HONORS CHEMISTRY CHAPTER 14. Polar Molecules

Transcription

1 HONORS CHEMISTRY CHAPTER 14 Polar Molecules

2 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

3 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 CH 3 Cl has an asymmetrical distribution of charge polar CH 4 symmetrical distribution of chg nonpolar A polar molec must have polar bonds & they can t be symmetrically arranged

4 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

5 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

6 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

7 14.2 Weak Forces Dispersion Forces also called London Forces 2 nonpolar molecs may be attracted to ea other Ex in H 2 molec, e- s move around the molec For an instant, both e- s may 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

8 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 normal conditions These forces are only effective over VERY short distances

9 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

10 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

11 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 vertices of a regular octahedron w/ central (+) ion in the middle 4 is also a common coord. # May be square planar corner of a square w/ central (+) ion in center May also be tetrahedral vertices & central (+) ion in center Coord # of 2 is found in complexes of Ag +, Au +, & Hg + Always linear w/ ea end & (+) ion in middle

12 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 NH 3 is also a common ligand (-) ions can also be ligands ex) F -, Cl -, I -, CN -, SCN -, S -2, CO 3-2, & C 2 O 4-2

13 14.3 Ligands Oxalate ion has 2 O atoms which attach to the (+) ion Bidentate (2-toothed) 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 14.4 Names & Formulas of Complex Ions Naming Complex Ions Rules 1. Ligands are named 1 st 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 If central ion has more than 1 poss oxid #, use Roman numeral to show correct oxid # 6. End w/ word ion.

15 14.4 Names & Formulas of Complex Ions Ex) Name: [CrCl(NH 3 ) 5 ] +2 Central ion is chromium (III), Ligands: 1 chloro, 5 ammines Name: pentaamminechlorochromium (III) ion Ex) Name: [IrCl 6 ] -3 6 chloro s, (-) ion ends in ate Name: hexachloroiridate (III) ion LEAVE ROOM FOR MORE EXAMPLES

16 14.4 Names & Formulas of Complex Ions Writing Formulas for Complex Ions 1. Symbol for central ion is 1 st 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(NH 3 ) 2 ] +2 Ex) carbonylpentacyanoferrate (II) ion [Fe(CN) 5 CO] -3 LEAVE ROOM FOR MORE EXAMPLES

17 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

18 14.5 Coordination Compounds In writing formulas for coordin. comps containing a complex ion, enclose the complex ion in brackets Ex) Name: [Ni(NH 3 ) 6 ]Br 2 Hexaamminenickel (II) bromide Write the formula for hexacarbonylchromium(0) [Cr(CO) 6 ] LEAVE ROOM FOR MORE EXAMPLES

19 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

20 14.6 Bonding in Complexes Isolated ions in 1 st 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 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

21 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

22 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

23 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

24 14.7 Fractionation Diff substs will 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

25 14.8 Chromatography Column Chromatography used for very delicate separations Complex substs Utilizes glass or plastic column packed w/ stationary phase like CaCO 3 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 a diff rate & substs are separated. Rate depends on: 1. Attraction of ea subst for stationary phase 2. Attraction for solvent 3. Solvent concentration

26 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

27 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

28 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

29 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