Dissociation of weak acids

When accurate measurements of the dissociation of weak acids were first made over a wide range of temperature, so many different types of behavior were found that the results could not easily be explained. It was only when the proposal was made to separate the quantum-mechanical part of the energy from the part that is sensitive to temperature, that the mechanism underlying the wide variety of behavior could to a large extent be understood. 

Near room temperature there is scarcely any difference between the two. When a deuteron has been removed from a molecule in D20, the electrostatic energy associated with the negative ion will scarcely differ from that associated with the field of a similar ion in H20 from which a proton has been removed. Furthermore, the energy associated with the electric field surrounding a (D30)+ ion in D20 will scarcely differ from that of the field surrounding a (H30)+ ion in 1I20. We must conclude then that the observed differences between the degrees of dissociation of weak acids in D20 and H20 are due entirely to a difference in the quantum-mechanical forces. 

The concentration of the acetic acid that has not dissociated is considered to be approximately equal to the initial concentration of the acid because the extent of the dissociation of weak acids is small. However, often it is not negligible and then to solve for the equilibrium concentrations, use of the quadratic equation or other special mathematics is needed. 

Although these effects are often collectively referred to as salt effects, lUPAC regards that term as too restrictive. If the effect observed is due solely to the influence of ionic strength on the activity coefficients of reactants and transition states, then the effect is referred to as a primary kinetic electrolyte effect or a primary salt effect. If the observed effect arises from the influence of ionic strength on pre-equilibrium concentrations of ionic species prior to any rate-determining step, then the effect is termed a secondary kinetic electrolyte effect or a secondary salt effect. An example of such a phenomenon would be the influence of ionic strength on the dissociation of weak acids and bases. See Ionic Strength... 

Use the reaction quotient, Q, to explain why the fraction of dissociation of weak acid, HA, increases when the solution is diluted by a factor of 2. 

We began a study of aqueous equilibria in Chapter 15, where we examined the dissociation of weak acids and... 

We ve already discussed the common-ion effect in connection with the dissociation of weak acids and bases (Section 16.2). To see how a common ion affects the position of a solubility equilibrium, let s look again at the solubility of MgF2 ... 

Ka acidity constant for the dissociation of weak acid radiation dose)... 

Typical examples are the dissociation of weak acids either in reaction of hydrogen evolution or regarding their oxidation when only the protonated forms are electroactive (e.g., oxalic acid) as well as the reduction of formaldehyde whose hydrate existing in aqueous solutions is electroinactive. 

Sketch two graphs (a) percent dissociation of weak acid HA versus initial concentration of HA ([HA]o), and (b) H+ concentration versus [HA]0. Explain both. 

This chapter demonstrates the usefulness of BasicBiochemDataS (1). In the future this database can be extended and many reactions can be added to this list of 229 enzyme-catalyzed reactions. At the present time Af is known for the species of 94 reactants. Future measurements of A, // ° and enthalpies of dissociation of weak acids will be used to calculate Af H° of species of more reactants so that the effects of temperature can be calculated for more reactions. When more heat capacities of species have been determined, it will be possible to make calculations of K over wider ranges of temperature. BasicBiochemDataS contains functions of pH and ionic strength for A, G ° of these 229 enzyme-catalyzed reactions so that A, G ° and A, A h can be calculated for these reactions at 298.15 K, pHs in the range 5 to 9, and ionic strengths in the range zero to 0.35 M. The index given in the Appendix lists the EC numbers for reactions that involve these 167 reactants. 

Liquid ammonia has a significant dielectric constant (1.49 D) and is an ionising solvent. Through its lone pair, the molecule is a strong proton acceptor, and the hquid facihtates the extensive dissociation of weak acids (p. 200) thus acetic acid is almost as completely dissociated in liquid ammonia as a mineral acid is in water ... 

---