THE AUTOIONIZATION OF WATER

Water acts as an acid (H donor) in its reaction with NH3, whereas it acts as a base (H acceptor) in its reactions with HCl and with HE 

Water can act either as an acid or as a base, depending on the other species present. 

EXAMPLE 10-1 Identifying Conjugate Acid-Base Pairs 

When an acid ionizes (loses ) the deprotonated product (often anionic) is the conjugate base, while a base that reacts with forms a protonated product that is the conjugate acid. 

In this reaction the H2P04 (aq) is the conjugate base to the H3P04(aq) acid. 

One of the most important chemical properties of water is its ability to act as either a Bronsted-Lowry acid or a Bronsted-Lowry base. In the presence of an acid, it acts as a proton acceptor in the presence of a base, it acts as a proton donor. In fact, one water molecule can donate a proton to another water molecule  

Because the forward and reverse reactions in Equation 16.12 are extremely rapid no water molecule remains ionized for long. At room temperature only about two out of every lO water molecules are ionized at any given instant. Thus, pure water consists almost entirely of H2O molecules and is an extremely poor conductor of electricity. Nevertheless, the auto ionization of water is very important, as we will soon see. 

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Kw ionization constant for the autoionization of water. kelvln the unit of the Kelvin temperature scale. 

Be(OH)2 produces less hydroxide ion than the autoionization of water. 

Since water is amphiprotic (meaning it can act as both an acid by releasing H+ and as a base by releasing OH-), every aqueous solution is characterized by autoionization, the process in which one H20 molecule transfers a proton to another water molecule. The autoionization of water always occurs, whether or not there are other acids or bases in solution. A special K is assigned to the reaction ... 

Note that, as NH3 replaces H20, the ammonium ion occupies the same position as H+ in the autoionization of water. And, of course, the amide ion takes the place of the hydroxide ion. 

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 see this behavior in the autoionization of water, which involves the transfer of a proton from one water molecule to another to produce a hydroxide ion and a hy-dronium ion ... 

Note that the zero value for [H+]0 is an approximation, since we are neglecting the H+ ions from the autoionization of water.)... 

Although the autoionization of water also produces OH- ions, the pH will be determined by the OH- ions from the dissolved NaOH. Thus in the solution... 

The autoionization of water is an endothermic reaction, so increases as the temperature is increased. This temperature dependence is plotted in Fig. 1. 

This reaction is responsible for the autoionization of water, which leads to small but measurable concentrations of hydronium and hydroxide ions at equilibrium. The equilibrium expression for this reaction is... 

The OH contribution from the autoionization of water is negligible here, as was the contribution of HaO from autoionization in the 0.10 M HCl and HCIO4 solutions. When only a very small amount of strong acid or base is added to pure water (for example, 10 mol L ), we have to include the autoionization of water to describe the concentration of hydronium and hydroxide ions accurately. 

The H30 arises both from the ionization of H2CO3 and from the subsequent ionization of HCO3, but because K i > K 2 it is reasonable to ignore the contribution of [H30 ] from the second ionization (as well as from the autoionization of water). These approximations will be checked later in the calculation. 

Suppose the autoionization of water is ignored and the method of Examples 15.3 and 15.4 is used. This gives [H30 ] = 7.9 X 10 M, which of course makes no sense, because it is lower than the concentration of hydronium ion in pure water. HCN is a very weak acid, but it is nonetheless an acid, not a base. 

Thiamine hydrochloride (vitamin Bi hydrochloride, HC12-H17ON4SCI2) is a weak acid with = 3.4 X 10. Suppose 3.0 X 10 g of thiamine hydrochloride is dissolved in 1.00 L of water. Calculate the pH of the resulting solution. [Hint This is a sufficiently dilute solution that the autoionization of water cannot be neglected.)... 

Calculate the pH of a 0.10 M aqueous solution of glycine at 25°C. Hint You may need to take account of the autoionization of water.)... 

Use the data from Table 15.1, together with Le Chatelier s principle, to decide whether the autoionization of water is exothermic or endothermic. 

This is called the autoionization of water. Kw is the equilibrium constant for this reaction. 

The reaction for the autoionization of water is shown below 2H20 -> H30+ + OH ... 

This autoionization (self-ionization) of water is an acid-base reaction according to the Bronsted-Lowry theory. One H2O molecule (the acid) donates a proton to another H2O molecule (the base). The H2O molecule that donates a proton becomes an OH ion, the conjugate base of water. The H2O molecule that accepts a proton becomes an H3O+ ion. Examination of the reverse reaction (right to left) shows that H3O+ (an acid) donates a proton to OH (a base) to form two H2O molecules. One H2O molecule behaves as an acid and the other acts as a base in the autoionization of water. Water is said to be amphiprotic that is, H2O molecules can both donate and accept protons. 

As we saw in Section 4-9, part 1, H30 and OH ions combine to form nonionized water molecules when strong acids and strong soluble bases react to form soluble salts and water. The reverse reaction, the autoionization of water, occurs only slightly, as expected. 

The autoionization of water (Section 10-5) was described in terms of Bronsted-Lowry theory. In Lewis theory terminology, this is also an acid-base reaction. The acceptance of a proton, H, by a base involves the formation of a coordinate covalent bond. 

What does autoionization mean How can the autoionization of water be described as an acid-base reaction What structural feamres must a compound have to be able to undergo autoionization ... 

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