Last time we looked at enthalpy of atomisation
…Which is defined as the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state.
And we’ve also looked at enthalpy of bond dissociation
…Which we’ve seen before is defined as the enthalpy change when one mole of a bond is broken to give separated atoms, which everything in the gas state.
And, sometimes these two enthalpy changes can be related
For example, the enthalpy of atomisation of chlorine is equal to half the bond dissociation enthalpy of a Cl-Cl bond
And that’s because, “to atomise chlorine gas such that we have 1 mol of chlorine atoms, we break.. ½ a mole of Cl-Cl bonds, form separated atoms, and everything is in the gas state
Similarly, the enthalpy of atomisation of oxygen is equal to half the bond dissociation enthalpy of an oxygen-oxygen double bond bond
And that’s because when we atomise oxygen, we break ½ a mole of oxygen-oxygen double bonds, form separated atoms, and everything is in the gas state
And lastly, by the same logic, the enthalpy of atomisation of fluorine is equal to…
The enthalpy of atomisation of fluorine is equal to half the bond dissociation enthalpy of a F-F bond
And that’s because when we atomise fluorine, we break ½ a mole of F-F bonds, forming separated atoms, and everything is in the gas state
So for all three of these elements we can relate their enthalpy of atomisation to their bond dissociation enthalpy
But be warned, this doesn’t work for every element.
For example, it doesn’t work for bromine..
And that’s because, even though we’re still breaking half a mole of br-br bonds
And even though we’re still forming separated atoms
Bromine isn’t a gas in its standard state, it’s a liquid, so this relationship doesn’t hold up
Nor does it work for sodium
Because sodium’s a metal in its standard state, and we can only calculate bond dissociation enthalpies for covalent bonds.
In fact, we can only relate bond dissociation enthalpy to enthalpy of atomisation if the element is gaseous and covalently bonded in its standard state.
So, for which of these elements can we relate enthalpy of atomisation and bond dissociation enthalpy?
These elements are gaseous and covalently bonded in their standards states, so we can relate their enthalpy of atomisation to their bond dissociation enthalpies
But these elements aren’t, so we can’t.
So, we’ve now fully covered the second step of our alternative route: atomisation
And next, we’ll look at the third step: removing electrons
But first, to sum up…
We can sometimes relate an element’s enthalpy of atomisation to its bond dissociation enthalpy, but only when
But only when it’s gaseous and covalently bonded in its standard state.
