Ionisation energy doubt

Question

Ionisation energy of Cl- is 349KJ/mole. Approximate heat liberated when an electron is added to gaseous atom

a)5x10^-20J
b)5.9x10^-19J
c)6.5x10^-20J
d)3.49x10^-26J

Answer 1

Prashant - This is a simple conversion from the energy per mole of Cl atoms gaining an electron, to ONE Cl atom gaining an electron. You use the term "ionization energy", but when we add electrons to neutral gaseous atoms, the energy associated with that change is more commonly referred to as the Electron Affinity (although you are forming an ion in the process).

 

We need to know the following information:

349 kJ = 1 mol Cl atoms undergoing this process, and

1 kJ = 1000 J, and

1 mol Cl atoms = 6.02 x 10²³ atoms (definition of a mole).

 

To add one electron to one gaseous Cl atom we can calculate:

 

1 Cl atom x (1 mol Cl / 6.022 x 10²³ Cl atoms) x (349 kJ / 1 mol Cl) x (1000 J / 1 kJ) =  5.80 x 10^-19 J

This most closely matches answer (b) provided, so they probably have a rounding error

 

That's a pretty small amount of energy, but this should make sense since it's only one atom!  :D