Acid-base properties of salt solutions

A salt is an ionic solid containing a cation other than H and an anion other than OH or cy. When a salt such as NaCl, K2CO3, or AllNOsls dissolves in water, the cation and anion separate from one another. 

To predict whether a given salt solution will be acidic, basic, or neutral, you consider three factors in turn. 

As we pointed out in Section 13.4, certain cations act as weak acids in water solution because of reactions such as 

Essentially all transition metal ions behave like Zn, forming a weakly acidic solution. Among the main-group cations, AP+ and, to a lesser extent, Mg, act as weeik acids. In contrast the cations in Group 1 show little or no tendency to react with water. 

If we say that to classify an ion as acidic or basic in water solution, it must change the pH by more than 0.5 unit in 0.1 M solution, then the cations derived from strong bases are  

Consider a solution of sodium cyanide, NaCN. A 0.1 M solution has a pH of 11.1 and is therefore fairly basic. Sodium cyanide dissolves in water to give Na and CN ions. 

A pH meter measures the pH of solutions of the salts NH4CI,NaCI, and Na2C03. 

Sodium ion, Na, is unreactive with water, but the cyanide ion, CN, reacts to produce HCN and OH . 

From the Br0nsted-Lowry point of view, the CN ion acts as a base, because it accepts a proton from H2O. You can also see, however, that OH ion is a product, so you expect the solution to have a basic pH. This explains why solutions of NaCN are basic. 

The reaction of the CN ion with water is referred to as the hydrolysis of CN . The hydrolysis of an ion is the reaction of an ion with water to produce the conjugate acid and hydroxide ion or the conjugate base and hydronium ion. The CN ion hydrolyzes to give the conjugate acid and OH ion. As another example, consider the ammonium ion, NH4, which hydrolyzes as follows  

Recall that a salt is an Ionic compound formed by the reaction between an acid and a base f Section 4.3]. Salts are strong electrolytes that dissociate completely Into ions. 

In Section 16.7, we saw that the conjugate base of a weak acid acts as a weak Brpnsted base in water. Consider a solution of the salt sodium fluoride (NaF). Because NaF is a strong electrolyte, it dissociates completely in water to give a solution of sodium cations (Na ) and fluoride anions (F ). The fluoride ion, which is the conjugate base of hydrofluoric acid, reacts with water to produce hydrofluoric acid and hydroxide ion  

This is a specific example of salt hydrolysis, in which ions produced by the dissociation of a salt react with water to produce either hydroxide ions or hydronium ions—thus impacting pH. Using our knowledge of how ions from a dissolved salt interact with water, we can determine (based on the identity of the dissolved salt) whether a solution will be neutral, basic, or acidic. Note in the preceding example that sodium ions (Na ) do not hydrolyze and thus have no impact on the pH of the solution. 

Analyze We are asked to determine dissociation constants for F, the conjugate hase of HF, and NH4, the conjugate acid of NH3. 

Plan We can use the tabulated K values for HF and NH3 and the relationship between K and K/, to calculate the ionization constants for their conjugates, F and NH4 . 

Check The respective K values for F and NH4 are listed in Table 16.5, where we see that the values calculated here agree with those in Table 16.5. 

Its conjugate acid is listed in handbooks as having a p T of 4.90. What is the base-dissociation 

Even before you began this chapter, you were undoubtedly aware of many substances that are acidic, such as HNO3, HCl, and H2SO4, and others that are basic, such as NaOH and NH3. However, our discussion up to this point in the chapter has indicated that ions can also exhibit acidic or basic properties. For example, we calculated for NH4 and Kb for F in Sample Exercise 16.17. Such behavior implies that salt solutions can be acidic or basic. Before proceeding with further discussions of acids and bases, let s examine the way dissolved salts can affect pH. 

Student Annotation HCOJ has an ionizable proton arxJ can also act as a Br0nsbed add. However, its tendency to accept a proton is stronger than its tendency to donate a proton  

Student Annotation Remember that for any conjugate acid-base pair (Equation 16.8)  

Up to now you ve seen that cations of weak bases (such as NH4 ) are acidic, anions of weak acids (such as CN ) are basic, anions of polyprotic acids (such as H2P04 ) are often acidic, and small, highly charged metal cations (such as are acidic. Therefore, when salts containing these ions dissolve in water, the pH of the solution is affected. You can predict the relative acidity of a salt solution frran the relative ability of the cation and/or anion to react with water. 

Because nearly aU salts are strong electrolytes, we can assume that any salt dissolved in water is completely dissociated. Consequently, the acid-base properties of salt solutions are due to the behavior of the cations and anions. Many ions react with water to generate H ( q) or OH (oq) ions. This type of reaction is often called hydrolysis. The pH of cm aqueous salt solution can be predicted qualitatively by considering the salt s cations and cuiions. 

Click Coached Problems for a self-study module on hydrolysis. 

Sodium chloride solution. When the NaCI is completely dissolved, the solution will contain only Na+. Cl-, and H20. and it will be neither acidic nor basic. 

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Example 15.15 illustrates how to predict the acid-base properties of salt solutions. EXAMPLE 15.15... 

SECTION 16.9 Acid-Base Properties of Salt Solutions 681... 

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