Does sunlight cause steel to rust?

brownej wrote: »

hmmm that rule of thumb is a generalisation. The Arrhenius equation is exponential and the rate of reaction is is governed by the activation energy constant (the Ea term). It's not necessarily that applicable to Rust though.

Here is what I was thinking. The activation energy of the reaction is not applicable in the instance of direct sunlight. I was think about things like sun bleaching, sunburn, what happens if you leave a plastic bottle with water in it in direct sunlight for too long (the water tastes strange - as if some of the plastic has dissolved in it).

For an exothermic or endothermic reaction to occur, it requires energy to activate the reaction. This is literally the energy required to break bonds and bang/stir the atoms around - so the reactants can come into range with each other and bind. If you rearrange Arrhenius' equation, what you have is a relationship of T, the temperature, of whatever is in the beaker, to his Ea. But that holds only for a beaker.

With an interface, the surface of a solid like a metal, to a mixture of gases, and sunlight (in particular, the high energy photons of sunlight), you do not think of the reactions in terms of Ea.

I was thinking of rusting in the same way as sun bleaching. High energy photons are hitting the metals surface, also as it's exposed to the air, high velocity atoms of oxygen will be zinging around (they're also propelled by the high energy photons). Also, I would imagine (I'm not sure of this) that due to the photoelectric effect, Iron losing electrons, would give the atoms a higher electronegativity - making the atoms more reactive than if they were in the shade.

There is another reason Ea is inappropriate in terms of rusting. Take a handful of nails and throw them in a pot of water, and boil them. They will not rust right before your eyes. But leave them for days, in still cool water, they will rust. So, I'm thinking the rust is caused by very high energy events, which are rarer (which is why the reaction takes longer). A way I suppose to test the hypothesis, would be to put some nails in two beakers, expose one to UV, and see if there's a difference in the rate of rusting.

Sun bleaching is really interesting too - for similarish reasons.

brownej wrote: »

I would probably have to say no to practically all of that.

You would, wouldn't you.

You are mixing up a whole pile of different reactions here.

No, I'm not mixing up a whole pile of different reactions. All reactions at an atomic level are fundamentally the same.

Chemistry is fundamentally mixing atoms, and unmixing them - and the mechanisms by which any reaction can happen are limited.

Arrhenius does not apply to rusting as temperature is not an accelerating factor in the reaction.

Reactions may not be dependent on temperature, but they are all dependent on motion. Bonds need to be broken and formed through collisions. Without motion, you have no reaction.

In exothermic reactions, when the atoms bond, their momentum increases. When a reaction is endothermic, they slow down. That's actaully a really puzzling thing. At least it puzzles me. It really puzzles me.

Iron exists as iron. It's an element it's pretty simple. It reacts with oxygen in the presence of moisture and voila you get rust.

Viola....and then the magic happens.

Your plastic bottle is made up of much more complicated hydrocarbon molecules. The strong sunlight in that case facilitates the breaking of some of those bonds in the plastic changing the nature of the plastic. The same thing happens with paint in strong sunlight etc. This type of reaction is very different to rusting.

And what I am saying is the reactions in the plastic bottle, are caused by the photoelectric effect. High energy photons. And since rusting happens slowly at low temperatures, I'm thinking, the mechanism is also the photo electric effect. Most of the thermal energy at low temperatures is low energy photons. Think of the thermal energy as the mixing energy - but since nothing much is happening at low temperatures. I'm thinking the mixing energy is in the high energy photons - which they are not many of, so the reaction is slow.

I'm nt sure what your talking about when you mention high energy events while taking about your rust experiments.

I'm talking about the photo electric effect. Take the water in a plastic bottle. At room temperature, the thermal photons will be at low energy(a very small portion will be high energy), they'll be able to jostle the hydrocarbons of the plastic bottle, but they won't break the bonds. Now, put the bottle in direct sunlight - now it's getting hit by lots of ultra violet light (high energy photons). These photons are so energetic, they're able to knock the electrons out of the ball park. This breaks the bonds in the polymer - and chemically changes the atoms without the need for another reagent. You end up with plastic molecules in the water.

If you want to experiment with rusting nails a better experiment might be to place nails in equal containers of water (from the same source) and add different concentrations of electrolytes like salt etc and see how that affects te rust formation. Make sure you have a control container with no water in it. Oh and use non galvanised nails. If your feeling very adventurous connect a battery and see what that does. The experiment could take a while by the way.

And what will the battery do?........

Capt'n Midnight wrote: »

Nope.

Covalent bonds are very different to ionic ones

You can't consider everything at once. For instance, leave aside the nature of the bond (covalent/ionic/other), before bonds are made and after they're split, you can think of them in terms of net electrostatic potential. And when they're forced into interacting, you can think of the bonds as stabilising, or balancing the electrostatic potential.

I'm not sure the whole idea of covalent and ionic bonds that chemists have is entirely correct. I've had a feeling, some of the ideas are the wrong way around.


This is a electrostatic map of benzene. I was looking for one for hydrogen - couldn't find it, but I like this one all the same.



The way it works out, the charge density of atoms and molecules, relative to each other is asymmetric. This allows them to fit together like bits of Lego. The nature of the asymmetry is insanely complicated - but it's there.

and both different to metals which are metallic because the electrons aren't localised and when you get to semiconductors it's complicated.

I know it's complicated. And deeper you go the worse it gets. But the asymmetries, again, make it kind of like Lego.

Only the outer electrons are involved, but apart from gravity all the interactions you have are with electrons.

No, you always have to consider the nuclear charge of the protons. Electrons try to position themselves in an equilibrium state around the charge of the nuclear protons. As it turns out, the positions in these states always leads to an asymmetry. In semiconductors you can create holes - when these are filled with free electrons, they turn the material from a conductor to an insulator.

When electrons change their energy levels the difference is converted to photons. (and visa versa) Einstein got his Nobel prize for partially explaining some of that.

No, I think that was Niels Bohr. Einstein got the Nobel for the photon electric effect (he didn't receive it for relativity).

The quantum jumping with absorption and release of a photon is interesting, it's where the spectral lines come from. Also lasing.

try it

I'm not putting a nail in salt water, with a battery running power through it. Don't tell me, the positive ions are drawn to the nail by the electric current - and the react with the surface iron.

I'd like to do some laser experiments though. I wonder by using high frequency lasers, would it be a more efficient method of doing high pressure chemistry - Do plants use the photoelectric effect in photosynthesis.

Would a CO2 laser be the most efficient method of breaking CO2 bonds.

Could you do the Haber process at a much lower temperature using lasers.

locum-motion wrote: »

Work on it a bit, change your 'feeling' into an actual theory, and - this bit's important - prove it, and you'll win yourself the Nobel prize for Chemistry.
After all, Linus Pauling* got it for figuring out the nature of the covalent bond.

I think there may be people ahead of me on that one. I must take a look at Pauling's book 'The Nature of the Chemical Bond'. His ideas and descriptions may be workable and usable.....but if he makes no mention of symmetry breaking, they're probably incorrect descriptions of what's happening.

New theories. It's not like 1900. Where you could read a few books, have a subscription to Annalen der Physik, peruse new designs for radio gizmos at your day job in the patent office, and be able to take on all the big shots. These days you have professional scientists, don't know what the hell it is the guy next door to him does, and vice versa.

*in the years before he went completely doolally and decided -without any evidence - that massive doses of vitamin C cured everything.

He might be the main krank who popularised vitamin supplements.

In fact, I think he was. If a Nobel prize winner says so, it must be true.