N = 727 Mean = 68% Diff T-Test P-Value SI 223 (31%) 71% No SI 504 (69%) 66% Test 2 - Letter Grade Distribution by SI Attendance

Transcription

1 CHEM 200/202 Exam 2 N = 727 Mean = 68% Diff T-Test P-Value SI 223 (31%) 71% No SI 504 (69%) 66% 5% <0.001*** Test 2 - Letter Grade Distribution by SI Attendance Percent Attendance A B C D/F No SI SI Letter Grade Only non-zero Exam scores were used to provide a more conservative analysis. Significance Code: 0.05*, 0.01**, 0.001***

2 ANNOUNCEMENTS If you have questions about your exam 2 grade, write to me or Chem200@mail.sdsu.edu. Chapter 7 homework due April. 4 th. Chapter 8 homework due April. 13 th. Chapter 9 home work due April. 20th. Exam 3 is 4/14 at 2 pm.

3 IONIC BOND STRENGTH Which compound has the stronger ionic bonds: NaCl or MgO? Charges: NaCl: +1 & -1 MgO: +2 & -2 HL: NaCl: -770 kj/mol MgO: kj/mol HL [(cation charge) (anion charge)]/internuclear distance

4 IONIC BOND STRENGTH Which compound has the stronger ionic bonds: NaCl or LiF? Charges: NaCl: +1 & -1 LiF: +1 & -1 HL: NaCl: -770 kj/mol LiF: kj/mol HL [(cation charge) (anion charge)]/internuclear distance Internuclear distance: NaCl > LiF

5 STRENGTH OF IONIC BONDS Ionic bonds are very strong (highly endothermic HL) much stronger than covalent bonds. The strength of ionic bonds increase with: Larger charges on the ions. Smaller ionic radii (shorter bond distances)

6 QUESTION Arrange the compounds, KI, CaBr2 and CaS in order of increasing melting point, using lattice energy as a guideline. Answer: CaS < CaBr2 < KI CaBr2 < CaS < KI KI < CaBr2 < CaS CaBr2 < KI < CaS Do not know A B C D E

7 THE BORN-HABER CYCLE

8 Li + (g) + F(g) H s3 H s4 Li + (g) + F - (g) Li + (g) + F - (g) - H L =? H x =? H s1 Li(g) + F(g) Li(g) + ½F2(g) H s2 Li(s) + ½F2(g) H f LiF(s) - H L = H f - H x H x = H s1 + H s2 + H s3 + H s4 - H L = H f - ( H s1 + H s2 + H s3 + H s4)

9 THE BORN-HABER CYCLE The enthalpy change ( H) that occurs when one mole of ionic solid separates into gaseous ions. MX(s) M + (g) + X - (g) HL[MX(s)]

10 BORN-HABER CYCLE FOR LITHIUM FLUORIDE - H L = H f -( H 1 + H 2 + H 3 + H 4)

11 LECTURE OBJECTIVES Chapter 7.6 Use the VSEPR theory to predict molecular structures/shapes. Determine the correct VSEPR arrangement from a formula or Lewis structure. Recognize the common bond angles in the various VSEPR arrangements. Identify variations in the common bond angles in VSPER arrangements due to lone pairs and multiple bonds. Use VSEPR structures to determine the polarity of molecules.

12 LEWIS STRUCTURES & MOLECULAR SHAPE Lewis structures show the connectivity (the number and type of bonds) in a molecule. Lewis structures do not directly give information on the shape of the molecule. H O H H O H O H H All are valid Lewis structures for water.

13 VALENCE SHELL ELECTRON-PAIR REPULSION (VSEPR) THEORY An empirical method for determining the molecular shape of a molecule for its Lewis formula. Molecular shape can be determined by the number and type of electron groups around each central atom. Electron groups: Lone pair Bonding groups - single, double, triple bonds

14 ELECTRON REPULSION As electrons have the same charge they repel each other. Electrons forming a bond will repel other bonding electrons. Lone pairs of electrons occupy more space than bonding electron pairs. Methane (CH4) H H C H H Lots of repulsion H H C H H Least of repulsion (but not 3D structure) These groupings of electrons push each other around to find the proper space for each other.

