Question

Define bond enthalpy in a polyatomic molecule. How does bond enthalpy vary with electronegativity of atoms? Also explain the Born Haber cycle with examples.

Answer 1

Born Haber process or Born Haber cycle refers to the analysis of energies in a reaction. It is primarily used in describing the formation of ionic compounds from different elements.

Complete step by step solution:
Bond enthalpy
It is defined as the amount of energy needed to break a mole of bond of a certain type between two atoms in a gaseous state. It is always directly proportional to the strength of bond between the molecules. In case of polyatomic molecules because of its differences in bond enthalpy polyatomic molecules have average bond enthalpy.
As electronegativity atoms increases the bond enthalpy also increases. It can be explained by taking the example of bond strength of hydrogen halide.
The bond enthalpy $\left( {in\,KJ/mol} \right)$ of the $H - X$ bond of 4 hydrogen halides are:
$HF......566$
$HCl......431$
$HBr......366$
$HI......299$
The electronegativity of the halogens decreases in the order $F > Cl > Br > I$ . Also the bond enthalpy of the $H - X$ bond also decreases in the same order from fluorine to iodine.
Born Haber cycle
Born Haber cycle is a method to analyse the energies of a reaction. This process is basically used in order to measure the lattice energy that cannot be calculated directly. The lattice enthalpy is generally defined as the change in enthalpy resulting in the production of an ionic compound from its gaseous ions which is also sometimes defined as the energy needed to break the ionic compound into gaseous ions.
Born Haber cycle represents a cycle of enthalpy change of a reaction that results in the formation of a solid crystalline ionic compound from the elemental atoms in their standard state and the enthalpy of formation of the solid compound such that the net enthalpy turns to be zero.

Note:
The electronegativity of two atoms will be higher and stronger will be the bond. Also, in most of the cases ionic bonds are stronger than the covalent bonds and in case of melting point ionic compounds have the highest melting point whereas covalent bonds have the lowest.