The present work deals with the sensitivity of aero-heating predictions to variations in chemical reaction rates in high enthalpy flows over re-entry capsules. With in this scope, numerical simulations are performed by varying the reaction rate constants over their uncertainty range. FIRE II re-entry capsule at 35 km altitude is chosen as the test case. At these conditions, recombination of oxygen atoms is the dominant chemical reaction in the thermal boundary layer next to the wall. The flow solutions computed by altering the reaction rates in this region are carefully analyzed to understand the physical effects that influence the surface heat transfer rate. It is found that the chemical state of the gas i.e. equilibrium, non-equilibrium or frozen, and the extent of recombination reactions in the boundary layer are the critical factors that determine the sensitivity of the heating rate to variations in the reaction rate constants. The analysis is carried out at different points on the vehicle surface, and the heating rate sensitivity is found to be the highest at the shoulder. This is due to the non-equilibrium chemical state of the gas prevailing in the shoulder expansion region. By comparison, the equilibrium state of the gas on the forebody and the frozen chemistry on the afterbody result in lower sensitivity of the heating rate to variations in the chemical reaction rates.