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If there is a piece of coal in the open air at room temperature, some of the molecules in the surrounding air will have sufficient velocity to oxidize the carbon in the coal. Will the surface of the coal oxidize at the atomic level? If so, how much?

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    $\begingroup$ A huge number solids will form an oxide layer sitting in open air, and it seems pure carbon would be no exception. Even diamond has essentially a mono layer of oxide on the surface. But it would be an extremely thin layer since the formation of the oxide radically reduces further reactivity. $\endgroup$ Commented May 12 at 2:04
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    $\begingroup$ @Matt Hanson The only problem might be that the oxide of carbon is carbon dioxide or monoxide which are both gases. $\endgroup$
    – freecharly
    Commented May 12 at 3:39
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    $\begingroup$ You can form carbonyl groups without fully oxidizing carbon. That’s one of the central functional groups in organic chemistry. $\endgroup$ Commented May 12 at 11:56
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    $\begingroup$ @MattHanson but the oxide is a gas at RTP, so wouldn't it just float away exposing more surface? $\endgroup$
    – OrangeDog
    Commented May 12 at 12:37
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    $\begingroup$ A large pile of coal can self-heat enough, from a room-temperature start, to ignite. Which is a meaningful rate of reaction. But if ignition fails, I don't think it will burn to ash even after a million years. Coal is far from pure carbon. It has a small proportion of more reactive bonds which can oxidize, much like a drying oil. Once these are used up the heat generation will plummet to zero for all practical purposes. $\endgroup$ Commented May 12 at 17:17

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Yes, but very slowly- probably in the range of ~thousandths of an inch of thickness per year. You can imagine this process as being one of corrosion by chemical attack by oxygen. Because the oxidation process follows ordinary chemical rate kinetics, the chemical reaction slows way down at low temperatures but never goes completely to zero.

In cases where the oxidation products are solid, and the diffusion rate of the reactants through the oxide is low, and the oxide is conformal, tightly adherent and without cracks or pores, then the growth of the oxide layer chokes off the reaction and we say the corroding surface is self-passivating. Aluminum, tantalum and chromium all are self-passivating.

Self-passivation does not occur when the reaction products are gaseous, or in the presence of other chemicals which destabilize the oxide.

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Probably not. Atmospheric oxygen at room temperature is a good oxidizer, but probably not good enough to etch carbon by oxidizing it fully to carbon dioxide. It will, however, partially oxidize the surface. Coal is amorphous carbon, which at molecular level has roughly graphite's crystal structure. Picture one sheet of graphite as a fusion of gazillions of benzene rings. The terminal rings that stick out would probably be oxidized by both atmospheric oxygen and water vapor to form carboxyl, carbonyl, hydroxyl, and epoxide groups, like in the picture below. For related subject, read on Wikipedia about mellitic acid, which is the result of oxidizing graphite with nitric acid.

enter image description here

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Yes, coal reacts with the air slowly to produce carbon dioxide. If you have a lump of coal in a room, the reaction happens so slowly that the lump does not noticeably shrink and the gasses produced are diluted away by the air.

The situation changes if you have huge quantities of coal, long timescales, and limited air. In a coal mine, air is kept moving by ventilation fans, or historically by lighting a fire at the bottom of the shaft so convection pulls air out this shaft and in through another shaft nearby. If ventilation stops, the atmosphere in the mine becomes unbreathable after a few days. The mix of gasses is known as blackdamp, and CO2 is the main constituent, often alongside CO and H2O. The rate of CO2 production depends on the temperature and type of coal.

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