Water as an acid and a base

A substance is said to be amphoteric if it can behave either as an acid or as a base. Water is the most common amphoteric substance. We can see this clearly in the ionization of water, which involves the transfer of a proton from one water molecule to another to produce a hydroxide ion and a hydronium ion. 

In this reaction one water molecule acts as an acid by furnishing a proton, and the other acts as a base by accepting the proton. The forward reaction for this process does not occur to a very great extent. That is, in pure water only a tiny amount of H3O+ and OH exist. At 25 °C the actual concentrations are 

Notice that in pure water the concentrations of [H30 ] and [OH ] are equal because they are produced in equal numbers in the ionization reaction. 

One of the most interesting and important things about water is that the mathematical product of the H30 and OH concentrations is always constant. We can find this constant by multiplying the concentrations of H3O+ and OH at 25 °C  

To simplify the notation we often write H30 as just Hi . Thus we would 

H A 1 H 0 H—C— Most acids are oxyacids, in which the acidic hydrogen is attached to an oxygen atom (several oxyacids are shown in the margin). The strong acids we have mentioned, except hydrochloric acid, are typical examples. Organic acids, those with a carbon-atom backbone, commonly contain 

Nitrous acid Hypochlorous acid ypg re usually weak. An example is acetic acid, CH3COOH, 

There are some important acids in which the acidic proton is attached to an atom other than oxygen. The most significant of these are the hy-drohalic acids HX, where X represents a halogen atom. Examples are HCl(fl j), a strong acid, and HF(a j), a weak acid. 

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Water as an Acid and a Base Water can donate and can also accept proton, hence it can act as an add and a beise. 

Dissociation of Water Water is an amphiprotic solvent in that it can serve as an acid or a base. An interesting feature of an amphiprotic solvent is that it is capable of reacting with itself as an acid and a base. 

Water-free nitric acid is amphoteric, ie, it acts both as an acid and a base, or better as an electron donor or electron acceptor. This view, already suggested in the early Hantzsch papers, was supported by Walden (Ref 14) and later by Dalmon (Ref 30). Then Usanovich (Ref 25) demonstrated that nitric acid acts as a base with sulfuric acid and as an acid with water. 

Water reacts both as an acid and a base. With bases it reacts as an acid NH3(aq) + H20(l) o NH4+(aq) + OH and with acids it reacts as a base ... 

If water molecules interact with each other as an acid and a base, autoprotolysis (autoionization) occurs, as in Eq. (3.7) ... 

A substance which can behave both as an acid and a base is known as amphoteric. In water such a substance would yield both H+ and OH- ions, but the product of the concentrations of these ions could not exceed the ion product of water ... 

A certain sodium compound is dissolved in water to liberate Na+ ions and a particular negative ion. What evidence would you look for to determine whether the anion is behaving as an acid or a base (without measuring the pH of the solution) Explain how the anion could behave simultaneously as an acid and a base. 

From the table, the bases between water and the hydroxide ion are the weak bases. They are the ones that ionize to 10%. Note that in the case of bases, the other boundary limit is demarcated by the hydroxide ion rather than the hydronium ion. This is so, because the hydronium ion is not a base thus, it cannot form as a boundary for the bases. One the other hand, H2O is both an acid and a base. Thus, it consistently forms as a boundary limit in both the acids and the bases. Compounds that act both as an acid and a base are called amphoteric substances. H2O is an amphoteric substance. Above water in the table, the compounds do not exhibit any observable basic behavior. Water, then, is the very limit of basicity. Notice the arrows pointing downward from the weakest to the strongest bases. 

Definition of Acids and Bases. —The old definitions of an acid as a substance which yields hydrogen ions, of a base as one giving hydroxyl ions, and of neutralization as the formation of a salt and water from an acid and a base, are reasonably satisfactory for aqueous solutions, but there are serious limitations when non-aqueous media, such as ethers, nitro-compounds, ketones, etc., are involved. As a result of various studies, particularly those on the catalytic influence of un-ionized molecules of acids and bases and of certain ions, a new concept of acids and bases, generally associated with the names of Br nsted and of Lowry, has been developed in recent years. According to this point of view an acid is defined as a substance with a tendency to lose a proton, while a base is any substance with a tendency to gain a proton the relationship between an acid and a base may then be written in the form... 

The reactions of salts in water become more complicated if the salt in question is amphiprotic, i.e. can function both as an acid and a base. Examples of amphiprotic anions are bicarbonate (sometimes called hydrogen-carbonate), HCOy, and bisulfite (or hydrogensulfite), HSOj. These species can donate or accept H+ ions in solution. 

Here the ammonium ion is the- conjugate acid of the base NH3, and H2O is the conjugate acid of the base OH . The fact that H2O is now acting as an acid, whereas in the HA ionization it acts as a base, is of particular importance. On account of its ability to act both as an acid and a base, H2O is said to be oinphoteric (Greek amphoteros, in both ways). In the self-ionization of water one molecule is acting as an acid.and another as a base, as is clear if we write the reaction as... 

Some substances, called amphoteric substances, can act both as an acid and a base. The simplest example is water. Water can split apart to form a hydrogen ion and a hydroxide ion ... 

Kanzaki et al. [89] reported EAN composed of C2H5NH3+ and NO3 ions, which behave as an acid and a base, respectively, and reported that H3O+ is a stronger acid than HNO3 in an EAN solution, imlike in water. 

You must recognize these substances as an acid and a base. Write the skeletal reaction following the general pattern of acid plus base goes to water plus salt. SOLUTION HN03(a ) -b Ca(OH)2(fl< ) H20(/) + Ca(N03)2(o ) acid base water salt... 

Water is amphoteric it can act as either an acid or a base. Even in pure water, water acts as an acid and a base with itself, a process called self-ionization. 

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