HONORS CHEMISTRY
CHAPTER 14
Polar Molecules
14.1 Polarity
• Electronegativity – an atom’s ability to attract e-’s involved
in bonding
• Diff for ea elem
• \ In a covalent bond betw 2 diff elems, one elem will
attract the shared pr more than the other
• The bond is polar covalent
• The atom w/ the higher electroneg will have a partial (-)
charge & the other will have a partial (+) charge
14.1 Polarity
• Polar bonds may produce polar molecules
• If there is a concentration of (-) chg on one end of the molec & (+)
chg on the other end, then the molec is a polar molec
• If (-) chgs are situated opposite ea other, they will cancel ea other
- nonpolar
• CH3Cl has an asymmetrical distribution of charge – polar
• CH4 – symmetrical distribution of chg – nonpolar
• A polar molec must have polar bonds & they can’t be
symmetrically arranged
14.1 Polarity
• Polar molecule is also called a dipole
• Has a dipole moment – prop of a dipole resulting from
asymmetrical chg distribution
• Depends on size of partial charges & distance betw them
• Dipole moment = Q x d Q = size of partial chg in coulombs, d =
distance in meters
• \ expressed in coulomb  meters
• The higher the dipole moment, the stronger the intermolecular
forces & \ the higher the melting point & boiling point
14.2 Weak Forces
• There’s a wide range of melting pts among covalent
comps
• Forces involved in some cases are van der Waals Forces
• Also called weak forces bec they’re much weaker than chem bonds
• Involve the attraction of the e-’s of one atom for the p+’s of another
• Intramolecular forces – (w/in a molec) – hold atoms
together in molecs - covalent bonds
• Intermolecular forces – (betw molecs) – hold molecs to ea
other – van der Waals forces
14.2 Weak Forces
• 3 types of van der Waals attractions:
• Dipole – Dipole Forces – 2 molecs of same or diff subst
which are both permanent dipoles are attracted to ea
other
• Dipole – Induced Dipole Forces – dipoles can attract other
molecs that aren’t normally dipoles
• When a dipole comes close to a nonpolar molec, its partial chg will
either attract or repel e- cloud of nonpolar molec
• E- cloud will move to one end of the nonpolar molec
• Becomes transformed into a dipole – Induced Dipole
• Can be attracted to permanent dipole
14.2 Weak Forces
• Dispersion Forces – also called London Forces
• 2 nonpolar molecs may be attracted to ea other
• Ex – in H2 molec, e-’s move around the molec
• For an instant, both e-’s may be @ the same end of the molec
• Becomes a Temporary Dipole – can cause the molec next to it to
become an induced dipole & an attractive force results
14.2 Weak Forces
• Many molecs exhibit 2 or 3 of these dipole/dispersion
interactions
• Liquid & solid states exist bec of the intermolecular forces
• Polar substs have higher boiling pts
• Many are solids @ normal conditions
• These forces are only effective over VERY short distances
14.2 Weak Forces
• van der Waals attractions result from any of the following
3:
• 1. Dipole – Dipole Forces
• 2. Dipole – Induced Dipole Forces
• 3. Dispersion Forces
• Dispersion forces are the most important – they are the only
attractive force betw nonpolar molecs
• Accounts for 85% or more of van der Waals forces in polar molecs
14.3 Ligands
• An important prop of polar molecs is their behavior toward
ions in soln
