I wanted to ask for examples of spontaneous chemical reactions meaning that their enthalpy would be negative (exothermic) and their variation of entropy would increase, therefore the Gibbs energy would be also negative and the reactions would be spontaneous at all temperatures (?).

I was thinking about the boiling of water as the reaction feels hot and the entropy change is positive as it goes from liquid to gas (entropy increases) but then I thought that to do that, we need an external energy and so the reaction would be non-spontaneous?

Another one could be dropping mentos into a carbonated drink. I can be sure that the reaction doesn't requiere an external force to happen BUT the reaction itself doesn't feel hot (I think I saw it once when they did it in primary school but I'm assuming it isn't releasing any heat?)

I know that when enthalpy is negative and entropy is negative, it would only be spontaneous if the temperature was low enough.

For the non spontaneous reactions, I can't really think about one that has positive enthalpy and negative entropy. I CAN think of a reaction that needs a push to "occur" like taking off a stain of a shirt or even a reaction of decomposition of water into h2 and o2.

I think I'm getting something wrong.


I was thinking about the boiling of water

That is a good example of a process that goes forward at higher temperature (water boils when heated above the normal boiling point) and goes in the opposite direction at lower temperature (steam condenses below the normal boiling point). This process also depends on the pressure, but my statements are valid at atmospheric pressure.

For this process, the entropy change is positive (water has more freedom to move in the gas phase compared to the liquid phase) and the enthalpy change is positive (endothermic, it takes energy to break the hydrogen bonds between water molecules).

Reactions that are always spontaneous

How about combustion of cyclohexane? The entropy change is positive (lots of gas molecules are formed - six carbon dioxide molecules per hexane) and the enthalpy change is negative (exothermic).

Reactions that are always non-spontaneous

Once you are certain that a reaction is always spontaneous, it is easy to find one that is always non-spontaneous - just swap reactants and products. So according to this argument, cyclohexane and oxygen will never form from carbon dioxide and water spontaneously. That sounds reasonable.

  • $\begingroup$ +1 for realizing VLE is just a type of reaction! $\endgroup$
    – B. Kelly
    Mar 4 '19 at 23:01
  • $\begingroup$ VLE: vapor liquid equilibrium... I had to look it up $\endgroup$ Mar 4 '19 at 23:46
  • 1
    $\begingroup$ Well, if we were all engineers, the world would be a worse place. $\endgroup$
    – B. Kelly
    Mar 4 '19 at 23:54

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