Polyprotic acids dissociation

Acids that contain more than one dissociable proton are called polyprotic acids. Polyprotic acids dissociate in a stepwise manner, and each dissociation step is characterized by its own acid-dissociation constant, Kal, Ka2, and so forth. For example, carbonic acid (H2C03), the diprotic acid that forms when gaseous carbon dioxide dissolves in water, is important in maintaining a constant pH in human blood. It undergoes the following dissociation reactions ... 

Monoprotic weak acids, such as acetic acid, have only a single acidic proton and a single acid dissociation constant. Some acids, such as phosphoric acid, can donate more than one proton and are called polyprotic weak acids. Polyprotic acids are described by a series of acid dissociation steps, each characterized by it own acid dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions and acid dissociation constants. 

Polyprotic bases, like polyprotic acids, also have more than one base dissociation reaction and base dissociation constant. 

A more challenging problem is to find the pH of a solution prepared from a polyprotic acid or one of its conjugate species. As an example, we will use the amino acid alanine whose structure and acid dissociation constants are shown in Figure 6.11. 

From a chemical point of view, the common amino acids are all weak polyprotic acids. The ionizable groups are not strongly dissociating ones, and the degree of dissociation thus depends on the pH of the medium. All the amino acids contain at least two dissociable hydrogens. 

When a polyprotic acid is dissolved in water, the various hydrogen atoms undergo ionisation to different extents. For a diprotic acid H2A, the primary and secondary dissociations can be represented by the equations ... 

Buffer mixtures are not confined to mixtures of monoprotic acids or monoacid bases and their salts. We may employ a mixture of salts of a polyprotic acid, e.g. NaH2P04 and Na2HP04. The salt NaH2P04 is completely dissociated ... 

Polyprotic acids (or mixtures of acids, with dissociation constants AT, K2, and AT3) and strong bases. The first stoichiometric end point is given approximately... 

The theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

It will be noted that there is a factor of approximately 105 between successive dissociation constants. This relationship exists between the equilibrium constants for numerous polyprotic acids, and it is sometimes known as Pauling s rule. This rule is also obeyed by sulfurous acid, for which ffj = 1.2 X 10 2 and K2 = 1 X 10 7. 

For multiple equilibria dissociation constants (such as polyprotic acids), K for the overall reaction is the product of the equilibrium constants for the individual reactions. Therefore,... 

Titrations curves for polyprotic acids have an inflection point for each hydrogen in the formula if the dissociation constant (Ka) for each hydrogen is very different from the others and if any dissociation constant is not too small. The titration curves of the polyprotic acids H2S04 and H3P04 are shown in Figures 5.6 and 5.7. Sulfuric acid has essentially one inflection point (like hydrochloric acid—compare with Figure 5.1(a)), while phosphoric acid has two apparent inflection points. Both hydrogens on the... 

For polyprotic acids, acids that can donate more than one proton, the IQ for the first dissociation is much larger than the IQ for the second dissociation. If there is a third IQ, it is much smaller still. For most practical purposes you can simply use the first IQ. 

Acids such as HCl, CH3COOH, and HF are monoprotic acids. They have only a single hydrogen atom that dissociates in water. Some acids have more than one hydrogen atom that dissociates. These acids are called polyprotic acids. For example, sulfuric acid has two hydrogen atoms that can dissociate. 

Sulfuric acid is a far stronger acid than the hydrogen sulfate ion, because much more energy is required to remove a proton from a negatively charged ion. The strength of a polyprotic acid decreases as the number of hydrogen atoms that have dissociated increases. 

As you know, polyprotic acids have more than one hydrogen atom that dissociates. Each dissociation has a corresponding acid dissociation constant. How can you calculate the pH of a solution of a polyprotic acid ... 

Problems that involve polyprotic acids can be divided into as many sub-problems as there are hydrogen atoms that dissociate. The ion concentrations that are calculated for the first dissociation are substituted as initial ion concentrations for the second dissociation, and so on. You can see this in the following Sample Problem. 

