We have developed a model to predict the chemical reactivity of carbon nanotubes (CNTs) quantitatively from their initial structures. The parameters, universal for each reaction, of the model can be obtained from a graphene sheet analysis. The chemical reactivity of hydrogenation, hydroxylation, and fluorination were predicted within 0.1-0.3 eV errors, compared with first principle simulation results. The model also predicted the enhanced chemical reactivity of mechanically bent CNTs. The predictions can be applied to the controlled functionalization of CNTs.