The Hydronium Ion

These definitions carry a very important implication a substance cannot act as an acid without the presence of a base to accept the proton, and vice versa. 

A reaction of an acid with a base is thus a proton exchange reaction if the acid is denoted by AH and the base by B, then we can write a generalized acid-base reaction as 

But the product BH+ is now capable of losing its newly-acquired proton to another acceptor, and is 

Amphiprotic species Many substances can both donate and accept protons examples are H2PO4, HCO3, NH3, and H2O. Such substances are said to be amphiprotic the dissolved species themselves are called ampholytes. 

Acid-base reactions Within the Arrhenius concept, neutralization of H+ by OH is the only type of acid-base reaction that can occur. The Brpnsted concept broadens our view, encompassing a wide variety of reactions whose common feature is the transfer of a proton from a donor to an acceptor (Table 2). 

The ionization of nitric acid in water can be rewritten to show the hydrogen ion as the hydronium ion  

Acid — Any substance that produces hydronium ion, H30+, when dissolved in water. 

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Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

Strong and Weak Bases Just as the acidity of an aqueous solution is a measure of the concentration of the hydronium ion, H3O+, the basicity of an aqueous solution is a measure of the concentration of the hydroxide ion, OH . The most common example of a strong base is an alkali metal hydroxide, such as sodium hydroxide, which completely dissociates to produce the hydroxide ion. 

The most numerous cases of homogeneous catalysis are by certain ions or metal coordination compounds in aqueous solution and in biochemistry, where enzymes function catalyticaUy. Many ionic effects are known. The hydronium ion and the hydroxyl ion OH" cat-... 

A second way to achieve constancy of a reactant is to make use of a buffer system. If the reaction medium is water and B is either the hydronium ion or the hydroxide ion, use of a pH buffer can hold Cb reasonably constant, provided the buffer capacity is high enough to cope with acids or bases generated in the reaction. The constancy of the pH required depends upon the sensitivity of the analytical method, the extent of reaction followed, and the accuracy desired in the rate constant determination. 

Specific acid catalysis is catalysis by the hydronium ion (in water) or the lyonium ion in general. Acid-catalyzed ester hydrolysis is an example. 

Throughout this section the hydronium ion and hydroxide ion concentrations appear in rate equations. For convenience these are written [H ] and [OH ]. Usually, of course, these quantities have been estimated from a measured pH, so they are conventional activities rather than concentrations. However, our present concern is with the formal analysis of rate equations, and we can conveniently assume that activity coefficients are unity or are at least constant. The basic experimental information is k, the pseudo-first-order rate constant, as a function of pH. Within a senes of such measurements the ionic strength should be held constant. If the pH is maintained constant with a buffer, k should be measured at more than one buffer concentration (but at constant pH) to see if the buffer affects the rate. If such a dependence is observed, the rate constant should be measured at several buffer concentrations and extrapolated to zero buffer to give the correct k for that pH. 

Often k" is smaller than k° because attack on the dianion by hydroxide is disfavored. Moreover, the maximum often occurs on the acid side of neutrality. In the pH region near the maximum it will, therefore, often be permissible to set k" = 0 in Eq. (6-80). The result is differentiated and the derivative is set equal to zero, giving, for the hydronium ion concentration at the maximum in the bell. 

Setting the first derivative to zero gives Eq. (6-83) for the hydronium ion concentration at the left-hand minimum. 

In wet corrosion ionisation of oxygen to hydroxyl must involve the hydronium ion or water. 

In view of the importance of the hydronium ion, HjO, and dissolved oxygen as electron acceptors in corrosion reactions, some values of the redox potentials E and chemical potentials n for the equilibria... 

Notice that water can act either as an acid or as a base, d.epending on the circumstances. In its reaction with HC1, water is a base that accepts a proton to give the hydronium ion, In its reaction with amide ion, -NH2, however,... 

The Lewis definition of a base as a compound with a pair of nonbonding electrons that it can use to bond to a Lewis acid is similar to the Bronsted-Lowry definition. Thus, H20, with its two pairs of nonbonding electrons on oxygen, acts as a Lewis base by donating an electron pair to an H+ in forming the hydronium ion, H30+. 

A species that can either accept or donate a proton is referred to as amphiprotic. An example is the H20 molecule, which can gain a proton to form the hydronium ion, H30+, or lose a proton, leaving the hydroxide ion, OH-. 