15 THE BASE ELECTRON-PAIR

16 LINEAR ELECTRON-GROUP ARRANGEMENT Examples: CS2, HCN, BeF2, CO2 linear

17 TRIGONAL PLANAR ELECTRON- GROUP ARRANGEMENT trigonal planar Two different forms: AX3: SO3, BF3, NO AX2E: SO2, O3, PbCl2... bent (v-shaped)

18 TETRAHEDRAL ELECTRON- GROUP ARRANGEMENT tetrahedral Three different forms: AX4: CH4, SiCl4, SO AX3E: NH3, PF3, H3O +... AX2E2: H2O, OF2, SCl2... trigonal pyramidal bent (v-shaped)

19 TRIGONAL BIPYRAMIDAL ELECTRON-GROUP ARRANGEMENT trigonal bipyramidal Four different forms: AX5: PF5, AsF5, SOF4... AX4E: SF4, XeO2F2, IF AX3E2: ClF3, BrF3... AX2E3: XeF2, I3 -, see-saw linear t-shaped

20 OCTAHEDRAL ELECTRON- GROUP ARRANGEMENT octahedral Three different forms: AX6: SF6, IOF5... AX5E: BrF5, TeF5 -, XeOF4... AX4E2: XeF4, ICl square pyramidal square planar

21 # of Groups # of Lone Pairs Molecular Shape 2 0 linear 3 0 trigonal planar 3 1 bent (v-shaped) 4 0 tetrahedral 4 1 trigonal pyramidal 4 2 bent (v-shaped) 5 0 trigonal bipyramidal 5 1 see-saw 5 2 t-shaped 5 3 linear 6 0 octahedral 6 1 square pyramidal 6 2 square planar linear trigonal planar tetrahedral trigonal bipyramidal octahedral

22

23 STEPS IN DETERMINING A MOLECULAR SHAPE 1. Molecular formula 2. Lewis structure - count all e - groups around the central atom (A) 3. Electron-group arrangement - note lone pairs and double bonds 4. Bond angles - predict the ideal bond angles then account for any deviations due to lone pairs or double bonds 5. Molecular shape (AXmEn) - count bonding & nonbonding e - groups separately

24 QUESTION Using VSPER theory, predict the shape of NI3. Answer: trigonal pyramidal Tetrahedral A T-shaped See-saw B C I N I Trigonal pyramidal D I Trigonal planar E

25 QUESTION Using VSPER theory, predict the shape of IF4 +. Answer: Tetrahedral A Square planar B See-saw C Octahedral D Trigonal planar E F I F F F 1+ see-saw

26 PROBLEM Identify the molecular shape for the following molecules: COCl2 Xenon tetrafluroide Chlorine trifluoride

27 QUESTION Which compound or ion is represented by this molecular view of the gas phase? Answer: A B C D NO2 - SCl2 CO2 SO2

28 SHAPES OF MOLECULES WITH MULTIPLE CENTRAL ATOMS The rules for the shapes of single atom center molecules remain effective with multiple central atoms on larger molecules.

29 IDEAL BOND ANGLES

30 BOND ANGLE DISTORTIONS The bond angles expected from the models (ideal) are not always meet in reality. Difference in electron group types will alter the bond angles. Relative size of electron groups: Lone pairs > multiple bonds > single bonds VSPER theory can predict if an actual bond will be smaller or larger than the ideal bond angle.

31 QUESTION What is the molecular shape of NH3 and the H-N-H bond angle? Answer: Tetrahedral (109.5 ) A Trigonal pyramidal (109.5 ) B Trigonal planar (120 ) C Trigonal pyramidal (>109.5 ) D Trigonal pyramidal (<109.5 ) E

32 MOLECULAR POLARITY Individual, covalent bonds, are polar when there is a difference in the electronegativity of the bonded atoms (e.g. Si-O, H-F...) Molecules can also have polarity, however, this is a product of the aggregate polarity of all the bonds. Molecular polarity is influenced by both the bond polarities and the molecular shape. The polarity of a molecule is termed the dipole moment (µ), and is measured in units debyes. Non-polar molecules do not have dipole moments (µ=0)

33 MOLECULAR POLARITY To determine the molecular polarity draw the proper molecular structure of the compound. Indicate the individual bond polarities with polarity arrows. Look at arrows as a tug-of-war, are they equally balanced or does one side win out over the other. Keep in mind that the structure is 3D,

34 POLAR MOLECULES Polar molecules are influenced by electric fields; non-polar molecules are not affected by electric fields. Polar molecules will self-orient to the applied electric field. Electric field OFF Electric field ON

35 MOLECULAR POLARITY Molecular polarity is the result of synergistic bond polarities. If the polarity of the bonds around the central atom result in an unequal distribution of electronegativity the molecule will be polar. The VSEPR structure of the molecule must be considered in determining molecular polarity. Net polarity: Net polarity:

36 QUESTION Which of the following is a polar molecule? ANSWER: A - BF3 B - XeF2 C - CS2 D - BrF3

37 WHICH OF THE FOLLOWING MOLECULES ARE POLAR? Water Ammonia Methane (CH4) Chloromethane (CH3Cl) Dichloromethane (CH2Cl2) Trichloromethane (CHCl3) Carbonyl sulfide (CSO) Xenon tetrafluoride