mode (SOSM) construction around an unstable equilibrium point. The problem formulation features the use of the reactor temperature, as opposed to the concentration, as the controlled output. The system in the controllability canonical normal form exhibits stable zero dynamics. It is shown how SOSM can be applied to estimate on-line unmeasured disturbances in the inlet concentration, with which the nonlinear controller fully decouples the output from disturbances achieving asymptotic tracking.