mick
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Homogeneous solutions
This has bugged me for years, ever since a good practical chemist pointed it-
Why are you stirring that homogeneous solution?
I have read it in research papers-The homogeneous solution was stirred for eg 1 hour to 4 days, after 4 days of stirring the reaction worked.
I think it would have worked with out the stirring just as well.
Some one has tried to tell me that his homogeneous solution had to be stirred realy fast for the reaction to work.
Can a mechanical or magnetic stirrer or shaking influence the reaction in a homogeneous solution.
mick
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chemoleo
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I guess stirring is simply necessary to provide maximum accessibility between the reagents, so that things are not limited by the diffusion rate.
This is particularly true for catalyst-based reactions.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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Proteios
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if you dont stir... the rate of your reaction may be bottlenecked by the notoriously slow process of diffusion.
For instance.....acid base titrations are homogenious..... but try doing those without stirring will take forever!
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mick
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I am just thinking along the lines that if you diluted sulphuric acid with water, once it is homogeneous no amount of stirring will change the
concentration
mick
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Dodoman
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| Quote: | Originally posted by mick
I am just thinking along the lines that if you diluted sulphuric acid with water, once it is homogeneous no amount of stirring will change the
concentration
mick |
The concentration won't change but the rate at which the reactants collide would definitely change. Thus increasing the probability of an active
collision occurring.
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unionised
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The mean kinetic energy, and hence the velocity, of the molecules in a liquid is defined by its temperature. Unless you stir it so vigourously that
you heat it up, the stirring will not make any difference.
It's a valid point, there isn't any point stiring a homogeneous reaction.
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Magpie
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I have wondered about this too and agree that stiring of a homogeneous solution seems pointless. I think where it is called out for such solutions it
is usually to promote heat transfer.
The single most important condition for a successful synthesis is good mixing - Nicodem
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JohnWW
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It's called "mixing" - the term so far not having been used in this thread. When one liquid reagent is added to another, and especially
if they have different densities, the rate of reaction would be severely reduced if the two are not intimately mixed at the molecular scale. Hence the
need for agitation. Otherwise, the reaction is limited by the rates of diffusion of the reagents between localized regions of concentration of one or
the other, especially if the reagents are highly viscous. For miscible liquids, mixing is irreversible (positive entropy), other than through
additional steps like distillation. This means that stirring or agitation is no longer needed after mixing of all the reagents needed is complete at
the molecular scale, unless the reaction mixture is being heated or cooled and it is desirable to homogenize the temperature in the reaction mixture
(which is usually the case).
Heterogenous reactions, between two immiscible liquids or phases, e.g. sulfonation and nitration of aromatic organic liquids, also require the two
liquids to be in intimate contact. The rate of the reaction, which can only occur at the boundary between the two phases, depends on the contact area
between the phases. But unlike homonegous reactions between miscible liquids, the mixing - which may for a time form an emulsion or dispersion (very
small droplets of one liquid in the other) if the the surface tension is sufficiently low - is reversible, and the two phases separate out as layers
once agitation is ceased; hence CONSTANT stirring or agitation is needed until the reaction is complete in such cases.
See Perrys Chemical Engineers Handbook, chapters 19 and 21, for further information, including the fluid mechanics of mixing and emulsions, and design
of mixing equipment.
John W.
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mick
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Thanks
I will go along with that. Stirring should not affect a true homogeneous solution but is needed for heat transfer. I have had a problem trying the
explain to people that when you are doing a catalytic hydrogenation you need to mix (got the word in) a solid , liquid and a gas and it is no good
giving it a gentle stir, if you shake the f*** out of it, the reaction could be finished in 30min rather than 2days or more.
mick
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unionised
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"It's called "mixing" - the term so far not having been used in this thread."
No John, it isn't. The thread is quite explicitly about a homogeneous solution so there's nothing "unmixed" to mix.
It might be about heat dissipation, though convection will often do that and boiling under reflux certainly will if the reaction is that exothermic.
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JohnWW
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Wrong. An homogenous reaction solution is composed of two miscible (or mutually soluble, in the mixture ranges used) liquid reagents. While forming a
single phase when one is added to the other in the cncentration ranges encountered, initial mixing until the mixture is homogenized is needed for the
reaction to proceed efficiently, to avoid local concentrations of unreacted reagents which otherwise would react only when they diffuse (due to
Brownian motion) into each other. In addition, continual mixing for the duration of the reaction is needed if heat has to be either applied to or
removed from the reaction vessel to enable the reaction to be completed (and completed safely), to homogenize the temperature of the mixed reaction
solution at the desired value.
John W.
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unionised
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No, right.
http://dictionary.reference.com/search?q=homogeneous
It looks like the definition of homogeneous means mixed. Once it is mixed you can stop stirring.
The original post talked of stiring for hours or even days. At that level, it is pointless (except, possibly, for heat control, and I doubt that for
reactions that take place over days).
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