Question

diff. b/w Bond dissociation enthalphy and latice enthalpy?

Answer 1

The term bond energy normally refers to the strength of the covalent bond between two atoms. I would take the question to refer to the homolytic bond dissociation energy to cleave the bond giving two radicals. This is invariably a positive value since bond breaking costs energy and bond formation gives out energy. Where the two atoms are different and therefore have different values of electronegativity there will be an ionic component to that bond strength but the covalent component is a function of the bond order and the efficiency of the atomic orbitals overlapping to form the bonding molecular orbitals. In contrast, the term lattice energy normally refers to electrostatic (coulombic) interactions between cations and anions in an ionic lattice. Again, formation of an ionic lattice is a favourable process and breaking that lattice will cost energy.