The aqueous hydrogen ion

When the aqueous hydrogen ion is written as H+(aq), it indicates only that when a free proton, is transferred to water, it becomes bound, in some way, to the water molecules. But what is the nature of this binding Ascertaining the structure of liquids is very difficult, but it is believed that the proton becomes bound primarily to just one water molecule in the ion, H30 (Section 3.2)  

A Lewis structure for H30 is shown as Structure 5.2 the ion 0+ has five outer electrons, and gains a noble gas structure by forming three electron-pair bonds. 

This ion, in its turn, can pass a proton to a water molecule to its right, and so on. Thus, there can be a net transfer of a proton between two points in an aqueous solution without one particular proton having to cover the entire distance. 

The most widely accepted proposal for the arrangement of water molecules around the ion H30+(aq). Each O—H bond of the H3O+ ion points towards and binds the oxygen atom of an adjacent water molecule through a hydrogen bond. The hydrogen bonds are shown as dashed purple lines. The whole unit has the formula [H9O4P. 

1 Hydrogen is the most abundant element in the universe. On Earth, it occurs in the very accessible forms of water, natural gas and crude oil. 

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These solutions have similar properties and are called acid solutions. The common species in the solutions is the aqueous hydrogen ion, H+(ag), and the properties of aqueous acid solutions are attributed to this ion. We shall investigate these solutions in Chapter 11. 

Since neither of the two half-reaction potentials is known absolutely, it is necessary to propose an arbitrary /cm, relative to which all half-reaction potentials may be quoted. The half-reaction chosen to represent the arbitrary zero is the hydrogen electrode1 in which the half-reaction is the reduction of the aqueous hydrogen ion to gaseous dihydrogen ... 

Only a very small amount of work has been concerned with proton transfer processes not involving the aqueous hydrogen ion. Because they are less important, the discussion of these cases will be deferred till a later section, even though they are in some ways theoretically less complicated. 

On the other hand, does the formula H30+ give sufficient expression to the extent of solvation of the aqueous hydrogen ion There is persuasive evidence that the formula H904+ (1), with three further molecules of water of solvation, is a better representation of the hydrogen ion in solution than Hs0+ (Eigen, 1963). Such a formula would imply... 

A particular case in point is that of the aqueous hydrogen ion itself. We have assumed that its fractionation behaviour is adequately described by a single parameter I characterizing the exchange of three equivalent hydrogen nuclei (see Section IVA). However, if we chose to include, in our formula of the hydrogen ion, water molecules in the vicinity of the H3O unit, as in formula 1, for example, then additional factors l — n + n[Pg.294]

C° correspond to the standard partial molar quantities, and for individual aqueous ions they are relative quantities, defined with respect to the aqueous hydrogen ion, according to the convention [89COX/WAG] that Af//°(H, T) = 0 and that T) = 0. Furthermore, for an ionised solute B containing any number of different cations and anions ... 

We can state the Arrhenius concept of an acid as follows An acid is a substance that, when dissolved in water, increases the concentration of hydronium ion, HsO aq). For simplicity, chemists frequently use the notation ld aq) for the H30 (aq ) ion, and call it the hydrogen ion. Remember, however, that the aqueous hydrogen ion is not a bare proton in water, but a proton chemically bonded to water—that is, H30" (a ). The H30 (a ) ion is itself associated through hydrogen bonding with a variable number of water molecules. One such species is shown in Figure 16.2. A base, in the Arrhenius concept, is a substance that, when dissolved in water, increases the concentration of hydroxide ion, OH (aq). ... 

Thus, the enthalpies and free energies of formation of other ions can be measured relative to the aqueous hydrogen ion. 

Values of Xqw the molar ionic conductivity or, in metric imits the equivalent conductance of individual ions, can be obtained from measured values of A extrapolated to give Aq. Table D.2 contains values of aqueous equivalent ionic conductivity for many ions foimd in aqueous solutions at 25°C. It should be noted that the values in this table are given in SI units. Values in the metric units of S -cm -moDi would be larger by a factor of 10. An appropriate value for the aqueous hydrogen ion, for example, would be 349.99 S-cm -mohb... 

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