Unfortunately, the experimental data do not provide a definite answer to the nature of HVaqj. The hydronium ion, shown in Figure 11-5, does exist in certain crystal structures. Spectroscopic studies indicate that several species are present in water. Thermal and electrical con-... 

Whenever H+(aq) might appear in an equation for a reaction, it is replaced by the hydronium ion, H30+, and a molecule of water is added to the other side of the equation. We write (55) in the form... 

N-substituted phenylhydroxylamine derivatives, e.g. N-acetyl and N-sulphonic acid, also form the para-aminophenol", but more bulky groups prevent reaction, it is thought by steric hindrance to the approach of the hydronium ion. -substituted phenylhydroxylamines, on the other hand form only the ortho product, it is thought via an intramolecular rearrangement, e.g. 

Although hydrogen ions are always attached to water molecules as hydronium ions, H,0, or more complex species, we write them here as H for simplicity. The (aq) should be taken to imply that the hydrogen ion is actually present as the hydronium ion. 

Because at equilibrium virtually all the HCl molecules have donated their protons to water, HCl is classified as a strong acid. The proton transfer reaction essentially goes to completion. The H30+ ion is called the hydronium ion. It is strongly hydrated in solution, and there is some evidence that a better representation of the species is H904+ (or even larger clusters of water molecules attached to a proton). A hydrogen ion in water is sometimes represented as H + (aq), but we must remember that H+ does not exist by itself in water and that H CC is a better representation. 

In this reaction, the base O2- accepts two protons from the hydronium ions present in the hydrochloric acid solution. 

We can determine an approximate value of the pH of an aqueous solution very quickly with a strip of universal indicator paper, which turns different colors at different pH values. More precise measurements are made with a pH meter (Fig. 10.11). This instrument consists of a voltmeter connected to two electrodes that dip into the solution. The difference in electrical potential between the electrodes is proportional to the hydronium ion activity (as will be explained in Section 12.10) so, once the scale on the meter has been calibrated, the pH can be read directly. 

EXAMPLE 10.4 Sample exercise Calculating the hydronium ion concentration from the pH... 

STRATEGY Calculate the hydronium ion concentration from the pH and then calculate the value of /C, from the initial concentration of the acid and the hydronium ion concentration. 

Proton transfer equilibrium is established as soon as a weak base is dissolved in water, and so we can calculate the hydroxide ion concentration from the initial concentration of the base and the value of its basicity constant. Because the hydroxide ions are in equilibrium with the hydronium ions, we can use the pOH and pKw to calculate the pH. 

All three concentration ratios can he written in terms of the hydronium ion concentration. We simply rearrange the expressions for the first and second acidity constants ... 

In aqueous solutions of very weak acids, the autoprotolysis of water must be taken into account if the hydronium ion concentration is less than 10 6 mol-L The expressions for Kw and Ka are combined with the equations for charge balance and material balance to find the pH. 

A note on good practice Keep in mind the approximations required for the use of the Henderson-Hasselbalch equation (that the concentrations of both the weak acid and its conjugate base are much greater than the hydronium ion concentration). Because the equation uses molar concentration instead of activities, it also ignores the interactions between ions. 

To verify the presence of Zn2+ ions in the solution remaining after the first two steps, we add H2S followed by ammonia. The base removes the hydronium ion from the H2S equilibrium, which shifts the equilibrium in favor of S2- ions. The higher concentration of S2 ions increases the Qsp values of any remaining metal sulfides, such as ZnS or MnS, above their fCsp values, and they precipitate. 

Alcohols are heterolysed into carbocations and water in the presence of the hydronium ion (6). From the equilibrium constant the free energy of heterolytic dissociation of the carbon-oxygen a bond, AGSe,(ROH + H30+), can be calculated. The AG°het(ROH + H3O+) value is related to the pi R+ of the carbocation by (26). 

Two important polyatomic cations appear in introductory chemistry. These are the ammonium ion, NHJ, and the hydronium ion, H3 O, both of which are shown in Figure 3-11. These cations always have H-1 charges. The... 

Many books abbreviate the hydronium ion as H (a g) or just H. We prefer H3 O because it serves as a reminder of the molecular structure of the hydronium ion and of the proton-transfer nature of acid-base reactions. 

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