We use molecular-dynamics simulations and importance sampling to obtain transition-state theory rate constants for thermal desorption of an n-alkane series from Pt(111). These simulations indicate that the binding of a large molecule to a solid surface is a complex phenomenon involving many local minima. The existence of local minima invalidates the interpretation of thermal desorption as a single, first-order rate process. Because the number of local minima increases with increasing chain length, n-alkane adsorption becomes increasingly dominated by entropy for longer chains. Consequently, the binding energy increases in a less-than-linear way with increasing chain length. Our results provide a quantitative explanation for results and trends in recent experimental studies.