[schmitgreg]

I am having trouble understanding the basic concept of a chemical reaction when you have two very stable elements. I completely understand the paper-pencil part of chemical reactions and what happens, but the reaction physically does not make sense to me. As simple of a rection as:

2H₂ + O₂ => 2H₂O

To my knowledge, the diatomic molecules H₂ and O₂ are quite stable. For instance, if you simply mix them, nothing happens. However, heat will initiate the reaction causing the formation of H₂O.

So then my question is, how does heat physically start the reaction of two stable molecules? Do the electrons have something to do with it? Thanks!

[Mitch]

They will react to form H2O if you mix them! The caveat is that they will react very-very slowly at room temperature, heat will speed that process right up. See suggestion number 2 in my signature

[schmitgreg]

I don;t think you are understanding my question. I completely understand reactions on paper. I understand how to predict outcomes of reactions. I understand that H₂ + O₂ will react to form H₂O.

The only thing that I do not understand is how heat is involved in making the molecules react. For instance:

I understand the concept of an acid, because it lacks electrons and therefore rips electrons out of other materials that do not want the electrons as much. I can physically understand that.

However, I don't understand how the heat works to make the molecules react. Do the electrons on the Oxygen get flung out so wide frrom the heat that it is easy for them to detach from the other Oxygen and reattach to the Hydrogen forming a bond? I am just interested at what physically happens...

Thanks!

[Mitch]

There are two ways to view chemistry. A molecular point of view or by a statistical/thermodynamic approach. I'm assuming you want to know the mechanism of hydrogen oxidation to water. I don't know if it is known, I'll see what I can find.

[schmitgreg]

Yeah, it's cool if I wasn't very clear. I guess a similar question I was asking is this:

If H₂ + O₂ with heat  forms H₂O Then:

Why doesn't water naturally decompose into H₂ and O₂

I guess O₂ and H₂ are just more chemically reactive than water? Anyway, thanks for the information!

[Mitch]

But water does decompose backwards to H2 and O2. Except the reaction is very slow.

[schmitgreg]

ahhhh. maybe i should talk to my chem teacher and try to understand it. i had no clue that water decomposed naturally.

[Yggdrasil]

There are two types of stability, thermodynamic stability and kinetic stability.

Thermodynamic stability has to do with the energy of the products and reactants.  A mixture of H₂ + O₂ will eventually produce H₂O because water has a lower energy (free energy to be specific) than hydrogen and oxygen.  Water is a fairly stable molecule because it has such a low energy, and therefore we call it thermodynamically stable.  In general, all processes will move toward minimizing free energy.

Although all processes go toward minimizing free energy, this principle does not say how fast this minimization takes.  In some cases, thermodynamically unstable compounds will rapidly decompose to products that are more stable.  However, sometimes it takes a very long time for thermodynamically unstable compounds to decompose.  For example, diamond has a higher free energy than graphite, so diamond will tend to convert to graphite over time.  But, why then do people say that "diamonds are forever"?  Because, the conversion from diamond to graphite is extremely slow.  Therefore, in this case, diamond does not stick around because it is thermodynamically stable, but because it is kinetically stable.

How do we explain kinetic stability?  Every chemical reaction has what is known as an activation energy (http://en.wikipedia.org/wiki/Activation_energy or better look it up in your chemistry textbook), the energy needed to overcome a barrier to reaction.  In order for two molecules to react, they must collide with sufficient force to cross over this barrier.  If the molecules don't have enough energy to cross the barrier, they can't react.  The energy barrier for the diamond to graphite change is very large, so basically none of the carbon atoms in diamond have enough energy to cross the barrier and become graphite.  This gives rise to the kinetic stability of diamond.

So what does this have to do with your question?  H₂ and O₂ are kinetically stable at room temperature.  The reaction of hydrogen and oxygen gas is very slow due to a moderately high activation energy.  However, heating the reaction or providing a spark up gives hydrogen and oxygen gas enough activation energy to overcome the energy barrier that prevents them from reacting.  In these cases, you can overcome the kinetic stability of the compounds and allow them to proceed towards a state of lower energy and greater thermodynamic stability.

I hope this clarifies things.

[schmitgreg]

I see. Then does the actual reaction of hydrogen and oxygen (minus the activation energy) release excess energy? I think it does because that energy would fuel another reaction and so on just like the Hindenburg.

[Yggdrasil]

Exactly.