Question

Discuss the relationship of calculating lattice energy and the Born-Haber cycle. Here are some suggestions of topics on which to elaborate upon in your explanations: Provide an explanation of the Born-Haber cycle. Explain the difference between ionization energy and electron affinity. Explain how the enthaply of formation is related to the Born-Haber cycle.

Answer 1

This cycle is known as 'Born Haber cycle'. The energy terms involved in building a crystal lattice such as sodium chloride may be taken in steps the elements in their standard state are converted to gaseous atoms ,then to ions and finally converted into crystal lattice .

Born–Haber cycles are used primarily as a means of calculating lattice energy, which cannot otherwise be measured directly. The lattice enthalpy is the enthalpy change involved in the formation of an ionic compound from gaseous ions (an exothermic process), or sometimes defined as the energy to break the ionic compound into gaseous ions (an endothermic process). A Born–Haber cycle applies Hess's law to calculate the lattice enthalpy by comparing the standard enthalpy change of formation of the ionic compound (from the elements) to the enthalpy required to make gaseous ions from the elements.

This latter calculation is complex. To make gaseous ions from elements it is necessary to atomise the elements (turn each into gaseous atoms) and then to ionise the atoms. If the element is normally a molecule then we first have to consider its bond dissociation enthalpy (see also bond energy). The energy required to remove one or more electrons to make a cation is a sum of successive ionization energies; for example, the energy needed to form Mg²⁺ is the ionization energy required to remove the first electron from Mg, plus the ionization energy required to remove the second electron from Mg⁺. Electron affinity is defined as the amount of energy released when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion.

The Born–Haber cycle applies only to fully ionic solids such as certain alkali halides. Most compounds include covalent and ionic contributions to chemical bonding and to the lattice energy, which is represented by an extended Born-Haber thermodynamic cycle.

The extended Born–Haber cycle can be used to estimate the polarity and the atomic charges of polar compounds.

The electron affinity is the enthalpy associated with the formation of mole of gaseous anions, from one mole of gaseous atoms, and one mole of electrons:

M(g)+e−→M−(g)+Δ

And ionization enthalpy is the enthalpy associated with the formation of one mole of gaseous cations, and one mole of electrons from one mole of gaseous atoms....

M(g)+Δ→M+(g)+e−

Electron affinities reasonably (why) INCREASE across a Period, from left to right as we face the Table, and decrease down a Group

Ionization enthalpies ALSO increase across the Period, and decrease down the Group.