Chemistry help

Why wouldn’t we times 242 as one mole of Cl2 produces 2 moles of 2Cl when atomised?

Reply 1

If Cl2(g) -> 2Cl(g) and the enthalpy change of atomisation is +242, would we not times the value by 2 as one mole of Cl2 produces two moles of Cl for when calculating the lattice enthalpy of the born haber cycle?

Reply 2

TypicalNerd

Original post

by Erd_g

That equation represents the bond enthalpy and not the atomisation enthalpy. The two concepts are related, since the process of atomisation is the formation of 1 mol of gaseous atoms, e.g.:

1/2 Cl2(g) —> Cl(g)

So in the case of chlorine, the atomisation enthalpy is half the bond enthalpy.

Looking at the value you have given, I believe +242 kJ/mol is the bond enthalpy and not the atomisation enthalpy.

In terms of scaling up the value, it depends on which compound you have and whether it’s the bond enthalpy or the atomisation enthalpy you are given.

If it’s for a compound containing two chloride ions per formula unit (such as MgCl2 or CaCl2), then you double the atomisation enthalpy or if you are given the bond enthalpy, you don’t scale it whatsoever.

For a compound containing one chloride ion per formula unit (such as NaCl), you don’t scale the atomisation enthalpy if given it, or if given the bond enthalpy, you halve it.