Endothermic, energy needs to be absorbed to remove the electron. Ionization energy of sodium (∆HIE° (Na): Removal of one mole of electron from a gaseous metal atom involve some energy change, known as ionization energy. It’s an endothermic step as the bonds between Cl-Cl atoms are being broken. Standard enthalpy of atomization of chlorine (∆Ha°): The energy required to atomize one mole of gaseous chlorine molecules into one mole of gaseous chlorine atoms is known as standard enthalpy of atomization of chlorine. This is known as standard enthalpy of formation and represented by ∆fH°. The overall process can be explained in following steps.įormation of sodium chloride: The formation of one mole of sodium chloride from its elements under standard conditions releases 411 kj/mol of heat. The net enthalpy change for the formation of NaCl ( ΔfH° ) is 411.2 kj/mol. ΔHlatt = -796 -178-242-(590+1145) –(2 x -364) ΔHlatt = - 796 - 1427 = - 2223 kJ/molīorn Haber Cycle NaCl The energy change during the formation of sodium chloride crystal from metallic sodium and chlorine gas can be used to calculate lattice enthalpy of NaCl(s). By applying the Hess’s Law, Heat of formation (Δ H°f) = Heat of atomisation (Δ H°at)+ Dissociation energy(Δ Hd°)+ (sum of Ionization energies)+ (sum of Electron affinities)+ Lattice energy Or can be rearranging as Lattice energy= Heat of formation- Heat of atomization- Dissociation energy- (sum of Ionization energies)- (sum of Electron Affinities) *Dissociation energy refers to the energy used to convert Cl 2 to Cl, and the dissociation energy is doubled as two Cl is required to form calcium chloride. The lattice energy of calcium chloride can be calculated by using the Born Haber cycle in which the sum of enthalpy in a cycle is zero. Lattice enthalpy: The combination of one Ca2+ ion and two chloride ions (Cl-) to form one mole of calcium chloride release lattice energy (ΔHlatt ). Stable state has less energy compared to other state, thus energy is released. Note that chlorine atom (2.8.7) gains electron to its valance shell to achieve a stableĮlectron arrangement.
½ Cl2 (g) → Cl (g)įor the formation of calcium chloride, two Cl (g) is required, therefore total atomisation enthalpy will be double that is 242 kJ/mol.Įlectron affinity of chlorine: This is the amount of energy released during the addition of electron in an isolated neutral gaseous chlorine atom. The reaction enthalpy is half of the bond dissociation enthalpy of chlorine. Ca (g) → Ca+(g) + e+Ītomization enthalpy of Chlorine: This step involves dissociation of Cl2 (g) into Cl(g) atoms. Note that calcium atom (2.8.8.2) loses electron to achieve a stable electron arrangement and stable state has less energy when compared to other state, thus energy is released as calcium atom loses electrons to form gaseous -2 ion. Hence the second ionization energy for calcium ion is 1145 kJ/mol. The removal of second electron requires more energy as it is difficult to remove electron from a cation.
Ionisation enthalpy of calcium: Calcium forms di-positive ion (Ca2+), therefore the energy involve in the removal of first electron is called as first ionization energy and valued 590 kJ/mol. Note that solid calcium absorbs heat energy to form gaseous calcium, thus the products have higher energy than the reactants, this is an exothermic reaction. Ītomization enthalpy of calcium: This step involves the conversion of solid calcium to gaseous state. Born Haber Cycle, Example:CaCl2 The Born Haber cycle for the formation of calcium chloride form its constituent elements involves following steps.