What is Lattice energy ?
The amount of energy released, when one gm. formula weight of ionic crystal is formed from the requisite number of gaseous cations and anions,is called lattice energy.
OR
It is defined as the amount of energy released when cations and anions are brought from infinity to their respective equilibrium sites
in the crystal lattice to form one mole of ionic compound from the requisite number of gaseous ions.
in the crystal lattice to form one mole of ionic compound from the requisite number of gaseous ions.
Lattice energy denoted by the symbol `U'.
Examples:
Unit: The unit of Lattice energy is KJ /mol .
Lattice energy is directly proportional to the multiple of ionic charge and inversely proportional to the radius of cation and anion.
Born-Lande equition:
The calculation of lattice energy of ionic compounds in direct method is not possible. But the theoretical value of lattice energy can be calculated by Born-Lande equition.
The Born-Lande equition is,
where ,
U= Lattice energy . A= Madelung constant. e = charge of electron
r = distance between cation and anion. n = Born exponent .
N= Avogadro's number.
Bond dissociation energy : Bond dissociation energy is defined as the amount energy required to break a particular covalent bond in a molecule. Which is denoted by D .
Sublimation energy: The amount of heat energy is required to change one mole of a substance from solid state to gaseous state
under standard temperature and pressure is called sublimation energy. Which is denoted by ΔHsub.
Ionisation potential:
It is
defined as the amount of energy is
required to remove the most loosely bounded electron from an isolated gaseous
atom of an element .Which is
denoted by I.P or I.E .
Electron affinity: Electron affinity is defined as the amount of energy released when an electron is added to an isolated gaseous atom of an element. It is denoted by the symbol E.A.
Born-Haber cycle :
The lattice energy of ionic crystal may be calculated by indirect method which is known as Born-Haber cycle. This method is based on Hess's law of thermo chemistry.
The Born-Haber cycle and calculation of lattice energy of few ionic compounds are shown below.
The Born-Haber cycle of sodium chloride crystal ( NaCl ) .
According to Hess's law,
The Born-Haber cycle of calsium chloride crystal , CaCl2 .
According to Hess's law,
The Born-Haber cycle of magnesium oxide ,MgO .
According to Hess's law,
The Born-Haber cycle of Al2O3 .
According to Hess's law,
Significance of lattice energy:
The higher the magnitude of lattice energy, the greater would be the stability of the ionic compound.
The higher the magnitude of lattice energy means larger energy of attraction between the cations and the anions.
Hence, the magnitude of properties such as melting point,boiling point, enthalpy of formation, enthalpy of
sublimation would be high for ionic compounds having high the magnitude of lattice energy.
Q.What is Lattice energy ?
The higher the magnitude of lattice energy means larger energy of attraction between the cations and the anions.
Hence, the magnitude of properties such as melting point,boiling point, enthalpy of formation, enthalpy of
sublimation would be high for ionic compounds having high the magnitude of lattice energy.
Lattice energy table for few ionic compounds .
Practice problems.
Q.What is Lattice energy ?
Q. How Lattice energy depends on ionic radius ?
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