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General Chemistry Principles, Patterns, and Applications, 2011

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Figure 17.5 An MRI Image of the Heart, Arteries, <strong>and</strong> Veins<br />

Summary<br />

A complex ion is a species formed between a central metal ion <strong>and</strong> one or more surrounding lig<strong>and</strong>s,<br />

molecules or ions that contain at least one lone pair of electrons. Small, highly charged metal ions have<br />

the greatest tendency to act as Lewis acids <strong>and</strong> form complex ions. The equilibrium constant for the<br />

formation of the complex ion is the formation constant (Kf). The formation of a complex ion by adding<br />

a complexing agent increases the solubility of a compound.<br />

K E Y T A K E A W A Y<br />

<br />

The formation of complex ions can substantially increase the solubility of sparingly<br />

soluble salts if the complex ion has a large K f.<br />

C O N C E PTUAL P R OBLEMS<br />

1. What is the difference between K eq <strong>and</strong> K f ?<br />

2. Which would you expect to have the greater tendency to form a complex ion: Mg 2+ or Ba 2+ ? Why?<br />

3. How can a lig<strong>and</strong> be used to affect the concentration of hydrated metal ions in solution? How is K sp affected?<br />

Explain your answer.<br />

4. Co(II) forms a complex ion with pyridine (C 5 H 5 N). Which is the Lewis acid, <strong>and</strong> which is the Lewis base? Use<br />

Lewis electron structures to justify your answer.<br />

N U M E R I C A L PR O BL E M S<br />

1. Fe(II) forms the complex ion [Fe(OH) 4 ] 2− through equilibrium reactions in which hydroxide replaces water in a<br />

stepwise manner. If log K 1 = 5.56, log K 2 = 4.21, log K 3 = −0.10, <strong>and</strong> log K 4 = −1.09, what is K f ? Write the<br />

equilibrium equation that corresponds to each stepwise equilibrium constant. Do you expect the<br />

[Fe(OH) 4 ] 2− complex to be stable? Explain your reasoning.<br />

2. Zn(II) forms the complex ion [Zn(NH 3 ) 4 ] 2+ through equilibrium reactions in which ammonia replaces<br />

coordinated water molecules in a stepwise manner. If log K 1 = 2.37, log K 2 = 2.44, log K 3 = 2.50, <strong>and</strong> log K 4 =<br />

2.15, what is the overall K f ? Write the equilibrium equation that corresponds to each stepwise equilibrium<br />

constant. Do you expect the [Zn(NH 3 ) 4 ] 2+ complex to be stable? Explain your reasoning.<br />

3. Although thallium has limited commercial applications because it is toxic to humans (10 mg/kg body weight is<br />

fatal to children), it is used as a substitute for mercury in industrial switches. The complex ion [TlBr 6 ] 3− is<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

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