We applied density functional theory (DFT) to investigate structural and electronic properties, as well as the reactivity of in-plane heterostructures composed of graphene and hexagonal boron nitride (h-BN). The calculations demonstrate a strong tendency of graphene and h-BN to minimize the number of C−N and C−B bonds and thus to segregate into homogeneous domains. A simple bond model, with parameters obtained from DFT calculations, is used to describe trends in the formation energies of the studied heterostructures. We show that the electronic properties of the BN clusters embedded into graphene qualitatively resemble those of graphene antidot lattices. The calculations also reveal that the h-BN monolayer doped with small graphene clusters is a material with the band gap tunable over an energy range of several electron volts, since the band gap values strongly depend on the size of embedded graphene quantum dots. The reactivity of the graphene/h-BN heterostructures is quantified using H atoms as a probe. We found a strong increase of the H binding energy in the heterostructures, where localized electronic states appear in the vicinity of the Fermi level. The highest value of 2.31 eV, calculated for the ideal zigzag graphene/h-BN interface, is approximately three times larger compared to the H atom binding energy at an infinite graphene sheet.