Standard enthalpy change of ionisation

The standard enthalpy change of ionisation, ΔHiono, is the change in enthalpy for removing one mole of electrons from one mole of atoms or ions in the gaseous state under standard conditions.

When one mole of electrons is removed from atoms to form one mole of monovalent cations, the change in enthalpy is called the first ionisation enthalpy, e.g.

Na(g)\rightarrow Na^+(g)+e^-\; \; \; \; \; \; \; \Delta H_{ion}^{\: o}=502.2\: kJmol^{-1}

When one mole of electrons is removed from monovalent cations to form one mole of divalent cations, the change in enthalpy is called the second ionisation enthalpy, e.g.

Mg^+(g)\rightarrow Mg^{2+}(g)+e^-\; \; \; \; \; \; \; \Delta H_{ion}^{\: o}=1457.2\: kJmol^{-1}

The standard enthalpy change of ionisation of a substance is calculated from the substance’s ionisation energy (the two are not the same as ionisation energy is defined at absolute zero). Ionisation energies are determined experimentally using photoelectron spectroscopy (PES) where the known energy of an incident photon on an atom equals to the atom’s first ionisation energy plus the measured kinetic energy of the ionised electron. These ionisation energies are then converted to ionisation energies at absolute zero before converting to standard enthalpies of ionisation, with both conversions using Kirchhoff’s law.

 

Question

Does the standard enthalpy of ionisation apply to anions?

Answer

Yes, e.g.

Cl^-(g)\rightarrow Cl(g)+e^-\; \; \; \; \; \; \; \Delta H_{ion}^{\: o}=355.2\: kJmol^{-1}\; \; \; \; \; \; 3

 

 

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