At very high temperatures and pressures (such as carbon dissolved in liquid iron at pressure) many of the bonds in any form of carbon are already broken and the two forms of carbon can interconvert (this is roughly how diamond is formed in nature and laboratories). This is because the mechanism to convert diamond t graphite would need to break a very large number of chemical bonds in diamond (every carbon connected to other carbons in an infinite tetrahedral grid) to get graphite (every carbon connected to 3 other carbons in an infinite hexagonal array using delocalised multiple bonds). Graphite is more stable under normal conditions but the conversion is not seen. But this is unfair on the variety of chemical behaviours.Īnother example where you might expect reversibility is the conversion of diamond to graphite. In many cases there is no such mechanism which makes many chemical changes impossible to reverse, which is why a simplistic view might distinguish physical from chemical change. The key to reversible reactions is that there is some mechanism that can undo the new bonds created in a chemical change. The whole point of this is a recognition that the underlying reaction is reversible and needs to be pushed in one direction. So they have a cool piece of equipment called a that removes water from the reaction to ensure they get the product they want. But to push the reaction to produce the ester, chemists need to remove the water or the reverse reaction just undoes the new bonds. Esterification, the reaction of an acid with an alcohol, makes new molecules and water, for example. Many chemical transformations can be reversed easily given the right conditions. It seems to be pitched at describing the difference between simple examples such as the melting of water (physical, easily reversible by putting the water in a freezer) and the simple reaction between chlorine and hydrogen to give common salt (chemical, not spontaneously reversible or dinner tables would be far more entertaining).īut the variety of possible chemical reactions is far bigger than that. The statement you challenge is a gross oversimplification and generalisation that fails to capture much of the subtlety of chemical reactions. "Molecular orbital analysis of the hydrogen bonded water dimer". Bo Wang, Wanrun Jiang, Xin Dai, Yang Gao, Zhigang Wang, and Rui-Qin Zhangb (2016).Chemical reactions may be essentially irreversible, and (as I look at the scar permanently left on my arm after a long-ago surgery) so can physical changes. Physical changes can be reversible, and so can chemical reactions. So there is no clear boundary between physical and chemical changes. The molecular orbital description of hydrogen bonds identifies this hydrogen bonding as a form of (delocalized) covalent bonding - weaker bonds than those rearranged when carbon and carbon dioxide are heated in a furnace, but again there is only a difference in degree and not in the fundamental type of process. But the same holds true with melting ice, which involves the breaking of hydrogen bonds. True, the bonds in carbon monoxide are arranged quite differently from those in the carbon and carbon dioxide from which it can be made. One might suppose that we could differentiate the above processes hecause the reaction of carbon with carbon dioxide surely involves the breaking and making of bonds. Moreover, and more to the point, both the "physical change" of melting ice and the "chemical reaction" of carbon with carbon dioxide can be reversed upon cooling in the latter case it can lead to a real industrial problem of furnace parts being covered with soot in the cooler parts of the furnace. True, carbon requires a higher temperature to react at an appreciable rate with carbon dioxide than ice requires to melt, but this amounts only to a difference in degree rather than a difference in kind of process. $\text$įirst off, both processes are advanced by heating the material. For the latter, we choose the reaction of elemental carbon at high temperature with carbon dioxide gas, forming carbon monoxide: Let us compare the melting of ice with something that is obviously a chemical reaction. Essentially anything that involves rearrangement at an atomic or molecular level, even something as seemingly straightforward as melting ice or dissolving sugar into water, has characteristics of a chemical reaction. I am actually pretty old to this subject and still I cannot really answer, because the boundary between physical changes and chemical reactions is indistinct.
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