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An H atom is bonded to an O atom. The O atom has 2 dots above it and 2 dots below it. The O atom is bonded to an A l atom, which has three additional O atoms bonded to it as well. Each of these additional O atoms has 4 dots arranged around it, and is bonded to an H atom. This entire molecule is contained in brackets, to the right of which is a superscripted negative sign.

The equations for the dissolution are:

Al ( OH ) 3 ( s ) Al 3+ ( a q ) + 3 OH ( a q )
Al 3+ ( a q ) + 4 OH ( a q ) Al ( OH ) 4 ( a q )
Net: Al ( OH ) 3 ( s ) + OH ( a q ) Al ( OH ) 4 ( a q )

Mercury(II) sulfide dissolves in a solution of sodium sulfide because HgS reacts with the S 2– ion:

HgS ( s ) Hg 2+ ( a q ) + S 2− ( a q )
Hg 2+ ( a q ) + 2 S 2− ( a q ) HgS 2 2− ( a q )
Net: HgS ( s ) + S 2− ( a q ) HgS 2 2− ( a q )

A complex ion consists of a central atom, typically a transition metal cation, surrounded by ions, or molecules called ligands . These ligands can be neutral molecules like H 2 O or NH 3 , or ions such as CN or OH . Often, the ligands act as Lewis bases, donating a pair of electrons to the central atom. The ligands aggregate themselves around the central atom, creating a new ion with a charge equal to the sum of the charges and, most often, a transitional metal ion. This more complex arrangement is why the resulting ion is called a complex ion . The complex ion formed in these reactions cannot be predicted; it must be determined experimentally. The types of bonds formed in complex ions are called coordinate covalent bonds, as electrons from the ligands are being shared with the central atom. Because of this, complex ions are sometimes referred to as coordination complexes. This will be studied further in upcoming chapters.

The equilibrium constant for the reaction of the components of a complex ion to form the complex ion in solution is called a formation constant ( K f ) (sometimes called a stability constant). For example, the complex ion Cu ( CN ) 2 is shown here:

A Cu atom is bonded to two C atoms. Each of these C atoms is triple bonded to an N atom. Each N atom has two dots on the side of it.

It forms by the reaction:

Cu + ( a q ) + 2 CN ( a q ) Cu ( CN ) 2 ( a q )

At equilibrium:

K f = Q = [ Cu ( CN ) 2 ] [ Cu + ] [ CN ] 2

The inverse of the formation constant is the dissociation constant ( K d ) , the equilibrium constant for the decomposition of a complex ion into its components in solution. We will work with dissociation constants further in the exercises for this section. Appendix K and [link] are tables of formation constants. In general, the larger the formation constant, the more stable the complex; however, as in the case of K sp values, the stoichiometry of the compound must be considered.

Common Complex Ions by Decreasing Formulation Constants
Substance K f at 25 °C
[ Cd ( CN ) 4 ] 2− 3 × 10 18
Ag ( NH 3 ) 2 + 1.7 × 10 7
[ AlF 6 ] 3− 7 × 10 19

As an example of dissolution by complex ion formation, let us consider what happens when we add aqueous ammonia to a mixture of silver chloride and water. Silver chloride dissolves slightly in water, giving a small concentration of Ag + ([Ag + ] = 1.3 × 10 –5 M ):

AgCl ( s ) Ag + ( a q ) + Cl ( a q )

However, if NH 3 is present in the water, the complex ion, Ag ( NH 3 ) 2 + , can form according to the equation:

Ag + ( a q ) + 2 NH 3 ( a q ) Ag ( NH 3 ) 2 + ( a q )


K f = [ Ag ( NH 3 ) 2 + ] [ Ag + ] [ NH 3 ] 2 = 1.7 × 10 7

The large size of this formation constant indicates that most of the free silver ions produced by the dissolution of AgCl combine with NH 3 to form Ag ( NH 3 ) 2 + . As a consequence, the concentration of silver ions, [Ag + ], is reduced, and the reaction quotient for the dissolution of silver chloride, [Ag + ][Cl ], falls below the solubility product of AgCl:

Q = [ Ag + ] [ Cl ] < K sp

More silver chloride then dissolves. If the concentration of ammonia is great enough, all of the silver chloride dissolves.

Questions & Answers

Modern Electronic Theory
Levi Reply
find the volume of oxygen produced from the electrolysis of acidified water of a current of a 2A was passed through the electrolysis of acidified water for 2 hours
Ngwu Reply
A group of atoms that are connected by chemical bounds
Valerie Reply
the atoms are found in the shell, it is called atomic bond.
what is molecules
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Moecules are groups of atoms
What is collision theory
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by converting
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what is gas pressure
Sandra Reply
the pressure is the amount of Gas that is compressed in the container. in other words because of the gas that is compressed in the container there is pressure on it.
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amadi Reply
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accuracy is the movement of an object in a particular direction without any obstructions.
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johnson Reply
a titration curve is the separation of a solvent in a curved direction.
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gas law states that the amount of molecule in the container is inversely proportional to the pressure.
what is the functional group in this compound:CH3COCH2CH2-O-CH2
Obafemi Reply
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Practice Key Terms 8

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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