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