Proton solvation

Ionisations 2, 3 and 5 are complete ionisations so that in water HCI and HNO3 are completely ionised and H2SO4 is completely ionised as a monobasic acid. Since this is so, all these acids in water really exist as the solvated proton known as the hydrogen ion, and as far as their acid properties are concerned they are the same conjugate acid species (with different conjugate bases). Such acids are termed strong acids or more correctly strong acids in water. (In ethanol as solvent, equilibria such as 1 would be the result for all the acids quoted above.) Ionisations 4 and 6 do not proceed to completion... 

H3O" is strictly the oxonium ion actually, in aqueous solutions of acid this and Other solvated-proton structures exist, but they are conveniently represented as... 

Specific acid catalysis is observed when a reaction proceeds through a protonated intermediate that is in equilibrium with its conjugate base. Because the position of this equilibrium is a function of the concentration of solvated protons, only a single acid-dependent term appears in the kinetic expression. For example, in a two-step reaction involving rate-determining reaction of one reagent with the conjugate acid of a second, the kinetic expression will be as follows ... 

Under these circumstances, a distinct contribution to the overall rate will be seen for each potential hydrogen-bond donor D—H. General acid catalysis is also observed when a ratedetermining proton transfer occurs fiom acids other than the solvated proton ... 

Many organic reactions involve acid concentrations considerably higher than can be accurately measured on the pH scale, which applies to relatively dilute aqueous solutions. It is not difficult to prepare solutions in which the formal proton concentration is 10 M or more, but these formal concentrations are not a suitable measure of the activity of protons in such solutions. For this reason, it has been necessaiy to develop acidity functions to measure the proton-donating strength of concentrated acidic solutions. The activity of the hydrogen ion (solvated proton) can be related to the extent of protonation of a series of bases by the equilibrium expression for the protonation reaction. 

In the phosphoric acid fuel cell as currently practiced, a premium (hydrogen rich) hydrocarbon (e.g. methane) fuel is steam reformed to produce a hydrogen feedstock to the cell stack for direct (electrochemical) conversion to electrical energy. At the fuel electrode, hydrogen ionization is accomplished by use of a catalytic material (e.g. Pt, Pd, or Ru) to form solvated protons. 

At the cathode, air is reduced catalytically by reaction with solvated protons to generate the product water... 

In practical cells, the acid concentration is very high (>95%) and the solvated protonic species are not actually known, i.e. 

The generally accepted measure of the acidity of any solution is the logarithm of the activity of solvated protons times — 1,... 

If the dielectric constant of an amphiprotic solvent is small, protolytic reactions are complicated by the formation of ion pairs. Acetic acid is often given as an example (denoted here as AcOH, with a relative dielectric constant of 6.2). In this solvent, a dissolved strong acid, perchloric acid, is completely dissociated but the ions produced partly form ion pairs, so that the concentration of solvated protons AcOH2+ and perchlorate anions is smaller than would correspond to a strong acid (their concentrations correspond to an acid with a pK A of about 4.85). A weak acid in acetic acid medium, for example HC1, is even less dissociated than would correspond to its dissociation constant in the absence of ion-pair formation. The equilibrium... 

Protolytic reactions can also occur in fused salts. The solvent participates in these reactions provided that at least one of its ions has protogenic and/or protophilic character. An example of a solvent in which the cation is aprotic and the anion protophilic is ethylpyridinium bromide (m.p. 114°C). The acid HA is protolysed in this solvent (HA -I- Br HBr + A"). Hydrogen bromide acts as a solvated proton and the acidity is expressed as... 

Ojamae L, Shavitt I, Singer SJ (1998) Potential models for simulations of the solvated proton in water. J Chem Phys 109(13) 5547-5564... 

The formation of protonated H+(H20)n species can affect the acidity of the non-solvated protonic sites. Therefore, as the acid strength of the protonic sites in zeolites plays a key role in the hydrocarbon transformation reactions, driving the rate of the hydrocarbon protonation [4-6], the presence of water vapor among the reactants can modify reaction rates of the individual reactions involving in the hydrocarbon transformations. 

This mechanism is given in equation (37). Absolute rate theory leads to equation (55), and making the same assumption as for the A1 case, equation (50), leads to the relevant rate equation, equation (56).145,161 This equation is derived on the assumption that all the acidity of the medium comes from solvated protons , H30+ in sulfuric acid it will require modification above 80 wt% acid as the medium acidity begins to be due to the presence of undissociated H2S04 molecules as well, see above.179... 

