Autoionization, of water

Since water is defined as both an acid and a base (Sec. 17.2), it is not surprising to find that water can react with itself, even though only to a very limited extent, in a reaction called autoionization  

An equilibrium constant for this reaction, called, does not have terms for the concentration of water otherwise it is like the other equilibrium constants considered so far. 

The value for this constant in dilute aqueous solution at 25°C is 1.0 x IQ- . Thus, water ionizes very little when it is pure, and even less in acidic or basic solution. You must remember this value. 

The equation for indicates that there is always some H3O+ and always some OH in any aqueous solution. Their concentrations are inversely proportional. A solution is acidic if the H3O+ concentration exceeds 

EXAMPLE 17.8. Calculate the hydronium ion concentration in pure water at 25°C. 

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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. 

However, generating doubly charged ions would be highly unfavorable compared to autoionization of water, ammonia, or hydrogen fluoride in which proton transfer occurs. 

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. 

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