I am a 14 year old school student and have been told that acid-base reactions occur in aqueous solutions only for they produce ions in aqueous solutions only and I can’t help but wonder if they can take place if these two kinds of compounds are added to each other in their pure forms or molten states.
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2$\begingroup$ If they can intimately mix then there will be reaction. In solid state this is very slow, it can occur in molten state. There are also other solvents that support the formation of ions. $\endgroup$– WaylanderSep 28 at 9:06
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$\begingroup$ To get an acid-base reaction, the reagents (and ions) must be mobile. They are not in the solid state. So mixing a solid acid an a solid hydroxyde will produce a reaction limited to the contact points, which can be neglected. If the reagents are molten, they are mobile. It looks like the situation in an aqueous solution. The reaction can take place, but it is not so fast as in aqueous solution. $\endgroup$– MauriceSep 28 at 9:35
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2$\begingroup$ If can happen even in gaseous state. $\endgroup$– PoutnikSep 28 at 9:41
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$\begingroup$ Good question! I'm not an expert at all here but a quick look suggests solid state acid-base reactions are an active area of research - for instance google.com/… discusses solid state acid-base in pharmaceuticals, and en.wikipedia.org/wiki/Solid_acid touches on mixed phased reactions $\endgroup$– Ian BushSep 28 at 14:54
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4$\begingroup$ Does this answer your question? Can acid-base reactions occur in a non-aqueous medium? $\endgroup$– MithoronSep 28 at 17:12
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3 Answers
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The key question we must ask ourselves is which definition of acids and bases we wish to employ. Since acid and base is a man-made label for chemical compounds, then we must familiarize ourselves with plenty of acid-base definitions that were proposed by different scientists over a century. You will study these definitions in higher classes. When we restrict ourselves to aqueous solutions, we are using a very narrow and limited definition of acid and bases, which is fine for elementary classes. Therefore the acid-base reactions can occur in any state.
Gas-phase: The classical reaction of ammonia gas with HCl gas to form a solid ammonium chloride. (At a deeper level, a trace of water makes the reaction very fast, and but water vapor is always present in air)
Solid-gas: Carbon dioxide reacting with solid sodium hydroxide to form sodium hydrogen carbonate.
Reactions in organic anhydrous solvents: Acid-base reactions occur all the time in organic solvents. No water is there (anhydrous).
Similarly molten salts will react with pure bases or if you can melt them. Nobody does it regularly but in rare synthetic cases, acid base neutralization has been observed Reaction of molten sebacic acid with a layered (Mg/Al) double hydroxide.
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Water is not required for acid-base reactions. The materials do not need to be ionized to react.
The comments have already given you some good examples, like the reaction between ammonia ($\ce{NH3}$) and hydrogen chloride ($\ce{HCl}$), both gases at room temperature. They react to form ammonium chloride, $\ce{NH4Cl}$, which is a solid. There is no water involved and the result of the reaction is a very fine white smoke. This is actually a fairly common classroom demonstration.
Some elementary chemistry classes give an incomplete definition of a base, which only includes hydroxides like $\ce{NaOH}$ or $\ce{KOH}$. That would exclude the previous reaction, which involves ammonia. This definition is bad and is never used by real chemists, only in the classroom. But even under this definition, water is not required. Pure, dry $\ce{HCl}$ gas will react readily with hydroxides in their solid form. And if you were stupid enough to try adding molten sodium hydroxide to a pure liquid acid like sulfuric acid, the reaction would be very violent, and the whole caustic mixture would instantly explode in your face.
In fact, one reason to do a reaction in solution is to reduce the intensity of the reaction. You can safely mix very dilute solutions of acids and bases, while mixing the same chemicals at higher concentration could release too much heat too fast and explode.
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$\begingroup$ Not to mention the "base" or "acid" could be water itself. We all know the precausions necessary when diluting, say, sulfiric acid (much heat release) or hydrochloric acid (less heat, but fumes) with water. $\endgroup$ Sep 28 at 17:50
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Looking more closely at the classic case of ammonia and hydrogen chloride gases without water:
Countess and Heicklen[1] carried out the reaction between gaseous ammonia and hydrogen chloride in anhydrous nitrogen, demonstrating the raction without need for either liquid or gaseous water. They give a rate constant for this reaction.
What actually happens is that the strongly polarized hydrogen from the hydrogen chloride first hydrogen-bonds to the nitrogen from the ammonia, thus forming ammonia-hydrogen chloride clusters. In the absence of water the initial $\ce{H3N–HCl}$ cluster is not ionic, but as the cluster grows further the hydrogen-bonded protons become fully transferred to make the $\ce{NH4^+}$ and $\ce{Cl^-}$ ions, which ultimately are assembled into the solid-phase lattice. This is discussed more fully here.
Reference
- Richard J. Countess and Julian Heicklen, "Kinetics of particle growth. II. Kinetics of the reaction of ammonia with hydrogen chloride and the growth of pariculate ammonium chloride", J. Phys. Chem. ,1973, 77, 4, 444–447.
