Tuesday, April 5, 2011

Physical chemistry

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Let me take you through a few Physical Chemistry sample questions. i will put these up soon on my website www.etutornetwork.com.
1. Draw the Lewis structures for each of the following ions or molecules. For each, give (i) the molecular shape, (ii) the electron pair geometry at the central atom, and (iii) the hybridization of the central atom.
(a) Br2O
(b) IOF3
(c) BrF3
(d) AsF3
(e) PCl4+
Name of the moleculeMolecular shapeElectron pair geometryHybridization
Br2OBend typeTetrahedranSp3
IOF3PyramidalTrigonal bipyramidalSp3d
BrF3TriangleTrigonal bipyramidalSp3d
PCl4 +PyramidalPyramidalSp3d
2. Consider the following molecules or ions: AsCl5, ICl4-, COCl2, and PBr3. Answer the following questions based on the Lewis structures and VSEPR theory prediction of their molecular shapes.
(a) Which one has only bond angles of 109.5°?
(b) Which one has only bond angles of 120°?
(c) Which one has bond angles of 90 and 180°?
(d) Which one has bond angles of 90, 120, and 180°?
a) PBr3 has tetrahedran shape and hence it has only bond angles of 109.5o
b) COCl2 has sp3 hybridization with triangle shape and hence the bond angles are all 120o
c) AsCl5 has trigonal by pyramidal structure in which the angles in the plane of triangle is 120o and angles perpendicular of plane are 90o
d) ICl4- has seasaw structure in which the angle between the bonds in plane is 120o and angle between between bonds in plane and perpendicular bonds is90o and the angle between perpendicular bonds is 180o
3. Consider the four compounds CH4, NH3, H2O, and HF. Use periodic trends in bond lengths and the electro negativity table in Chang to answer the following questions:
(a) Which molecule is expected to have the longest bonds?
(b) Which molecule is expected to have the most polar bonds?
(c) Which molecule is expected to have the least polar bonds?
a) All the among compounds are hydrides of the different atoms.  The bond lengths depend on size of that atom.  Hence in the list atomic size of carbon will be the greatest.  Hence the bond length in methane will be greatest.
b) Polar nature of bonds depend electro negativity of that atom with respect to hydrogen.  Generally electronegativity increases along a period and the last element will have more electro negative values.  Hence HF will have more polar bonds
c) In the above list Carbon is the first element in the period with very small electro negative value comparing to hydrogen.  So CH4 will have very least polar bond.
4. Atom hybridization changes in many chemical reactions. In each of the following reactions, determine what change, if any, occurs to the hybridization of the underlined atom.
(a) HN=NH + H® H2N-NH2
(b) XeO2 + 1/2 O® XeO3
a) for the first reaction the double bond in nitrogen is changed to single bond.  So the hybridization is changed from sp2 to sp3
b) There is no change in hybridization as the hybridization is sp3d in both cases.  The extra oxygen is added by accepting one pair of electron from Xe
5. Which one of the following molecules is nonpolar?
SOF2,  AsF3 , NO2Br,  SF6,  SO2
In the above given list the structure of SF6 have regular symmetrical octahedran structure. So it is the only non polar compound.  Others have unpaired electrons over the central metal atom which make the bond polar.
6. Use molecular orbital theory to predict the following properties of the F2
- ion: (a) electron configuration; (b) bond order; (c) magnetic character (paramagnetic or diamagnetic); (d) whether the bond length is longer or shorter than in the F2 molecule; (e) whether the bond strength is greater or less than in the F2 molecule. Use the M.O. diagram for F2 in Table 10.5 of Chang as a starting point for this question.
a)If we look at the molecular orbital diagram, there are 19 (18+1) electrons in the molecular orbital of the ion.  They are arranged as σ1s2, σ *1s2, σ 2s2, σ *2s2, σ 2p2, Π2p4,  Π*2p4 , σ*2p1
Here * represents antibonding orbital.
b) bond order= number of electrons in bonding orbital - number of electrons in antibonding orbital/2
= 10-9/2 = 0.5
c)magnetic character: as the molecular orbital σ*2p1 contains one unpaired electron it is paramagnetic.
d) In F2 molecule the bond order is 1 (10-8/2) so bond order decreases and hence bond length increases.
e) As the bond order decrease when comparing with F2 molecule the bond is less stable then F2 molecule.
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