Chemical Bonding

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Interatomic Bonding Tutorial Suggested Solutions

1.
|Substances |Type of bonding |Type of structure | |H2O |Covalent |Simple molecular | |SiCl4 |covalent |simple molecular | |RbCl |ionic |giant lattice/ionic | |Si |covalent |giant molecular | |SiO2 |covalent |giant molecular | |HCl |covalent |simple molecular | |P4 |covalent |simple molecular | |S8 |covalent |simple molecular | |Al2O3 |ionic |giant lattice/ionic | |AlF3 |ionic |giant lattice/ionic | |AlCl3 |covalent |simple molecular |

2. (a) Krypton (noble gas) and Rubidium (Group I metal)
Kr has a simple molecular structure and Rb has a giant metallic structure. Larger amount of energy is needed to overcome the stronger electrostatic attraction between Rb+ and its mobile electrons than the weaker van der Waals' forces between Kr atoms . Hence, boiling point of Rb is higher than that of Kr.

b) AlF3 and PF3
AlF3 has a giant ionic structure and PF3 has a simple molecular/ covalent structure. Larger amount of energy is needed to overcome the stronger electrostatic attraction between Al3+ and F( ions in AlF3 than the weaker intermolecular forces (or van der Waals' forces/ permanent dipole –permanent dipole attraction) between PF3 molecules. Hence, melting point of AlF3 is higher than that of PF3.

c) Diamond and silicon
Both diamond and silicon have giant molecular/ covalent structure. Since C is a smaller atom than Si, the C─C bond length is shorter than that of Si─Si bond, giving rise to stronger C─C covalent bond (bond energy of C─C bond is 350 kJ mol-1 versus Si─Si bond 222 kJ mol-1). Larger amount of energy is needed to overcome the stronger C─C bond than the weaker Si─Si bond. Hence, melting point of diamond is higher than that of silicon. 3. AlCl3 tends to dimerises to form Al2Cl6. At higher temperatures this Al2Cl6 dimer dissociates into trigonal planar AlCl3. [pic]

4.Simple covalent/molecular structure.
Strong covalent bond between Se and O atoms and weak intermolecular forces between selenium dioxide molecules. udents to its (1) relatively low melting point and (2) Non-conductor of electricity due to absence of charged carriers.]

5.ACJC Prelim 01/2/2(a)
Briefly explain the following observations.

AlF3 conducts electricity when molten but AlCl3 does not. AlF3 is essentially ionic, made up of oppositely charged Al3+ and F( ions. In the molten state, these charged ions are mobile and free to move about to carry charge and conduct electricity.

AlCl3, on the other hand, is essentially covalent. This is due to the polarisation of the large electron cloud of Cl( by the small and highly charged Al3+. In the molten state, AlCl3 exists as discrete molecules and there is an absence of charged particles in any state to conduct electricity.

NH4F sublimes when heated.
Solid NH4F decomposes into gaseous NH3 and HF on heating (solid to gas = sublimation).
NH4F(s) ( NH3(g) + HF(g)

6. IBoron trifluoride and not methane, will react with trimethylamine to form a 1:1 complex. The N atom on (CH3)3N has a lone pair for donation whilst the B atom on BF3 has an empty orbital to accept this pair of electrons to form a dative...
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