MadSci Network: Physics

Re: Volatility of water at high temperatures

Date: Sun Jan 5 13:25:50 2003
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Physics
ID: 1039712116.Ph


The interactions of matter and energy have been intensely studied over the past 2 centuries and you have picked an interesting and difficult area. I will try to outline some of the pertinent physical laws and can suggest your local college library for books on the subject.

Substances heated above certain temperatures or subjected to fast, large, energy input decompose or react by a variety of chemical mechanisms or pathways. Such reactions, unless carried out under strictly controlled conditions, are usually not reversible; what you get after the reaction can be very different from what was there before. Nothing can be heated to Infinity, so to speak. Besides the need for unlimited energy, at higher and higher energy inputs, new reactions always seem to occur.

The following terms are described in any good Physical Chemistry book and most can be found by a diligent search on the Internet.

As a material is heated the energy is distributed among the modes that are available to it according to the Law of Equipartition of Energy.

The Maxwell-Boltzmann Distribution Function describes the population of molecules in the various states.

A material that attains temperature equilibrium in itself has a distribution of energy levels and a black body temperature with an emission of energy according to the Planck Radiation Law. erols

Most substances at room temperatures have a wide range of molecular kinetic energies, a relatively wide range of rotational energy levels [quantized], a small range of vibrational levels [quantized], and no appreciable higher electronic energy levels [quantized]. As the temperature is raised the average molecular velocities increase and the populations of the rotational, vibrational, and electronic levels move to higher quantum levels. Again the Boltzmann distribution describes these populations. The closer the energy levels lie the more easily they are affected by heat input. A material absorbs a photon when the energy of the photon corresponds to the energy difference between 2 appropriate energy levels in the substance. Absorption of discrete energy or rapid input of a large amount of energy results in a non-equilibrium situation and unusual things can happen if the excited states have sufficient lifetime. As the temperature of material increases, or energy is absorbed by a non-equilibrium method, the ability of chemical reactions to occur increases and the rates of possible chemical reactions increase.

At a given temperature a chemical reaction at equilibrium is described by the mass action law and defined quantitatively by the Equilibrium Constant.

Studies of reactions that are reversible and can attain equilibrium enable calculation of the equilibrium constant. The equilibrium constant can determine how far a particular system is from equilibrium, but usually gives little information about how fast a particular reaction will reach equilibrium unless there is additional information about the kinetics or mechanism of the reaction.

The reaction H2 + Ĺ O2 = H2O is described in detail in the following Mad Scientist answer ID: 981054892.Ph. The data suggest that the reaction is reversible with significant amounts of H2 and O2 present near 2500K. The equilibrium constant can also be calculated from electrode reactions at room temperature. Rough calculations from the calculated equilibrium constant and heats of reaction indicate that the pressures of water, H2, and O2 would be 1 atmosphere near 5000K[approximate temp of the sunís surface]. Somewhere between 2500 and 5000K the equilibrium begins to have no meaning because water, hydrogen and oxygen molecules break down into atoms and ions.

Information on lightning can be found here:

The people here are actively studying atmospheric processes and events:

Lightning is an important factor in nitrogen fixation in the nitrogen cycle

An electrical discharge between two areas of separated charge causes Lightning. The potentials are in the thousands to millions of volts, the current flow is high and the time frame is probably short. The temperatures in the bolt itself are in the tens of thousands of degrees and are not equilibrated. That means any material in the bolt is in the plasma state, highly ionized, and far from chemical and thermal equilibrium. The bolt has expanded rapidly and must collapse when the electrical potential is reduced. Then the various high energy chemical species probably combine somewhat statistically to give molecules in high energy states that undergo further reaction. There seems to be limited readily available information on the chemical reactions that happen. I can surmise that most of the possible compounds of H, O, and N are formed to some extent; most return to N2, O2, and H2O because of their stability. O3 and NO are formed. Any trace elements, especially carbon, present are probably incorporated into a myriad of compounds. There seems to be controversy brewing over the chemical and physical properties of plasmas and discharges.

It must be a topic of interesting and difficult research.

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