Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Urza on 15/10/2011 00:02:15
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I was just wondering what the first visible reaction to result from heating the atmosphere would be, and what temperature it would likely occur at, assuming STP.
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assuming STP
No heating takes place.
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Good point.
Standard Temperature and Pressure (http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure)is essentially an artificially created set of standards that really only applies to laboratory conditions. There are actually several different definitions, but most commonly 1 ATM (or about 1ATM), and either 0°C or 20°C which could either be cooling or warming, and a pressure change depending on where one is, and what season it is.
Certainly one would not expect something like the atmosphere to start polymerizing with a ½°C change in atmospheric temperature.
If temperatures do increase, there will be some state changes, but then again we are already seeing some of these.
Ice Melting beyond what normally occurs on a seasonal basis. However, seasonal freeze/thaw cycles will continue.
Greater evaporation of water from the oceans, and perhaps also from the continents. And, increased water carrying capacity of the air. You can determine whether that means wetter or dryer conditions, but it certainly will not appear the same as the typical drought as the underlying causes will be different.
If the oceans warm, the partial pressure of CO2 will increase. However, we are currently overwhelming the equilibrium by increasing the atmospheric CO2, so we will continue to drive CO2 into the oceans, rather than having it released from the oceans.
The partial pressure of methane will also increase, just releasing more methane from the oceans. As long as the ocean temperature gradients remain, the clathrates and methyl hydrates will remain intact. However, they may be equilibria driving either accumulation of methane, or slow release of methane.
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Alright, I guess to rephrase the question, suppose a heating element of some sort was exposed to what we would normally consider standard atmospheric conditions at sea level, What would the first chemical reaction to take place? Would it be the formation of ozone, or maybe the formation of some sort of nitrogen-oxygen compound?
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Oh... I think this is a Bored Chemist Question.
I believe that the Nitrogen Oxides (NO, N2O, NO2, etc) are at a higher energy than the constituent parts, so it tends to slowly revert to its constituent parts, N2 & O2 in the atmosphere.
Ahh, found it in Wikipedia. The formation of Nitrogen Oxides is an Endothermic Reaction. (http://en.wikipedia.org/wiki/NOx#Formation_and_reactions). So, increasing the temperature may alter the equilibrium slightly, but NOx is not favored. It forms in small quantities in gasoline engines due to very high temperatures and pressures, beyond what we'll be likely to see in the atmosphere until the sun enters its red giant phase.
As mentioned above, you will likely see changes in partial pressures, especially with gasses including CO2 and Methane dissolved in the oceans. There is also a potential of melting of methyl hydrates both in the oceans and the permafrost. And, of course, methane will slowly oxidize to water and CO2 in the atmosphere.
You may be interested in CO2 Weathering, in which I believe carbonates can replace silicates in rock.
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This is a very interesting question, but its terms are not quite clear. The Earth's atmosphere is a very complicated mixture of gases and particles of condensed matter. It is continuously subjected to radiation from the sun (at least for half the time), and the energy from that radiation drives a lot of chemical reactions in the normal atmosphere.
The composition of most of the major components of the atmosphere is constant for the 99% plus of the atmosphere below 85 km altitude, but typical concentrations of the trace gases vary significantly with height, and, in some cases, with latitude.
The major components of the atmosphere are about 78% nitrogen, 21% oxygen, 1% argon, and variable water vapour/particles. Water vapour levels range from ().5% to 4% at the surface, falling to near zero vapour at or above 12 km. Water particles are mostly visible as clouds of various types, and their distribution is obviously localized and highly variable.
So do we just want to talk about reactions of these four components? (There are none that are thermally driven below 2000°C). Or do we want to include minor gases like CO2 (about 400 parts per million), methane (1.6 ppm), hydrogen (0.5 ppm), carbon monoxide (0.1 ppm) ,,, and just how far do we go?
And what about the solid particles in the atmosphere. Among the first atmospheric reactions to show a noticeable change in a global warming scenario might be
NaCl + H2O [revarrow] NaOH + HCl
And finally what about other parts of the world in contact with the atmosphere? At surface level, the most significant "atmospheric" reactions involve the interface between atmosphere and biosphere. With a record temperature of 45°C in the official weather station, and extremely low humidity, we learnt about that here two summers ago in terrible forest fires -- as has also been happening in many other places.
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Among the first atmospheric reactions to show a noticeable change in a global warming scenario might be
NaCl + H2O [revarrow] NaOH + HCl
[???]
Are you talking about free salt in the atmosphere? As far as I can tell, it is a minor atmospheric constituent once away from the coastlines.
