Chem1000A Spring 2007 Practice Assignment 6 - Answers

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2. Looking at electronegativity differences, ∆χ, predict whether the following compounds are ionic or covalent. (a) TeF6 ∆χ = 3.98-2.10 = 1.88 covalent (polar) (b) CF4 ∆χ = 3.98-2.55 = 1.43 covalent (c) AlF3 ∆χ = 3.98-1.61 = 2.37 ionic (d) NaF ∆χ = 3.98-0.93 = 3.05 ionic (e) B2H6 ∆χ = 2.20-2.04 = 0.16 covalent 3. (a) Draw Lewis structures of the following molecules. (b) Specify electron-pair geometries and molecular geometries of the following molecules according the VSEPR model. (c) Indicate the bond dipole moments and the overall molecular dipole moments (if the molecule is polar). Specify whether the molecule is polar or non-polar. (i) ClO2 - (ii) XeO4 (iii) BrF4 - (iv) NO2F (v) SeF4 (vi) SF3 + (vii) OH2 (viii) OF2 (ix) CFCl3 (x) XeF2 (xi) SF5 - (xii) XeO3F2 (xiii) PO2F2 - (ii) (i) .. : Cl : O.. : - : O : Xe O O O.. .. : : .. : O.. : .. .. .. + : Cl .. Cl O : : O .. - O O .. electron-pair geometry: tetrahedral (SN = 4) molecular geometry: bent(the two ClO bonds are equivalent) polar molecule: molecular dipole moment is pointing towards the side of the oxygens, away from Cl electron-pair geometry: tetrahedral (SN =4) molecular geometry: tetrahedral bond dipole moments cancel each other, therefore, no mol. dipole moment XeO 4 is non-polar (iii) .. .. - .. F.. : .. : F.. .. Br .. F F .. : : .. electron-pair geometry: octahedra (SN = 6) molecular geometry: square planar (lone pairs on opposite sides of the plane) bond dipole moments cancel each other, therefore, no molecular dipole moment BrF 4 - is non-polar 2-5
(iv) .. : .. F .. : : .. O: N O - S F .. F F.. : .. : : : .. .. + .. O .. F F : : .. : : .. .. .. : .. F .. - : O: N O.. : electron-pair geometry: trigonal planar (both NO bonds are equivalent due to resonance) molecular geometry: trigonal planar The bond dipole moment of NF is significantly larger than that of the NO bonds, hence, the NF dipole is only partially compensated by the NO dipoles. The resulting molecular dipole moment points in the direction of the fluorine. NOF 2 is polar. (vi) electron-pair geometry: tetrahedral molecular geometry: trigonal pyramidal The SF bond dipole moments point have all a component pointing down. SF 3 + is polar. (viii) electron-pair geometry: tetrahedral molecular geometry: bent The OF bond dipole moments point towards F. F 2 O is polar. + + F N 3-5 O O + (v) .. .. O H H Cl Cl .. : : : .. .. .. : F : .. Cl .. : .. : F : F Se F .. F: : .. .. : : .. .. : + electron-pair geometry: trigonal bipyramidal (lone pair goes into the equatorial position) molecular geometry: see saw the axial SeF dipoles will cancel each other. The equatorial SeF dipoles will add up to a molecular dipole moment, pointing to the side of the two equatorial fluorines. SeF 4 is polar. (vii) electron-pair geometry: tetrahedral molecular geometry: bent The OH bond dipole moments point towards O. H 2 O is polar. (ix) electron-pair geometry: tetrahedral molecular geometry: tetrahedral The CCl bond dipole moments are smaller than that of the CF bond. Hence, the overall molecular dipole moment points in the direction of F. CFCl 3 is polar. + +
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Chem1000A Spring 2007 Practice Assignment 6 - Answers

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