• When ionic comp dissolves in water, surface ions of the crystal are
surrounded by polar water molecs which adhere to surface
• These water molec / ion clusters have greatest stability when there is a
small ion w/ high charge in the center
• Complex ion – formed when polar molecs or (-) ions
cluster around a central (+) ion
• Ligands – polar molecs or (-) ions that are attached to the
central (+) ion
14.3 Ligands
• Coordination Number - # of pts of attachment of the
ligands around a central (+) ion in a complex
• Most common coord # is 6 – octahedral – ligands lie @ vertices of
a regular octahedron w/ central (+) ion in the middle
• 4 is also a common coord. #
• May be square planar – ligands @ corner of a square w/ central (+) ion
in center
• May also be tetrahedral – ligands @ vertices & central (+) ion in center
• Coord # of 2 is found in complexes of Ag+, Au+, & Hg+
• Always linear w/ ligands @ ea end & (+) ion in middle
14.3 Ligands
• Ligands can be either molecs or (-) ions
• Molecular ligands are always polar & always have an unshared pr
of e-’s that’s shared w/ central ion
• Most common ligand is water – hydrated comps are
composed of (+) ion surrounded by water ligands & the (-)
ions
• NH3 is also a common ligand
• (-) ions can also be ligands – ex) F-, Cl-, I-, CN-, SCN-, S-2,
CO3-2, & C2O4-2
14.3 Ligands
• Oxalate ion has 2 O atoms which attach to the (+) ion
• Bidentate (2-toothed) – attaches @ 2 pts
• Carbonate ion is also bidentate
• 2 bidentate ligands can form a tetrahedral complex
• 4 bidentate ligands can form an octahedral complex
• There are also tridentate & quadridentate ligands
14.4 Names & Formulas of Complex Ions
• Naming Complex Ions – Rules
1. Ligands are named 1st followed by central ion
2. Use a prefix before name of ligand to indicate how many of that
ligand is in the complex
• Di, tri, tetra, penta, hexa, etc – no prefix is used for 1
3.
4.
If more than 1 type of ligand is in complex, names are listed
alphabetically w/o regard to numerical prefixes
If the whole complex ion has a (+) charge, the central ion is
named in the usual way
• If complex ion has (-) charge, central ion must end in a t e.
• For some elems, Latin stems are used in negatively charged complex ions
• Table 14.4 p. 360
5.
6.
If central ion has more than 1 poss oxid #, use Roman numeral
to show correct oxid #
End w/ word ion.
14.4 Names & Formulas of Complex Ions
• Ex) Name: [CrCl(NH3)5]+2
• Central ion is chromium (III),
• Ligands: 1 chloro, 5 ammines
• Name: pentaamminechlorochromium (III) ion
• Ex) Name: [IrCl6]-3
• 6 chloro’s, (-) ion – ends in ate
• Name: hexachloroiridate (III) ion
• LEAVE ROOM FOR MORE EXAMPLES
14.4 Names & Formulas of Complex Ions
• Writing Formulas for Complex Ions
1. Symbol for central ion is 1st
2. Negative ligands
3. Neutral molecules
• w/in thoe 2 groups (2&3), the ligands are listed alphabetically according
to their symbols.
• Ex) diamminepalladium (II) ion
• [Pd(NH3)2]+2
• Ex) carbonylpentacyanoferrate (II) ion
• [Fe(CN)5CO]-3
• LEAVE ROOM FOR MORE EXAMPLES
14.5 Coordination Compounds
• Formed by:
1. A neutral compound is formed if the charge of the central ion in
the complex is matched by the charges of the ligands.
2. Sometimes a complex is formed in which neither the ligands nor
the central atom has a charge.
• Name of central atom is followed by a zero (0)
3.
