Autoprotolysis constants

The dissociation, or autoprotolysis constant for a solvent, SH, relates the concentration of the protonated solvent, SH2, to that of the deprotonated solvent, S . For amphoteric solvents, which can act as both proton donors and proton acceptors, the autoprotolysis reaction is... 

The AutoprotolySis of Methanol. The table gives the value log K = —16.6 for the autoprotolysis constant at 25°C. From this value we find... 

In Table 38 the autoprotolysis constant of sulfuric acid is given by — log K = 3.1, from which we calculate... 

In dilute aqueous solutions (the only ones we consider in this chapter), the solvent, water, is very nearly pure, and so its activity may be taken to be 1. The resulting expression is called the autoprotolysis constant of water and is written Kw ... 

STRATEGY When Ba(OH)2 dissolves in water, it provides OH ions most hydroxides of Groups 1 and 2 can be treated as fully dissociated in solution. Decide from the chemical formula how many OH ions are provided by each formula unit and calculate the concentrations of these ions in the solution. To find the concentration of H,Oj ions, use the water autoprotolysis constant Kw = [H,0 1 [OH ]. 

The values of pH and pOH are related. To find that relation, we start with the expression for the autoprotolysis constant of water Kw = [H3Oh [Of I ]. Then we take logarithms of both sides ... 

Now consider a very dilute solution of a strong base, such as NaOH. Apart from water, the species present in solution are Na+, OH, and H30+. As we did for HCl, we can write down three equations relating the concentrations of these ions by using charge balance, material balance and the autoprotolysis constant. Because the cations present are hydronium ions and sodium ions, the charge-balance relation is... 

The calculation of pH for very dilute solutions of a weak acid HA is similar to that for strong acids in Section 10.18. It is based on the fact that, apart from water, there are four species in solution—namely, HA, A, H,0 +, and OH. Because there are four unknowns, we need four equations to find their concentrations. Two relations that we can use are the autoprotolysis constant of water and the acidity constant of the acid HA ... 

The pK for the autoprotolysis (more precisely, the autodeuterolysis, because a deuteron is being transferred) of heavy water (D20) is 15.136 at 20.°C and 13.8330 at 30.°C. Assuming AH° for this reaction to be independent of temperature, calculate A.Sr°for the autoprotolysis reaction. Suggest an interpretation of the sign. Suggest a reason why the autoprotolysis constant of heavy water differs from that of ordinary water. 

Given that the autoprotolysis constants of ethanol and methanol are 10 191 and 10-16-77, respectively, the background kQbs values for the lyoxide-catalyzed ethanolysis and methanolysis reactions of paraoxon at pH 7.3 (ethanol) and 8.3 (methanol) are 8.1 x 10-15 and 3.5 x 10 us 1, respectively... 

As with all physicochemical processes, the extent of Equation (6.2) may be quantified by an equilibrium constant K. We call it the autoprotolysis constant, as defined by... 

The water term in the denominator of Equation (6.3) is always large when compared with the other two concentrations on the top, so we say it remains constant. This assumption explains why it is rare to see the autoprotolysis constant written as Equation (6.3). Rather, we usually rewrite it as... 

Table 6.2 Values of the autoprotolysis constant Kw as a function of temperature...

Water dissociates to form ions according to Equation (6.2). The ionic product of the concentrations is the autoprotolysis constant Kw, according to Equation (6.4). Taking logarithms of Equation (6.4) yields ... 

A medicine or skin lotion is often described as pH neutral as though it was obviously a good thing. A solution is defined as neutral if it contains neither an excess of solvated protons nor an excess of hydroxide ions. Equation (6.4) tells us the autoprotolysis constant Kw of super-pure water (water containing no additional solute) is 10-14 (moldm-3)2. Furthermore, we saw in Worked Example 6.1 how the concentration of the solvated protons was 10-7 mol dm-3 at 298 K. 

Blood plasma is that part of the blood remaining after removal of the haemoglobin cells that impart a characteristic blood-red colour. According to Table 6.4, most people s plasma has a pH in the range 7.3-7.5. So, what is the concentration of solvated protons in such plasma We met the autoprotolysis constant Kw in Equation (6.4). Although we discussed it in terms of super-pure water, curiously the relationship still applies to any aqueous system. The product of the concentrations of solvated protons and hydroxide ions is always 10 14 at 298 K. 

Kw autoprotolysis constant of water AC/ change in internal energy, e.g. during... 

As a result, inert and aprotic solvent toluene is suitable for the titration of weak bases in non-aqueous media as solvent, although benzene which is more carcino-genic aromatic hydrocarbon used widely in literature for non-aqueous titrations. The major advantage of toluene is tliat it does not compete for protons with the reactant in the titrations because of its autoprotolysis constant approaching zero. The major disadvantages of solubility can be removed by using small amount of amphiprotic solvents. 

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