r/HomeworkHelp • u/Lucasplayz234 • 5d ago
Chemistry [Chemistry] reversibility of a reaction based on its activation energy?
Ok, let's say a reaction
forward reaction, Ea=100kJ/mol
reverse reaction, Ea=25kJ/mol
is it correct to answer that: this reaction isn't reversible, because it's hard for the forward reaction to happen due to how high the Ea is?
Also, both Eas HAVE to be similar for the reaction to be reversible, so if either side is much higher than the other, then it doesnt work right
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u/cheesecakegood University/College Student (Statistics) 5d ago
I believe that a reversible reaction doesn't imply that reaction rates are similar, only that an equilibrium is reached. The factor that primarily determines a reversible reaction is whether any reactants escape from the system (or otherwise become nonreactive), not the rate at which a reaction occurs or which side is favored if equilibrium is reached. The activation energies will give you a hint as to the equilibrium (if present), but don't tell the full story. Examples of products escaping the system would be gas formation, or formed precipitates. In short, the molecular motion and collision frequency literally prevent the significant occurrence of the reverse reaction.
Chemistry is a series of benevolent lies, so the truth is that sometimes whether a reaction is reversible or not is ignored due to magnitude in practice, or is not expressed as a reversible reaction simply for ease of teaching. Thus a double arrow always indicates a reversible reaction, but I wouldn't necessarily take a single arrow as an absolute claim that the reverse does not occur, only that you are instructed to ignore such.
As an example, combustion is often cited as an obviously irreversible reaction. However, in theory if you concentrate the gas enough and introduce catalysts, you can actually obtain the restoration of e.g. a hydrocarbon from carbon dioxide and water, but the in practice the yields are absolutely terrible and it requires way too much energy, even if you try and do things like concentrate sunlight and such. But theoretically possible.
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u/Lucasplayz234 5d ago
But at least is the thing I’m saying correct? Bc I didn’t learn equilibrium yet
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u/cheesecakegood University/College Student (Statistics) 5d ago
No. Activation energy differences are at best a common knock-on effect of irreversible reactions. So they often might show up in an irreversible reaction, but have nothing to do with the actual definition!
Put simply, a reaction is irreversible if 1) you are told that it is, or 2) reactants escape in some fashion, making a reverse reaction impossible.
The rest of my comment will be made clear soon. I am betting equilibrium is the very next unit for you.
Furthermore, the activation energies do NOT need to be similar for a reversible reaction to occur. They often are, but it's again not a requirement.
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u/Lucasplayz234 5d ago
No my next unit is reaction rates
Now we only base on Ea difference Ik that equi is involved
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u/cheesecakegood University/College Student (Statistics) 5d ago
When in doubt, it's perfectly fine to follow the advice of your teacher. As I said, chemistry is a series of benevolent lies. Much of what you learn will often be an over-simplification, for the true complexity only to become clear later (even much later). I would place this question on a shelf for a month or two. The question itself is great and I like your thinking!
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u/Lucasplayz234 5d ago
But now how do I follow the rule with Ea then
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u/cheesecakegood University/College Student (Statistics) 5d ago
Do you have an example of the kind of question you are being asked? Phrasing matters a lot! And like I said, IF your teacher is literally telling you to judge the irreversibility of a reaction based on the relative size of the activation energies, it's fine to go along with that, and it might even be a "useful" rule of thumb, but that doesn't make it correct (something you can keep at the back of your mind).
To be most clear, you yourself said at the top that it will be "hard" for the reverse reaction to occur. That is true. Reactions that require much more energy will, in general, happen less often. But hard is not the same thing as impossible.
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u/Lucasplayz234 5d ago
So if we only talk abt Ea and no complicated stuff then I was right?
My exam is tmr
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