Relative Strengths of Acids and Bases

A weak acid only partially dissociates in aqueous solution and therefore exists in the solution as a mixture of the undissociated acid and its conjugate base. The conjugate base of a weak acid shows a slight ability to remove protons from water. The conjugate base of a weak acid is a weak base.) 

The ions H30 (oq) and OH aq) are, respectively, the strongest possible acid and strongest possible base that can exist at equilibrium in aqueous solution. Stronger acids react with water to produce H30 (fl ) ions, and stronger bases react with water to produce OH aq) ions, a phenomenon known as the leveling effect. 

Given that HCIO4 is a strong acid, how would you classify the basicity of CIO4  

We can think of proton-transfer reactions as being governed by the relative abilities of two bases to abstract protons. For example, consider the proton transfer that occurs when an acid HA dissolves in water  

If H2O (the base in the forward reaction) is a stronger base than AT (the conjugate base of HA), it is favorable to transfer the proton from HA to H2O, producing H3O and A . As a result, the equilibrium lies to the right. This describes the behavior of a strong acid in water. For example, when HCl dissolves in water, the solution consists almost entirely of H30 and Cl ions with a negligible concentration of HCl molecules  

The inverse relationship between the strengths of acids and their conjugate bases is illustrated in FIGURE 16.3. Here we have grouped acids and bases into three broad categories based on their behavior in water  

If ions are added to water, what reaction, if any, occurs  

Strong acids — H2SO4 HN03 H30+(c 7) HS04 H3PO4 HF 

Section 9.3 distinguished between the relatively few strong acids and the many weak acids. Strong acids are those that ionize almost completely, whereas weak acids ionize but slightly. Hydrochloric acid is a strong acid 0.10 M HCl is almost 100% ionized. Acetic acid is a weak acid only 1.3% of the molecules ionize in 0.10 M HC2H3O2. 

In a Br0nsted-Lowry sense an acid behaves as an acid by losing protons. The more readily protons are lost, the stronger the acid. A base behaves as a base by gaining protons. The stronger the attraction for protons, the stronger the base. 

Look at the ionization equations for hydrochloric and acetic acids, one written above the other, with the stronger acid first  

Use Table 17.1 to list the following acids in order of decreasing strength (strongest first) HC2O4-, NH4+, H3PO4. 

Find the three acids among those shown in the table and list them from the strongest (first) to the weakest (last). 

Using a table of acid-dissociation constants is the surest way to quantify relative strengths of weak acids, but you can often classify acids and bases qualitatively as strong or weak just from their formulas  

2 Oxoacids in which the number of O atoms exceeds the number of ionizable protons by two or more, such as HNO3, H2SO4, and HCIO4 for example, in H2SO4, 4 O s - 2 H s = 2 

Weak acids. There are many more weak acids than strong ones. Four types, with examples, are 

Carboxylic acids (general formula RCOOFI, with the ionizable proton shown in red), such as CH3COOH and CgHsCOOH 

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This is always the case for any two acids, and by measuring the positions of the equilibrium the relative strengths of acids and bases can be determined. Of course, if the two acids involved are close to each other in strength, a measurable reaction will occur from both sides, though the position of equilibrium will still be over to the... 

EXAMPLE 10.3 Assessing the relative strengths of acids and bases... 

We can use the information in Tables 10.1 and 10.2 to determine the relative strengths of acids and bases in solution. For example, suppose we need to decide which member of (a) HF or HI03 (b) NOz or CN is the stronger acid or base in water. We need to know that the higher the K, of a weak acid, the stronger the acid and the weaker its conjugate base. Similarly, the... 

Long before we ever took a chemistry course we all had some knowledge of the relative strengths of acids and bases. If you asked any of your friends if they... 

These equations represent a transfer of a proton from A, (Acid,) to B2 (Base2). Reactions between acids and bases are hence termed protolytic reactions. All these reactions lead to equilibrium, in some cases the equilibrium may be shifted almost completely in one or another direction. The overall direction of these reactions depends on the relative strengths of acids and bases involved in these systems. 

THE STABILITY OF COMPLEXES In the previous section hints were made about the differences in stabilities of various complexes. In order to be able to make more quantitative statements and comparisons, a suitable way has to be found to express the stability of complexes. The problem in many ways is similar to that of expressing the relative strength of acids and bases. This was done on the basis of their dissociation constants (cf. Section 1.16), obtained by applying the law of mass action to these dissociation equilibria. A similar principle can be applied for complexes. 

Table 19.1 Relative Strengths of Acids and Bases and meir Conjugates...

When a weak acid is titrated with a weak base, the titration curve shows a continuously and gradually changing pH with the addition of base. No region with a sharp shift in pH is obtained for small additions of titrant. If a sharp shift occurs, it is still less than 2 pH units and not detectable by indicators. On both sides of the equivalence point, buffers are present, and at the equivalence point, the pH depends on the relative strengths of acid and base. 

Usually these reactions have been studied in water or in other protic solvents such as the alcohols. Thus, the acid-base properties of the solvent are important in determining the relative strength of acids and bases which are solutes in water. This leads to the definition of two other types of proton transfer reaction, namely, the protolysis reaction,... 

It should be noted that the terms acid and base in the Brpnsted-Lowry theory refer only to the function of the given substance in a protolytic reaction. The same substance under certain conditions may react as an acid, and in other cases it may demonstrate basic properties. Also, the relative strength of acids and bases is considerably affected by solvation of the reagents and reaction products. 

Ka and Kb are particular types of equilibrium constants that give us an idea of the relative strengths of acids and bases, respectively. The acid-dissociation constant, Ka, is the equilibrium constant for the ionization of a weak acid to a hydrogen ion and its conjugate base. Likewise, the base-dissociation constant, Kb, is the equilibrium constant for the addition of a proton to a weak base by water to form its conjugate acid and an OH ion. 

Figure 3 Relative strengths of acids and bases on the proton ladder ...

Chapter 6 has been reorganized to highlight contemporary aspects of acid-base chemistry and to include a broader range of measures of relative strengths of acids and bases. 

The double arrow in the acetic acid ionization reaction indicates that the dissociation is incomplete, that is, a certain amount of undissociated acetic acid exists in solution with the dissociated acetic add. Acetic acid is called a weak add because it ionizes only to a limited extent in water. HCl and HNO3 are strong acids because they are, for all practical purposes, completely ionized in water. The relative strengths of acids and bases are discussed in detail in Section 11.2. 

Since interaction with the solvent causes such complications, especially for alkyl substitution near the acid-base site, it is natural to ask whether any information can be obtained about the relative strengths of acids and bases in the gas phase. Measurements of thermal equilibria are not possible because of the very low equilibrium concentrations of ionic species, but many ions of interest can be generated by electron bombard-... 

By measuring the position of the equilibrium, the relative strengths of acids and bases can be determined and a sequence of acid-base strengths can be established. Such measurements give the so-called thermodynamic acidity. Acid strengths (relative to water) of some important acids are presented in Table 1 with their pl a values listing the acids in decreasing order of acidity (7). 

By comparing various acid-base reactions, you can construct a table of relative strengths of acids and bases (see Table 16.2). To see how you might do this, look again at what we have called a strong acid. Recall that an acid is strong if it completely ionizes in... 

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