All polyprotic acids, except sulfuric acid, are weak. Their second dissociation is much weaker than their first dissociation. For this reason, when calculating [HsO" ] and pH of a polyprotic acid, only the first dissociation needs to be considered. The calculation is then the same as the calculation for any weak monoprotic acid. In the Sample Problem, [HP04 ] was found to be the same as the second dissociation constant, Ka. The concentration of the anions formed in the second dissociation of a polyprotic acid is equal to... 

The only common strong polyprotic acid is sulfuric acid. Nevertheless, it is strong only for the first dissociation. Like the second dissociation of other polyprotic acids, the second dissociation of sulfuric acid is weak. 

The standard notation for successive acid dissociation constants of a polyprotic acid is Kt, K2, K2, and so on, with the subscript a usually omitted. We retain or omit the subscript as dictated by clarity. For successive base hydrolysis constants, we retain the subscript b. The preceding examples illustrate that Kal (or K ) refers to the acidic species with the most protons, and Kbl refers to the basic species with the least number of protons. Carbonic acid, a very important diprotic carboxylic acid derived from COz, is described in Box 6-4. 

The definition of pH is pH = —log[H+] (which will be modified to include activity later). Ka is the equilibrium constant for the dissociation of an acid HA + H20 H30+ + A-. Kb is the base hydrolysis constant for the reaction B + H20 BH+ + OH. When either Ka or Kb is large, the acid or base is said to be strong otherwise, the acid or base is weak. Common strong acids and bases are listed in Table 6-2, which you should memorize. The most common weak acids are carboxylic acids (RC02H), and the most common weak bases are amines (R3N ). Carboxylate anions (RC02) are weak bases, and ammonium ions (R3NH+) are weak acids. Metal cations also are weak acids. For a conjugate acid-base pair in water, Ka- Kb = Kw. For polyprotic acids, we denote the successive acid dissociation constants as Kal, K, K, , or just Aj, K2, A"3, . For polybasic species, we denote successive hydrolysis constants Kbi, Kb2, A"h3, . For a diprotic system, the relations between successive acid and base equilibrium constants are Afa Kb2 — Kw and K.a Kbl = A w. For a triprotic system the relations are A al KM = ATW, K.d2 Kb2 = ATW, and Ka2 Kb, = Kw. 

The equilibrium constant, K , is called hnc stria dissociation constant. Similarly for a polyprotic acid (i.e. phosphoric acid), the equilibrium reactions are f ... 

Acetic acid, CH3C02H, contains four protons, only one of which is dissociable. Hence, acetic acid is not a polyprotic acid. 

Polyprotic acid solutions contain a mixture of acids—H2A, HA-, and H20 in the case of a diprotic acid. Because H2A is by far the stronger acid, the principal reaction is dissociation of H2A, and essentially all the H30+ in the solution comes from the first dissociation step. Worked Example 15.11 shows how calculations are done. 

Polyprotic acids contain more than one dissociable proton and dissociate in a stepwise manner. Because the stepwise dis-... 

The extent of the second dissociation is so small that it does not either lower [HS-] or raise [H+] as calculated from the first dissociation significantly. Notice that in a solution of a polyprotic acid the concentration of the conjugate base resulting from the second dissociation is equal to K2. This result is general whenever the extent of the second dissociation is less than 5% (an application of the five percent rule). 

Polyprotic acids have unique values for each dissociation Ka 1, Ka2, etc. 

H3PO4 is a polyprotic acid. It may further dissociate to transfer more than one... 

The correct answer is (D). The first thing to notice here is that H3P04 is a polyprotic acid. Each mole will yield three moles of H+ ions. In 0.020 L of 0.10 M solution, there are 2.0 X 10 3 moles of H3P04. After complete dissociation, this will yield 6.0 X 10 3 moles of... 

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