sophiecentaur said:
If you breathe Oxygen at too high partial pressure, it is toxic. It's highly poisonous to some life forms - anaerobic bacteria, for example - so it must be potentially bad for us. That's a crazy thought.
The interesting thing, from my perspective, is why oxygen isn't
more reactive. Why don't we all spontaneously burst into flames and burn up? Chemically, that means there must be a high energetic barrier to trigger that reaction. But why? The reaction we learn in school for burning carbon seems simple enough:
C + O
2 --> CO
2
The thing is, that reaction doesn't happen. Not in a single step like that, it's impossible! Because it's not a balanced equation. Chemically balanced, yes, But there's a subtlety here: The angular momentum isn't the same on both sides of the arrow, and angular momentum is conserved!
See, oxygen is a triplet biradical. Biradical meaning it has two unpaired electrons. A triplet means these electrons have the same spin angular momentum (which isn't
entirely like angular momentum, but for the sake of this discussion that can be ignored). There are two possible values for it, up or down. And oxygen has two "up" (or "down", doesn't matter) electrons without corresponding electrons with the opposite spin. This is because of the Pauli principle, which says you can't have two electrons with the same spin in the same state. So by having the same spins, the two electrons are more spatially spread-out than they'd have been otherwise, and have lower energy.
But carbon, be it diamond or graphite, has all of its electron spins paired up. So does CO
2 and most molecules, in fact. Which means that oxygen
can't do the reaction above, because it means one of the spins has to change orientation, which would violate conservation of angular momentum. The only way it can change the spin orientations is by breaking apart the molecule and forming radicals (which have a single unpaired electron, which is then more or less free to flip its spin).
So basically, you have to do the reaction O
2 --> 2O. You have to break the oxygen-oxygen bond into free oxygen atoms, before the oxygen atoms can then react with carbon. So there's a very high activation energy for that reaction.
On the other hand you have singlet oxygen - which is oxygen where the two electrons are paired. It has higher energy. About 22 kcal/mol higher, which is quite a bit, about as much as a C-C single bond. Due to conservation of angular momentum, it can't immediately lose that energy and become ground-state triplet oxygen. So it's a fairly stable thing, despite having lots of energy and being
very reactive. Because unlike ordinary triplet oxygen, it can react directly with ordinary molecules that have all their electrons paired up. It's a dangerous and harmful form of oxygen, and it's sometimes produced in our bodies. In those contexts it (together with superoxo and peroxo oxygen) is called "Reactive Oxygen Species" (ROS). So we've evolved special ROS-scavenging molecules, often with transition-metal atoms (which often have unpaired spins), which go around and find singlet oxygen molecules and flip their spins back so they don't harm us. It's pretty neat.
