Hydroxonium ions

Since free protons cannot exist, acidic properties can only be shown when the solvent can act as a proton acceptor, i.e. as a base. Thus aqueous solutions of acids contain the hydroxonium ion,... 

In practice, however, the amount of hydrochloric acid employed is less than 5 per cent, of the amounts indicated by either of the above equations. Various explanations have been advanced to account for this one is that the following reaction is catalysed by acid or by hydroxonium ions ... 

Because water is not protonated in these solutions, its addition reduces the concentration of ions, and therefore the electrical conductivity. The conductivity reaches a minimum in solutions containing 97% of acid, but rises on further dilution as a result of the formation of nitrate and hydroxonium ions. ... 

NOj ions/ Addition of water to nitric acid at first diminishes its electrical conductivity by repressing the autoprotolysis reactions mentioned above. For example, at -10° the conductivity decrea.ses from 3.67 x 10 ohm cm to a minimum of 1.08 x 10" ohm" cm at 1.75 molal H2O (82.8% NjOs) before rising again due to the increasing formation of the hydroxonium ion according to the acid-base equilibrium... 

Actually the hydrogen ion H+ (or proton) does not exist in the free state in aqueous solution each hydrogen ion combines with one molecule of water to form the hydroxonium ion, H30+. The hydroxonium ion is a hydrated proton. The above equations are therefore more accurately written ... 

The ionisation may be attributed to the great tendency of the free hydrogen ions H+ to combine with water molecules to form hydroxonium ions. Hydrochloric and nitric acids are almost completely dissociated in aqueous solution in accordance with the above equations this is readily demonstrated by freezing-point measurements and by other methods. 

Strictly speaking the hydrogen ion H+ exists in water as the hydroxonium ion H30 + (Section 2.4). The electrolytic dissociation of water should therefore be written ... 

In all cases the reaction of the solution can be quantitatively expressed by the magnitude of the hydrogen ion (or hydroxonium ion) concentration, or, less frequently, of the hydroxide ion concentration, since the following simple relations between [H + ] and [OH-] exist ... 

Any strong acid that may be present is first neutralised. Then, by selecting an appropriate base, whose conjugate acid has a Ka of about 10 5, the equilibrium for the tripositive cations will be forced to the right the base is too weak, however, to remove the hydroxonium ions from the equilibrium of the dipositive cations. Since a large excess of the basic ion is added, a basic salt of the tripositive metal usually precipitates instead of the normal hydroxide. Acetate or benzoate ions (in the form of the sodium salts) are the most common bases that are employed for this procedure. The precipitation of basic salts may be combined with precipitation from homogeneous solution, and thus very satisfactory separations may be obtained. 

Research into the mechanism of diazotization was based on Bamberger s supposition (1894 b) that the reaction corresponds to the formation of A-nitroso-A-alkyl-arylamines. The TV-nitrosation of secondary amines finishes at the nitrosoamine stage (because protolysis is not possible), but primary nitrosoamines are quickly transformed into diazo compounds in a moderately to strongly acidic medium. The process probably takes place by a prototropic rearrangement to the diazohydroxide, which is then attacked by a hydroxonium ion to yield the diazonium salt (Scheme 3-1 see also Sec. 3.4). 

These special features are explained by an interaction between the proton and one of the water molecules, which is not merely electrostatic but also covalent. This yields a new chemical species, the hydroxonium ion, HjO. The existence of such ions was demonstrated in the gas phase by mass spectrometry and in the solid phase by X-ray diffraction and nuclear magnetic resonance. The H -H20 bond has an energy of 712kJ/mol, which is almost two-thirds of the total proton hydration energy. 

Hydrogen can be evolved not only as a result of the discharge of hydroxonium ions, HjO, but also by discharge of other proton donors, HA, that may be present in the solution, including the water molecules themselves ... 

Steady photoemission currents can be realized when acceptors (scavengers) for the solvated electrons are present in the solution. A good scavenger should be nonelectroactive at the potenhal of interest, should react quickly with solvated electrons, and the reaction products should be either nonelectroactive or reducible. A reachon with acceptors implies that the current of reoxidation of the solvated electrons becomes lower, and thus a steady photoemission current appears. The acceptors most often used are nitrous oxide, N2O, and hydroxonium ions, HjO. In the former case, OH radical is produced in the scavenging process, which undergoes further reduction on the electrode, thus doubling the photocurrent ... 

Hydrogen evolution at metal electrodes is one of the most important electrochemical processes. The mechanisms of the overall reaction depend on the nature of the electrode and solution. However, all of them involve the transfer of proton from a donor molecule in the solution to the adsorbed state on the electrode surface as the first step. The mechanism of the elementary act of proton transfer from the hydroxonium ion to the adsorbed state on the metal surface is discussed in this section. 

An important observation on the proton behavior in chemical compounds is that it is a quantum particle. In particular, the frequency of its valence vibrations in molecules such as hydroxonium ion is on the order of Q 10 s [i.e., the energy of corresponding vibrational quantum TiQ. ( 0.3 eV) is much higher than the thermal... 

Therefore, electrons adjust their state to any instant position of the proton and solvent polarization in both the initial (hydroxonium ion) and final (adsorbed hydrogen atom) states. The proton in the hydroxonium ion sees an average electron cloud but feels any instant configuration of solvent polarization. 

H30 + is also called the hydroxonium ion because it expresses (analogously with the ammonium ion) the donation of a proton to a lone pair of electrons of the oxygen it explains also the term alkoxonium ion for ROH (reaction 4.4) like the alkylammonium ion for RNH3+ (reaction 4.8) CH3COOH2f or AcOH may be called the acetoxonium ion... 

The H2S+ ion is generally termed a lyonium ion and the S" ion is termed a lyate ion. The symbol H2S+ (for example H30+, CH3COOH2+, etc.) refers only to a proton solvated by a suitable solvent and does not express either the degree of solvation (solvation number) or the structure. For example, two water molecules form the lyonium ion H30+, termed the oxonium (formerly hydronium or hydroxonium) ion, and the lyate ion OH", termed the hydroxide ion. 

Water adsorption on high silica zeolites. Formation of hydroxonium ions and hydrogen-bonded adducts... 

In a water solution, a proton (hydrogen ion) is hydrated, forming the hydroxonium ion H30. For the sake of simplicity we write H+ instead of H30. For further simplicity, we assume that the diffusion of protons from the bulk of the solution to the electrode [outer Helmholtz plane (OHP)],... 

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