Last time we saw that the fourth and final step of our alternative route was adding electrons
But…. while we’ve seen how to calculate the enthalpy change of removing electrons using first ionisation energy
We haven’t seen an equivalent measurement for adding electrons…
So, we define the first electron affinity as the enthalpy change when one mole of electrons is added to one mole of gaseous atoms
And we represent it like this [N(g) + e – → N – (g)]
We define the second electron affinity as the enthalpy change when one mole of electrons is added to one mole of gaseous ions with a one minus charge
We represent it like this [N – (g) + e – → N 2- (g) ]
And, we define the third electron affinity as…
As the enthalpy change when one mole of electrons is added to one mole of gaseous ions with a two minus charge
And we represent it like this [N 2- + e – → N 3- (g) ]
Now, just like ionisation energies, chemists also have access to tables of data on electron affinities
And that means we can use them to calculate the enthalpy change of adding electrons
So the enthalpy change of this reaction [Cl(g) + e – → Cl – (g)] is equal to the 1st electron affinity of chlorine
The enthalpy change of this reaction [S(g) + 2e – → S 2- (g)] is equal to the electron affinity of sulfur plus the second electron affinity of sulfur
And the enthalpy change of this reaction is equal to
The enthalpy change of this reaction is equal to the electron affinity of nitrogen plus the second electron affinity of nitrogen, plus the third electron affinity of nitrogen.
So, to sum up…
Chemists calculate the enthalpy change of adding electrons using electron affinities.
The first electron affinity is defined like this.
And we represent it like this.
And the second electron affinity is defined like this.
And we represent it like this. And subsequent electron affinities, like the third fourth fifth sixth and so on, follow a similar pattern.
