In order to test the reliability of plane-wave and Gaussian-orbital based DFT methods for calculating reaction energies and activation barriers, detailed calculations are performed for several reactions involving gas phase silanes and a simple model of H-2 desorption from the Si(100)2X1 surface, This study is motivated in particular by apparent discrepancies between the results of cluster-model and slab-model calculations of the activation energy for H-2 desorption from the Si(100)2x1 surface. The DFT results obtained with several different exchange-correlation functionals are compared with the results of calculations with the generally reliable QCISD(T) method and, where possible, with experiment. It is found that the functionals usually employed in plane-wave DFT calculations significantly underestimate the activation energies. The Becke3LYP functional, on the other hand, is found to give reaction and activation energies close to experiment and to those from QCISD(T) calculations.