In studying the properties of solutions of substances such as HC1 and HN03, Arrhenius was led to the idea that the acidic properties of the compounds were due to the presence of an ion that we now write as H30+ in the solutions. He therefore proposed that an acid is a substance whose water solution contains H30+. The properties of aqueous solutions of acids are the properties of the H30+ ion, a solvated proton (hydrogen ion) that is known as the hydronium ion in much of the older chemical literature but also referred to as the oxonium ion. 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

In Equation 50 the chemical potential of non-electrolyte A depends on the usual choice of standard-state conventions described above, and the chemical potentials of both H2(g) and H+(sod are taken to be zero (this defines e.s.s., the electrolyte standard state). By setting the standard-state free energy of the solvated proton equal to zero, this standard-state convention... 

Note, in using Equations 50 and 53 above, that tabulations of thermodynamic data for electrolytes tend to employ a 1 molar ess concentration for all species in solution. For situations defined to have a standard-state pH value different from 0 (which corresponds to a 1 molar concentration of solvated protons), the standard-state chemical potentials for anions and cations are determined as... 

Worked Example 4.10 Consider the dissociation of ethanoic (acetic) acid in water to form a solvated proton and a solvated ethanoate anion, CH3COOH + H20 CH3COO ... 

We see how solvated protons impart the subjective impression of a sour, bitter flavour to the ethanoic acid in vinegar. In fact, not only the sour flavour, but also the majority of the properties we typically associate with an acid (see Table 6.1) can be attributed to an acidic material forming one or more solvated protons H+(aq) in solution. 

Pure water is a mixture of three components H20, and its two dissociation products, the solvated proton (H30+) and the hydroxide ion (OH-). 

It is safer in many instances to assume the solvated proton has the formula unit [H(H20)4]+, with four water molecules arranged tetrahedrally around a central proton, the proton being stabilized by a lone pair from each oxygen atom. 

When water contains no dissolved solutes, the concentrations of the solvated protons and the hydroxide ions are equal. Accordingly, from our definition of neutral above, we see why pure water should always be neutral, since [H30+(aq)] = [OH (aq)]. 

Worked Example 6.1 What is the concentration of the solvated proton in super-pure... 

The concentration of solvated protons in super-pure water is clearly very small. 

Carbonic acid, H2C03(aq), never exists as a pure compound it only exists as a species in aqueous solution, where it dissociates in just the same way as ethanoic acid in Equation (6.1) to form a solvated proton and the HCOj(aq) ion. Note how we form a solvated proton H30+(aq) by splitting a molecule of water, rather than merely donating a proton. Carbonic acid is, nevertheless, a Lowry-Brpnsted acid. 

The carbonic acid produced in Equation (6.5) is a proton donor, so the solution contains more solvated protons than hydroxide ions, resulting in rain that is (overall) an acid. To make the risk of pollution worse, acid rain in fact contains a mixture of several water-borne acids, principally nitric acid, HNO3 (from nitrous oxide in water), and sulphurous acid, H2SO3 (an aqueous solution of sulphur dioxide). 

The solvated hydroxide ion in Equation (6.9) is formed in addition to the hydroxide ions produced during water autoprotolysis, so there are more hydroxide ions in solution than solvated protons, yielding excess hydroxide in solution. We say the solution is alkaline. As an alternative name, we say hydroxide is a base (see p. 241). 

All aqueous solutions naturally contain hydroxide ions in consequence of the auto-protolytic reaction in Equation (6.2). As we have seen, there will be equal numbers of solvated protons and solvated hydroxide ions unless we add an acid or base to it. A solution containing more solvated protons than hydroxide ions is said to be an acid within the Lowry-Brpnsted theory, and a solution comprising more hydroxide ions than solvated protons is said to be a base. 

The solvated proton on the left of Equation (6.12) acts as an acid, since it donates a proton at the same time as the ethanoate ion behaves as a base, because it accepts a proton. To complicate the situation, the reaction is one half of a dynamic equilibrium, i.e. it proceeds in both the forward and backward directions. In the backward direction, we notice how this time the ethanoic acid acts as an acid and the water acts as a base. 

The reaction in Equation (6.12) illustrates the coexistence of two acids and two bases. We say the ethanoate ion and ethanoic acid represent a conjugate pair, and the solvated proton and the water form a second conjugate pair. Within the ethanoic-ethanoate pair, the ethanoic acid is the conjugate acid and the ethanoate anion is the conjugate base. Similarly, H30+ is a conjugate acid to the... 

Between these two acids, there is up to a million-fold difference in the number of solvated protons per litre. We cannot cope with the unwieldy magnitude of this difference and tend to talk instead in terms of the logarithm of the concentration. To this end, we introduce a new concept the pH. This is defined mathematically as minus the logarithm (to the base ten) of the hydrogen ion concentration ... 

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