Up Learn – A Level Chemistry (aqa) – Constructing Born-Haber Cycles
Exothermic and Endothermic Electron Affinity
First electron affinities are exothermic. Second electron affinities and higher are endothermic.
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Constructing Born-Haber Cycles
Last time we saw that classifying electron affinities as either exothermic or endothermic is more complex than the other enthalpy changes in born haber cycles.
For example, if we look at electron affinity data for oxygen, we can see that the 1st electron affinity of oxygen is exothermic [EA1(O) = -142 kJ mol-1]
But the 2nd is endothermic [EA2(O) = +844 kJ mol-1]
And so’s the 3rd, 4th, 5th, 6th and so on… [EA3(O) = +…, EA4(O) = +… etc – no need to include numbers but just make the plusses get larger and larger]
Similarly… the 1st electron affinity of phosphorus is exothermic [EA1(P) = -72 kJ mol-1]
But the second is endothermic [EA2(P) = +468 kJ mol-1]
And so’s the rest! [EA3(P) = +886 kJ mol-1 EA4(O) = +… etc – no need to include numbers but `just make the plusses get larger and larger]
And lastly, the 1st electron affinity of chlorine is exothermic [EA1(Cl) = -349 kJ mol-1 ]
But the rest are endothermic. [EA2(Cl) = +… kJ mol-1 EA3(O) = +…, EA4(O) = +… etc – no need to include numbers but just make the plusses get larger and larger ]
So, based on this data it seems like there are some clear-cut rules for deciding whether an electron affinity is exothermic or endothermic
It seems like all first electron affinities are exothermic, but every other electron affinity is endothermic.
And, as it happens, those rules are actually true.
All the 1st electron affinities you’ll see at A-Level are exothermic
And all second and subsequent electron affinities are endothermic.
So, with that in mind, which of these electron affinities are exothermic?
These electron affinities are exothermic, but these are endothermic.
So, now that we’ve seen the rules, we next need to understand why they’re true.
Now remember, the first electron affinity involves adding one electron into each atom.
So, to understand why first electron affinities are exothermic, we need to think about the forces that act on the electron that’s added in
Now, whenever an electron approaches an atom it experiences two forces:
An attraction to the protons in atom’s nucleus
And a repulsion from the atom’s electrons
Now, since the number of protons and electrons are always equal in atoms, it’s reasonable to expect the attraction and repulsion to cancel each other out.
But that’s not actually what happens.
In fact, for reasons we can’t really get into, the attraction actually outweighs the repulsion.
In other words, the overall force between the external electron and atom is attractive.
So, because the electron is attracted towards the atom, if we wanted to move it away again, we’d need to put in some energy.
And, conversely, it also means as the electron gets closer to the nucleus energy is given out
In other words, adding electrons to atoms is exothermic.
On the other hand, for second or subsequent electron affinities, electrons are no longer added into atoms
They’re added into negatively charged ions, and this changes everything.
Negatively charged ions have more electrons than protons
And that means the repulsion from the ion’s electrons now outweighs the attraction towards the ions nucleus
In other words, the overall force between the external electron and the anion is repulsive.
So, since the overall force is repulsive, this means energy is required to add that electron in.
In other words, adding electrons to negatively charged ions is endothermic.
So, to sum up…
There are two rules we’ll use to decide whether an electron affinity is exothermic or endothermic
1st Electron Affinities are exothermic
And all 2nd and higher electron affinities are endothermic
And that’s because the overall force between an external electron and an atom is attractive
But the overall force between an external electron and an anion is repulsive.