And, NaOH+HCl, is neutral, of course.
The ocean salinity naturally varies somewhat with the major glacial cycles.
The acidification of the oceans is a different reaction based on disolving carbon dioxide into the oceans.
H2O + CO2 [revarrow] H2CO3 [revarrow] H+ + HCO3‾
Keeping in mind that pH is a negative logarithmic scale, so very small changes in the H+ ion concentration can make significant changes in the pH.
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If you make the reasonable assumption that there is water vapour present then thermodynamically, the oxygen and nitrogen should combine to make oxides on nitrogen which would dissolve in the water to make nitric acid.
Fortunately for us this process is very slow.
If you heat the air to a high temperature it happens to some extent. In fact the equilibrium is more favourable at low temperatures- but the reaction is even slower.
I have heard (but never checked) that if you run air through an electric arc (which heats it to a very high temperature) then into a glass vessel you can see the yellow colour of the NO2 formed.
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Among the first atmospheric reactions to show a noticeable change in a global warming scenario might be
NaCl + H2O [revarrow] NaOH + HCl
[???]
Are you talking about free salt in the atmosphere? As far as I can tell, it is a minor atmospheric constituent once away from the coastlines.
And, NaOH+HCl, is neutral, of course.
The ocean salinity naturally varies somewhat with the major glacial cycles.
The acidification of the oceans is a different reaction based on disolving carbon dioxide into the oceans.
H2O + CO2 [revarrow] H2CO3 [revarrow] H+ + HCO3‾
Keeping in mind that pH is a negative logarithmic scale, so very small changes in the H+ ion concentration can make significant changes in the pH.
Yes I am talking about free salt in the atmosphere.
Yes it is a very minor, but very important constituent of the atmosphere. Without free salt in the atmosphere it would be much more difficult for clouds to form.
Yes, NaOH + HCl is neutral. And the equilibrium in that reaction is usually driven well towards the salt plus water rather than the acid plus alkali. But the higher the temperature, the greater the trend for the HCl to be driven off into the gas phase while the NaOH remains firmly in the particulate phase. So as the temperature rises, the gaseous atmosphere contains larger amounts of trace HCl due to this effect, while the halites (salt particles) become more alkaline, and more hygroscopic (absorbent of water).
To further clarify, I was not talking about ocean acidification, which you correctly point out is a totally separate and different process; I was talking about a change in the balance of an atmospheric reaction involving the suspended halite nanoparticles and the trace gas composition of the atmosphere below 12 km. It may well be the only reaction that produces a measurable change in the chemical make-up of the atmosphere in the 0°C to 40°C range.
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If you make the reasonable assumption that there is water vapour present then thermodynamically, the oxygen and nitrogen should combine to make oxides on nitrogen which would dissolve in the water to make nitric acid.
Fortunately for us this process is very slow.
If you heat the air to a high temperature it happens to some extent. In fact the equilibrium is more favourable at low temperatures- but the reaction is even slower.
I have heard (but never checked) that if you run air through an electric arc (which heats it to a very high temperature) then into a glass vessel you can see the yellow colour of the NO2 formed.
I am going to attempt a calculation. Hope I get it right!
Here are some thermodynamic values, taken from standard tables:
Standard free energy of formation:
N2(gas) & O2(gas) = 0 -- These are simple substances that make up the conventional standard state of the element.
H2O(liquid) = –237.2 kJ/mol
HNO3(aqueous) = –110.5 kJ/mol
So, for the proposed reaction N2 + 2 O2 + H2O [revarrow] 2 HNO3 the Gibbs free energy of reaction is positive under standard conditions. It has a value of +16.2 kJ/mol.
This means that the reactants on the left side of this reaction are quite strongly favoured over the products on the right. However, the relatively small positive value means that in an equilibrium mixture, minor but significant amounts of aqueous nitric acid will be present: the equilibrium constant at 25°C is 0.0016. This would result in an equilibrium concentration of 0.058 M of nitric acid in atmospheric water droplets. That would correspond to a pH of 1.7 in atmospheric water droplets -- roughly equal to what we see in the worst "acid rain" incidents.
So we are indeed fortunate, as you say, that
(1) the lightning induced formation of nitric acid is on a small scale, and overall a much slower process than rain-out in the water cycle,
(2) that the direct formation of nitric acid at ordinary temperatures is an immeasurably slow process, and
(3) that the equilibrium is less favourable for nitric acid formation at the higher temperatures where it is actually formed.
However, it is also the case that nitrogen gas plus oxygen gas plus liquid water is thermodynamically favoured over aqueous nitric acid at ordinary temperatures.