<< Chapter < Page Chapter >> Page >
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 )

with

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

What is whizatron?
Frendick Reply
What is stoichometry
ngwuebo Reply
what is atom
yinka Reply
An indivisible part of an element
ngwuebo
the smallest particle of an element which is indivisible is called an atom
Aloaye
An atom is the smallest indivisible particle of an element that can take part in chemical reaction
Alieu
is carbonates soluble
Ebuka Reply
what is the difference between light and electricity
Joshua Reply
What is atom? atom can be defined as the smallest particles
Adazion
what is the difference between Anode and nodes?
Adazion
What's the net equations for the three steps of dissociation of phosphoric acid?
Lisa Reply
what is chemistry
Prince Reply
the study of matter
Reginald
what did the first law of thermodynamics say
Starr Reply
energy can neither be created or distroyed it can only be transferred or converted from one form to another
Adedeji
Graham's law of Diffusion
Ayo Reply
what is melting vaporization
Anieke Reply
melting and boiling point explain in term of molecular motion and Brownian movement
Anieke
Scientific notation for 150.9433962
Steve Reply
what is aromaticity
Usman Reply
aromaticity is a conjugated pi system specific to organic rings like benzene, which have an odd number of electron pairs within the system that allows for exceptional molecular stability
Pookieman
what is caustic soda
Ogbonna Reply
sodium hydroxide (NaOH)
Kamaluddeen
what is distilled water
Rihanat
is simply means a condensed water vapour
Kamaluddeen
advantage and disadvantage of water to human and industry
Abdulrahman Reply
a hydrocarbon contains 7.7 percent by mass of hydrogen and 92.3 percent by mass of carbon
Timothy Reply
Practice Key Terms 8

Get the best Chemistry course in your pocket!





Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chemistry' conversation and receive update notifications?

Ask