My question is that if we raise the temperature of a substance carrying chemical bond …….then will the bond be made again by itself by cooling down the temperature?
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2$\begingroup$ Happens all the time during melting and resolidification. $\endgroup$– Jon CusterCommented Aug 4, 2023 at 11:29
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3$\begingroup$ It depends on exactly what is heated and then cooled back down. Caramel can be made by appropriately heating a sugar and then cooling it back down to room temperature. More generally, thermal decomposition is a thing. $\endgroup$– Ed VCommented Aug 4, 2023 at 12:39
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1$\begingroup$ @EdV Yes, I think the answers might be assuming that the reactions are taking place (figuratively or literally) in a vacuum... $\endgroup$– MichaelCommented Aug 4, 2023 at 19:57
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$\begingroup$ @Michael Agreed, but “a substance carrying chemical bond” is kind of broad, given that tens of millions of chemical substances are known and the number goes up every day. ;-) $\endgroup$– Ed VCommented Aug 4, 2023 at 20:48
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$\begingroup$ Of course. An example: chemistry.stackexchange.com/questions/175408/… $\endgroup$– Nilay GhoshCommented Aug 6, 2023 at 11:11
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4 Answers
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This is exactly what chemical equilibrium teaches. Take a reaction, for example $$ 2\text{NH}_3 \leftrightharpoons \text{N}_2 + 3\text{H}_2 $$ which is endothermic. Increasing $T$ moves the equilibrium to the right, which means N-H bonds are broken to form N-N and H-H bonds. The opposite happens when temperature is decreased.
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It is certainly possible. If you heat water so that it boils into steam then you are basically supplying enough thermal energy to break the hydrogen bonds that exist between water molecules in the liquid state. If you cool the steam so that it condenses into liquid water again then the hydrogen bonds are re-established.
answered Aug 4, 2023 at 11:31
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The Key Concept is "Chemical thermal reversible reaction" , which is a type of the more general "reversible reaction" category.
[[ Most reactions are irreversible : Burning Cotton/Paper/Petrol/Wax will generate Smoke/Ash/Water/Gases , which can not be cooled to get back those Items , in general ]]
In general terms , "Chemical thermal reversible reactions" are like this , where ⇋ indicates the reversibility :
$$Chemical A + Chemical B + heat\ \ \ \ \unicode{x21CB}\ \ \ \ Chemical C + Chemical D$$
The reaction may occur when Certain Conditions ( Pressure / temperature / Concentration levels / ETC going higher ) are suitable to convert the Original Chemicals.
The reverse reaction may occur when those Conditions are reversed ( Pressure / temperature / Concentration levels / ETC going lower ) to give back the Original Chemicals.
Your Query is about "heat" being the triggering Condition. These are quite common.
Diverse Examples :
[1] Ammonium chloride + heat ⇋ ammonia + hydrogen chloride
[2] $(NH_4)_2SO_4 + heat\ \ \ \ \unicode{x21CB}\ \ \ \ NH_3 + H_2SO_4$ ( at $235^0C$ )
[3] blue hydrated copper(II) sulphate + heat ⇋ white anhydrous copper(II) sulphate + water
[4] If limestone is heated above $900^0C$ , it will decompose into calcium oxide (lime) and carbon dioxide. If the lime (calcium oxide) is cooled, it will recombine with carbon dioxide to give back the limestone.
$CaCO_3\ \ \ \ \unicode{x21CB}\ \ \ \ CaO + CO_2$
Check out this Article , from where I got these Examples , for more Details :
https://docbrown.info/page04/4_74revNH3a.htm
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There are many thermally activated reactions that do not really seem to be reversible.
Metal oxidation or corrosion is the process of formation of an oxide film at the surface of a metal via reactions at the surface. It is known that all metals (except gold) are less stable than their oxides, this is why a metal exposed to an agressive environement (like air or liquid electrolyte) will be oxidized or corroded. This reaction happens faster at higher temperatures. Actually, in mining industries it is almost never the case that a metal can be extracted from earth in its pure form. For example aluminium is extracted as aluminium-oxide and then converted to pure aluminium via application of high voltages (electropolishing). Thus, this reaction can be reversed, but only with high voltages, not with low temperatures.
Hydrocarbon cracking is the process by which long-chain hydrocarbons and other organic molecules are broken down into smaller pieces via cleavage of carbon-carbon bonds. In thermal cracking this breakdown of large hydrocarbons into smaller ones is achieved by heating. Again here I have never seen this reaction being run backwards by cooling.
