Chemistry · Thermodynamics · NEET
They measure the same event: pulling one electron off a gaseous atom, M(g) -> M+(g) + e-. But they are defined at different temperatures. Ionization energy (the older term, symbol E0) is defined at absolute zero, 0 K. Ionization enthalpy (the modern term, symbol delta_i H) is defined at the working temperature T. Because heat capacities of reactant and products differ, the enthalpy value is a little larger. So the numbers are close but not identical.
NCERT explains this in the Thermodynamics unit. The words 'energy' and 'affinity' were the old habit. Modern chemistry defines these quantities as reaction enthalpies (delta_r H), because real reactions happen at some temperature T, not at absolute zero. So 'ionization enthalpy' and 'electron gain enthalpy' are the correct thermodynamic names. For NEET, treat them as the updated names for the same properties.
For the ionization reaction M(g) -> M+(g) + e-(g), one gas particle becomes two gas particles. Each ideal gas species has Cp = (5/2)R. So delta_r Cp = +(5/2)R (products minus reactant, an extra particle appears). Using delta_r H(T) = delta_r H(0) + integral of delta_r Cp dT, you get: ionization enthalpy = ionization energy (E0) + (5/2)RT. The extra gas particle is why enthalpy is a bit higher than energy.
Both describe adding an electron to a gaseous atom, M(g) + e- -> M-(g). Electron affinity (A) is the old term at 0 K. Electron gain enthalpy (delta_eg H) is the modern term at temperature T. Here two gas particles become one, so delta_r Cp = -(5/2)R, and the relation flips sign: electron gain enthalpy = -A - (5/2)RT. Notice the minus signs come from the sign convention, explained below.
Electron affinity (A) is usually defined as a positive number = the energy released when an atom gains an electron. But electron gain enthalpy (delta_eg H) uses the thermodynamic sign convention: energy released means delta_eg H is negative (exothermic). That is why NCERT writes delta_eg H = -A - (5/2)RT. Same physical process, opposite sign because one counts 'energy released' and the other counts 'enthalpy change'.
Rarely. The (5/2)RT correction is small (about 6 kJ/mol at 298 K) and NEET usually just asks you to know that ionization enthalpy and electron gain enthalpy are the modern thermodynamic names, and to use them inside a Born-Haber cycle. Know the relation exists and its sign; deep integral calculations are not asked.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Use ionization enthalpy (delta_i H). It is the current NCERT and IUPAC term. But if a question uses 'ionization energy', treat it as the same property for answering.
Ionization enthalpy = ionization energy (E0) + (5/2)RT, and electron gain enthalpy = -(electron affinity A) - (5/2)RT.
Each species is treated as an ideal monatomic gas, for which Cv = (3/2)R and Cp = Cv + R = (5/2)R. The change in number of gas particles gives the +(5/2)R or -(5/2)R for delta_r Cp.
Inside the Born-Haber cycle for NaCl. Steps use sublimation enthalpy, ionization enthalpy of Na (+496 kJ/mol), bond dissociation of Cl2, and electron gain enthalpy of Cl (-348.6 kJ/mol) to find lattice enthalpy.
No. (5/2)RT at 298 K is about 6.2 kJ/mol, small compared to ionization enthalpies of hundreds of kJ/mol. That is why the terms are often used loosely, but the correct thermodynamic name is 'enthalpy'.