Complex ions can form ionic comps (like monatomic &
polyatomic ions)
• All charges in comp must add up to 0
14.5 Coordination Compounds
• In writing formulas for coordin. comps containing a
complex ion, enclose the complex ion in brackets
• Ex) Name: [Ni(NH3)6]Br2
• Hexaamminenickel (II) bromide
• Write the formula for hexacarbonylchromium(0)
• [Cr(CO)6]
• LEAVE ROOM FOR MORE EXAMPLES
14.6 Bonding in Complexes
• Most (+) ions may form complexes – (groups 1 & 2 are
unstable)
• Transition metals form the most important & interesting complexes
• Have partially filled d sublevel involved in bonding
• (+) ions are small w/ high charge – high charge density on central
ion
• Favorable to formation of complex ions - more stable
14.6 Bonding in Complexes
• Isolated ions in 1st transition series have 5 degenerate 3d
orbitals
• E-’s may move from 1 orbital to another w/out a change in energy
• In complexes, ligands affect the energies of diff 3d orbitals
• In octahedral complexes, d orbitals are split into 2 groups
• 2 orbitals are in a higher energy group
• 3 orbitals are in a lower energy group
• See figure 14.17 p. 364
• The intense colors of many of these complexes are due to e-’s
moving betw the split d orbitals
• Split is only a small energy gap
• Energy is absorbed as e-’s move from low energy group to high
energy group – causes color
14.6 Bonding in Complexes
• When central ion is (+), ligands are – ions or polar molecs
• Suggests bonding structure of complex ion is similar to that of salts
• Electrostatic or ionic
• Ligands have an unshared pr of e-’s that can be donated
• Central ion always has unoccupied orbitals where these e- prs can
be placed
• Suggests bonds are covalent
• Coordinate Covalent Bond – covalent bond in which both
e-’s in shared pr come from the same atom
• Chemistry is the same as regular covalent bonds
• Bonds of most complex ions have both covalent & ionic
character – covalent character is dominant
14.7 Fractionation
• - the overall separation of parts from a whole by any
process
• Separations are called fractions
• Chromatography – a method of separation based on the
polarity of substs
• Name comes from fact that the fractions are usually diff colors
• Gas Chromatography – does not involve color
• Still depends on fractionation
14.7 Fractionation
• In chrom., a mobile phase w/a mixture of substs to be
separated passes over a stationary phase which has an
attraction for polar materials
• Mobile Phase – consists of a mixture to be separated dissolved in a
fluid (liquid or gas)
• Stationary Phase – consists of a solid or a liquid adhering to the
surface of a solid
14.7 Fractionation
• Diff substs will travel @ diff rates bec of varying polarity
• A polar subst will have an attraction for both the solvent (mobile) &
stationary phase
• Stationary phase will attract some substs more strongly than others
• Slowest moving substs will have the greatest attraction for stationary
phase
• Subst w/ least attraction for stationary phase will migrate the fastest
• \ substs can be separated
14.8 Chromatography
• Column Chromatography – used for very delicate
separations
• Complex substs
• Utilizes glass or plastic column packed w/ stationary phase like
CaCO3
• Mobile phase w/ material to be separated is added to top of column
• Fresh solvent is poured onto top of column & allowed to percolate thru
column
• Ea subst in mobile phase travels down the tube @ a diff rate &
substs are separated.
• Rate depends on:
1. Attraction of ea subst for stationary phase
2. Attraction for solvent
3. Solvent concentration
14.8 Chromatography
• Substs w/ high attraction for stationary phase will not
travel as far as substs w/ less attraction
• w/ constant percolation, subts are separated into zones
14.8 Chromatography
• Paper chromatography – separations carried out on paper
• Paper is placed in an atmosphere of water vapor or solvent vapor
• Drop of soln to be separated is placed on paper
• One end of paper is placed in solvent
• Solvent moves up thru paper by capillary action
• Separations come out as a series of colored spots
• Fast, simple, & has a high resolving power
• Need a control strip to identify comps
• Very useful
14.8 Chromatography
• Gas Chromatography – process used to analyze volatile
liquids & mixtures of gases
• Gases to be analyzed are carried by inert gas (usually He) in
mobile phase
• Fractionated on stationary phase like column chromat.
• After separated, gases are carried by inert gas thru a tube which
puts electricity thru it
• Various amts of contamination in inert gas produces various currents
• Recorded and interpreted by computer.
• See p. 269 of pkt
14.8 Chromatography
• Thin Layer Chromatography – combines techniques of
column & paper chromatog.
• Glass or plastic plate is coated w/ very thin layer of stationary
phase – like column chromatog
• Spot of unknown mixture is applied – like paper chromatog
• Glass plated is placed in an atmosphere of solvent vapor & solvent
• From here it’s like paper chromatog
• Used for separating biological materials
• See p. 369 of pkt
• Read in pkt about High Performance Liquid
Chromatography and Ion Chromatography
• p. 368