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Hydrochloric acid is always discussed as dissolved in water, and acidity is measured in terms of pH by the concentration of the hydronium ion.

Is pure HCl still an acid, and if so, how can we measure its acidity?

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    $\begingroup$ HCl without water is gas. It is called hydrogen chloride and it is not an acid. $\endgroup$ – Nilay Ghosh Jun 13 at 4:50
  • $\begingroup$ @Nilay Ghosh but HCl recives lone pair from oxygen in water when dissolves in water ( lewis acids recieve lone pair no?) $\endgroup$ – Yousuf Ithfi Jun 13 at 4:58
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    $\begingroup$ Does this answer your question? Is HCl(l) acidic? $\endgroup$ – Mithoron Jun 13 at 13:50
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If we look at the classic case of ammonia and hydrogen chloride in the gas phase, the answer is yes -- but it's more complicated than one might think. The salt does form, but it requires either a proton-transfer medium such as water vapor or the formation of particles out of the gas phase to actually get the ions.

The acid-base reaction is formally,

$$\ce{HCl + NH3->(NH4)^+Cl^-}$$

but according to an an ab initio calculation by Cazar et al. [1] a single pair of molecules do not actually form the indicated ions. They form instead a hydrogen-bonded species in which the would-be transferred proton remains primarily bonded to the chlorine atom. Only with the introduction of water molecules do we find the formation of ammonium and chloride ions in the gas phase.

An empirical study by Countess et al. [2] does show the formation of ammonium chloride salt as condensed particles in anhydrous nitrogen. They give a rate constant in terms of molecules per unit volume for formation of the monomeric gas-phase species, which is polar and attracts other monomers or reactant molecules to nucleate the particles. It is during this polymerization phase where the ions are definitively formed; Chaban et al. [3] compute that even two ammonium chloride entities are sufficient to form an ionic structure.

References

  1. Robert A. Cazar, Alan J. Jamka, and Fu-Ming Tao, "Ab Initio Investigation of Proton Transfer in Ammonia−Hydrogen Chloride and the Effect of Water Molecules in the Gas Phase", J. Phys. Chem. A., 1998, 102, 26, 5117–5123.

  2. 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.

  3. Galina M. Chaban, R. Benny Gerber, and Kenneth C. Janda, "Transition from Hydrogen Bonding to Ionization in (HCl)n(NH3)n and (HCl)n(H2O)n Clusters:  Consequences for Anharmonic Vibrational Spectroscopy", J. Phys. Chem. A 2001, 105, 36, 8323–8332.

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