Ionic product constant of water

Tabie 16.1 The ionic product constant of water at various temperatures... 

The ionic product constant of water (K ) may be used to calculate the hydroxide ion concentration in solutions of acids. It may also be used to calculate the hydronium ion concentration in solutions of bases. 

A wide variety of acid—base equilibria problems can be solved using the relationships between pH, hydrogen ion concentration and hydroxide ion concentration in conjunction with the ionic product constant of water. 

The relationship between temperature and the ionic product constant of water... 

At 60°C, the ionic product constant of water is 9.55 x 10" moFdm . Calculate the pH of a neutral solution at this temperature. 

This table gives values of pKw on a molal scale, where Kw is the ionic activity product constant of water. Values are from W. L. Marshall and E. U. Franck, 7. Phys. Chem. Ref. Data, 10 295 (1981). 

For pure water, [H+] = [OH ] and pH = 7. Any solution with pH = 7 is by definition a neutral solution. No matter what other solutes occur in a given solution, the product of hydrogen and hydroxide ion activities will always be 1CT14 at 25 °C. This may be noted that the value of this equilibrium constants alter with temperature, as do all equilibrium constants. For this reason at 230 °C, K = 10 11/1 and a neutral solution would have a pH of 5.7. This brief diversion specifically focusing attention on the ionic compositional aspects of water is quite relevant with regard to its role played as a leaching agent. 

The product of these two concentrations is known as the ionization constant of water, Kyf (or as the ionic product of water, or maybe sometimes as the autoprotolysis constant, Kap)... 

Significantly, the ionic product (dissociation constant) of water also is greatly influenced by temperature, increasing by three orders of magnitude from 1013-99 at 25 C to 10"-30 at 300 C (P). Water is therefore a much stronger acid and base at elevated temperatures than at ambient. These properties of water can be exploited with the microwave reactors, so biomimetic reactions that would normally be acid-catalyzed, can be earned out in the absence of added acidulant. 

Table 2.4 Thermodynamic constant of water ionic product (-temperature range 0-800 C and pressme range 0.1-500 MPa Shvarov, 1992).

Figure 3.2b shows the variation of ionic product or autodissociation constant of water, K, with temperature, where = [H ][OH Values of log initially increase with temperature from -13.9 at 25 °C up to a maximum value of -11.01 at 250 °C and then steadily decrease until the critical point, after which there is a substantial fall in to below -22.4. The increase in thermal energy upon heating results in two competing effects i) increased heterolytic scission of H2O into H and OH thereby increasing and ii) the reduction in dielectric constant as a result of the disruption of... 

Similarly, the solubility product constants of ionic precipitates (substances chemists usually describe as insoluble) are also extremely small. An apparently insoluble substance is never totally insoluble. In actuality, an equilibrium occurs in which there are an extremely small number of ions present. An example of this is Mg3(P04)2. If you were to drop a few grains of this solid into water and shake, it would appear to not dissolve. However, small... 

The stoichiometric stability constants, as defined by Eqs. (2.6) and (2.8), are related through the protolysis constant of water relevant to the experimental conditions (ionic strength and temperature) used to study the stoichiometric reaction. Stability constants that include an asterisk denote conditions where water is used as the reactant and no asterisk is used to denote where hydroxide ion is the reactant (i.e. see Eqs. (2.6) and (2.8)). Equation (2.5) can also be used to denote the reaction for the protolysis of water. In this circumstance, / = 0 and = 1 in the equation, with the right-hand product of the reaction being the hydroxide ion. Stepwise hydrolysis reactions for monomeric species can be written as... 

The typical strong acid of the water system is the hydrated proton H30+, and the role of the conjugate base is minor if it is a sufficiently weak base, e.g. Cl-, Br-, and C104. The conjugate bases have strengths that vary inversely as the strengths of the respective acids. It can easily be shown that the basic ionisation constant of the conjugate base KR canj is equal to Kw/KA conj, where Kw is the ionic product of water. 

The ionic product varies with the temperature, but under ordinary experimental conditions (at about 25 °C) its value may be taken as 1 x 10 14 with concentrations expressed in molL-1. This is sensibly constant in dilute aqueous solutions. If the product of [H + ] and [OH-] in aqueous solution momentarily exceeds this value, the excess ions will immediately combine to form water. Similarly, if the product of the two ionic concentrations is momentarily less than 10-14, more water molecules will dissociate until the equilibrium value is attained. 

The hydrolysis constant is thus related to the ionic product of water and the ionisation constant of the acid. Since Ka varies slightly and Kw varies considerably with temperature, Kh and consequently the degree of hydrolysis will be largely influenced by changes of temperature. 

In aqueous solutions, H+ and OH ions are present, owing to the dissociation of water molecules. In dilnte solntions, the activity of water is constant. Hence, for the activities of these ions an eqnation of the type (3.17) is obeyed, too. The ionic product... 

Kw ionic product of water Ka = dissolution constant of weak acid ... 

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