Acid equilibrium ionisation constant

Different equilibrium dissociation constant and equilibrium ionisation constant equations may then be derived with reference to the kinetic scheme (Scheme 8.16) on the basis of stasis, such as Equation (8.93), which dehnes an equilibrium dissociation constant equilibrium ionisation constant for acidic functional group 1, or 8.95 that defines the equilibrium ionisation constant K di for basic functional group 2 ... 

In contrast, equilibrium properties have been successfully discussed in terms of the field effect. Notable instances are those of the ionisation constants of saturated dibasic acids, - and of carboxyl groups held in... 

K is the equilibrium constant at a particular temperature and is usually known as the ionisation constant or dissociation constant. If 1 mole of the electrolyte is dissolved in Vlitres of solution (V = l/c, where c is the concentration in moles per litre), and if a is the degree of ionisation at equilibrium, then the amount of un-ionised electrolyte will be (1 — a) moles, and the amount of each of the ions will be a moles. The concentration of un-ionised acetic acid will therefore be (1 — a)/ V, and the concentration of each of the ions cl/V. Substituting in the equilibrium equation, we obtain the expression ... 

Weak acids are not completely ionised in aqueous solution and are in equilibrium with the undissociated acid, as is the case for water, which is a very weak acid. The dissociation constant Ka is given by the expression below ... 

AG° can also, of course, be defined in terms of Ka, the equilibrium constant written in terms of activities, a. [Note This symbol should not be confused with that for the acid ionisation constant, also Ka], In this present case the standard state defined for A G° is that associated with unit activities (a = 1) rather than with unit pressure ( P° = 1 bar), or in the case where gases are involved, with unit fugacities (/ = 1). We can then write that ... 

Ill] A. Albert and E. P. Serjeant The Determination of Ionisation Constants - A Laboratory Manual, 3rd ed., Chapman and Hall, London, New York, 1984. [112] D. D. Perrin, B. Dempsey, and E. P. Serjeant pK -Prediction for Organic Acids and Bases, Chapman and Hall, London, 1981. [113] A. Streitwieser, E. Juaristi, and L. L. Nebenzahl Equilibrium Carbon Acidities in Solution, in E. Buncel and T. Durst (eds.) Comprehensive Carbanion Chemistry, Part A, p. 323ff., Elsevier, Amsterdam, 1980. [114] J. H. Futrell Gaseous Ion Chemistry and Mass Spectrometry, Wiley-lnterscience, New York, 1986. [115] R. T. Mclver Chemical Reactions without Solvation, Scientific... 

In the case of an acid dissociation, the equilibrium constant for the reaction is termed Ka, and is called the ionisation constant, the dissociation constant or, sometimes, the acidity constant. The above equation can now be rewritten as... 

Ka is a constant for a given compound at a given temperature. Clearly, the farther the above equilibrium lies to the right-hand side, the more completely the acid will ionise and the greater will be the value of JCa. 

Let us consider the proton transfer from acetone to a series of bases [3] and compare the logarithm of the rate constant with the pK of the conjugate acid of the base. A linear relationship is observed (Fig. 1) which is an example of a Bronsted correlation. If we had plotted log kf (a measure of the free-energy difference between ground and transition state) versus log k /kf (a measure of the free energy of the reaction) it is clear intuitively that the transition state would be product-like for a slope of unity and reactant-like for zero slope. It is difficult to measure equilibrium constants such as in Eqn. 1 but ionisation constants are easily estimated (using pH-titration equipment for example) so that the majority of comparisons are with these. Inspection of Eqns. 1 and 2 shows that the only identities are the base and the acid comparison of oxonium ion with acetone and water with the conjugate base of acetone is doubtful. 

So far the four metal ions have been compared with respect to their effect on (1) the equilibrium constant for complexation to 2.4c, (2) the rate constant of the Diels-Alder reaction of the complexes with 2.5 and (3) the substituent effect on processes (1) and (2). We have tried to correlate these data with some physical parameters of the respective metal-ions. The second ionisation potential of the metal should, in principle, reflect its Lewis acidity. Furthermore the values for Iq i might be strongly influenced by the Lewis-acidity of the metal. A quantitative correlation between these two parameters... 

ApA < 1. In Fig. 2 the region of curvature is much broader and extends beyond — 4 < ApA < + 4. One explanation for the poor agreement between the predictions in Fig. 3 and the behaviour observed for ionisation of acetic acid is that in the region around ApA = 0, the proton-transfer step in mechanism (8) is kinetically significant. In order to test this hypothesis and attempt to fit (9) and (10) to experimental data, it is necessary to assume values for the rate coefficients for the formation and breakdown of the hydrogen-bonded complexes in mechanism (8) and to propose a suitable relationship between the rate coefficients of the proton-transfer step and the equilibrium constant for the reaction. There are various ways in which the latter can be achieved. Experimental data for proton-transfer reactions are usually fitted quite well by the Bronsted relation (17). In (17), GB is a... 

The same approach can be applied not only to the bulk equilibrium constants (K) but also to the microscopic connection processes (given the symbol k). Recall that the macroscopic equilibrium constant is simply the sum of all the microscopic equilibrium constants. For example, if an acid (H2A) has two non-equivalent ionisable protons there are two distinct but equivalent ways to remove a proton to produce HA- and hence there are two microscopic equilibrium constants kx and k2) for this deprotonation process. Thus the macroscopic acid dissociation constant, K = k1+k2. Don t get confused between microscopic equilibrium constants and rate constants, both of which have the symbol k. So, in terms of... 

Ethanoic acid on the left hand of the equation is called the free acid, whereas the carboxylate ion formed on the right hand side is called its conjugate base. The extent of ionisation or dissociation is given by the equilibrium constant (Keq) ... 

If the ionisation of a weak acid is represented as described above, we may express an equilibrium constant as follows ... 

Another source of confusion concerning strengths of acids arises with Ka and pKa. The term Ka is the dissociation constant for the ionisation of an acid, and hence the larger the value of Ka, the stronger is the acid (since the equilibrium constant lies farther to the right-hand side). 

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