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.  If more than 1 type of ligand is in complex, names are listed alphabetically w/o regard to numerical prefixes

4.  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.  If central ion has more than 1 poss oxid #, use Roman numeral to show correct oxid #

6.  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