Within the framework of isospin-dependent quantum molecular dynamics model,
isotopic distributions in the reactions of Sn-112 + Sn-112 and Sn-124 + Sn-
124 at 40 and 100 MeV/u are investigated. It is found that the isotopic dis
tribution has a significant isospin effect at 40 MeV/u,but the isospin effe
ct disppears at 100 MeV/u. By investigating mechanical and chemical instabi
lities at different energies,we find that the energy dependence of isospin
effects on isotopic distributions results from the competition between mech
anical instabilities and chemical instabilities at different energies.