We report density functional calculations of the surface properties and chemistry of UO₂(111) performed within the generalized gradient approximation corrected with an effective Hubbard parameter (GGA + U within Dudarev's formalism) to account for the strong on-site Coulomb repulsion between U 5f electrons. The variation of the properties of periodic slab models, with collinear ferromagnetic and antiferromagnetic arrangements of the uranium magnetic moments, was investigated while ramping up the effective Hubbard parameter from U_eff = 0 eV, corresponding to standard density functional theory, up to U_eff = 4 eV, the value that correctly reproduces the antiferromagnetic ground state of bulk UO₂. The chemical interactions of molecular water, dissociated water, dissociated oxygen and co-adsorbed molecular water and monatomic oxygen with the UO₂(111) surface were also studied as functions of the U_eff parameter. Calculations reveal that some of the key electronic and chemical properties controlling the surface reactivity are very sensitive to the value of this strong electron correlation parameter.