Chemical Formulas: Interpreting Single Atom Reactions

[tirrel]

This is a very basic question. When I see a chemical formula, like for example combustion of methane: $CH_4+2O_2->CO_2+2H_20$ and I compute the standard enthalpy of reaction $H_r$, do I have always to think about this value $H$ and about the chemical reaction as describing a "certain amount", big from the microscopic point of view

, of methane reacting with a certain amount of oxygen? Told another way, is it possible to interpret the chemical formula as describing a reaction between a single atom of methane with a singole molecule of oxygen (with no other methane molecules or oxygen present?)
My answer would be no, because we have always to talk about substances in thermodynamic equilibrium but I wanted to ask the opinion of somebody more expert than me.

 

[Quantum Defect]

This is a very basic question. When I see a chemical formula, like for example combustion of methane: $CH_4+2O_2->CO_2+2H_20$ and I compute the standard enthalpy of reaction $H_r$, do I have always to think about this value $H$ and about the chemical reaction as describing a "certain amount", big from the microscopic point of view

, of methane reacting with a certain amount of oxygen? Told another way, is it possible to interpret the chemical formula as describing a reaction between a single atom of methane with a singole molecule of oxygen (with no other methane molecules or oxygen present?)
My answer would be no, because we have always to talk about substances in thermodynamic equilibrium but I wanted to ask the opinion of somebody more expert than me.

You are right. Enthalpies (and other thermodynamic quantitites) are for macroscopic quantities of material. If you talk about the energy for a reaction of a single molecule of methane with two molecules of oxygen producing a molecule of carbon dioxide and 2 molecules of water, you can. Physical chemists do this kind of experiment, but if you want to learn something about the energetics of the reaction you would need to stipulate the energies (internal) of all reactants and products -- i.e. what rotational, vibrational states are we talking about? You would also need to say something about the kinetic energies of the reactants and products. In the case you give, this would probably not be too meaningful, since the reaction is quite complex. Look at Glassman's book, "Combustion" to see the complexity of this simple chemical reaction.