RawWork
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Why reactions happen?
For example why carbon doesn't react with oxygen at room temperature and sodium does. Is only one parameter playing role here? Is it because of
ionization energy (in case of elements) and/or bond dissociation energy in case of compounds, molecules...?
I don't care so much about displacement reactions, but about simple one element with another element reactions.
If more then one parameters play role, what are they? Also if we ignore physical barriers like protective layer of newly formed product like oxide on
metals...
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aga
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Lots of stuff likes to react (i.e. play electron-shuffle) with lots of other things.
Given that we live in a narrow range of possible temperatures, we tend to see a narrow range of reactions, both slow and really fast.
If we could observe matter in the range of absolute zero, right up the inside of super-massive star, then we'd see a lot more reactions.
Basically the atoms have to encounter each other (kinetics) have enough energy to 'afford' the electron shuffle (activation energy) and end up with
less energy than they started with (entropy).
Well, something like that.
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Magpie
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This is basically it. Electrons want to go to their lowest energy state.
The single most important condition for a successful synthesis is good mixing - Nicodem
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LearnedAmateur
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Yeah, thermodynamics plays probably the biggest role in reactions, I.E would it prefer to exist in state A or B - take ester + NaOH for instance, it
may exist as either the reactants or products (hence why saponification usually requires reflux), but the energy change is such that an alcohol is
unlikely to displace sodium from the carboxylate group, whereas OH-, being a strong base, is much more likely to rip off the alcohol, leaving the
carboxylate ion floating around as a result.
As for carbon and oxygen, the atoms of oxygen must hit the carbon at a threshold speed, AKA have enough kinetic energy, to overcome the activation
energy. If you look at a Boltzmann distribution, you’ll see that a small number of particles will actually have enough energy to overcome Ea (a slow
reaction at lower temperature), and increasing temperature/Ek will shift the bulk of the particles past the threshold (increased rate at higher
temperature). The rate of a reaction can be calculated using the Arrhenius equation, linked below because I CBA to write it out properly.
To be perfectly honest, it’s a pretty vague question considering the essentially infinite number of reactions to be made in chemistry. You also have
to consider that physical effects do in fact play a key role - not just oxide layers, but also steric hinderance. In most cases, this is the result of
a bulky inert group which limits access to a reactive moiety. Take for instance amines as an example, primary amines always react the fastest for a
general reaction because the nitrogen atom is at the end of the molecule, then adding another group or two will add more stuff in the way of incoming
reactants. Then increasing the dimensions of these groups (e.g -CH3 compared to -Ar) will also mean there is less spatial freedom for reactants to hit
the nitrogen from.
Sorry if some of this is confusing, just got in from work and I’m completely knackered.
In chemistry, sometimes the solution is the problem.
It’s been a while, but I’m not dead! Updated 7/1/2020. Shout out to Aga, we got along well.
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Ozone
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Look up enthalpy, entropy, and Gibb's free energy (and see how they are connected). Then, look up catalysis, Arrhenius kinetics, and how that relates
to activation energy.
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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aga
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"Why reactions happen?"
I get it now - it's a trick question.
The reactions i observe in the lab happen because i mix stuff up, sometimes with some chemi-maths too.
Is the prize a banana ?
I like bananas.
They're edible, mostly, yellow, and are good companions in the vacuum of Space.
Not many fruits can do that, then pass thru pretty much any Gut and make Mars go all Bananas too.
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zed
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Huh? Well, mostly because opposites attract.
But, sometimes because of narcissism, or fer a plethora of other reasons.
Note the great affinity of Nitrogen, for itself. As illustrated, by our twins.
https://www.youtube.com/watch?v=lDUPqfOmLpA
Banana prizes? Sounds good.
[Edited on 20-3-2018 by zed]
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phlogiston
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Good question, and it has been answered already, but just to boil it down a bit for you to the two most important factors as I suspect it may seem
overwhelming at first:
Thermodynamics A reaction can only occur if it is 'energetically favorable'. As Ozone wrote, you will want to look into something
called 'Gibbs free energy'.
Kinetics The reactants must be able to reach each other efficiently (small particle size if solids, mixed well, not too dilute in
solution, etc), overcome an 'activation energy' (some extra energy or a catalyst may be required). Even if a reaction is thermodynamically possible,
it may not take place at an appreciable rate if kinetic factors prevent it.
[Edited on 21-3-2018 by phlogiston]
[Edited on 21-3-2018 by phlogiston]
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"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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