Chapter 4: Chemical bonding
For similar sized molecules, the relative strength of the attractive forces between them are: • • •
temporary dipole-induced dipole forces (weakest) permanent dipole-dipole forces hydrogen bonding (strongest).
c Name the strongest type of attractive force between each of these pairs of molecules. Use the list above to help you. i
CH3Cl and CH3Br
ii CH3NH2 and CH3OH iii CH3CH2CH2CH2CH2CH3 and CH3CH2CH2CH2CH3 iv CH3COCH3 and CH3COCH2CH3 v CH3Br and CH3NH2 d Draw diagrams of each of these molecules to show how the atoms are arranged. On each diagram show the direction of the dipole as δ+ +→ δ−. If no net dipole is present, write ‘none’. i
CH2Cl2
ii CBr4 iii NH3 iv ClBr e Suggest in terms of intermolecular forces why: i
water has a higher enthalpy change of vaporisation than pentane, C5H12, even though pentane has a higher molar mass.
ii pentane is a liquid at r.t.p. but butane C4H10 is a gas at r.t.p. iii CH3NH2 has a higher boiling point than CH3Cl.
Exercise 4.5 Bonding and orbitals This exercise focuses on covalent bonding, the hybridisation of atomic orbitals and how these affect the properties of simple molecules. Remember that: • •
Hybridisation of orbitals is a mixing of s and p orbitals depending on the types of orbitals available. Graphite has layers of hexagonally arranged carbon atoms.
a Atomic orbitals overlap in various ways. Three types of hybrid orbital are sp, sp2 and sp3. i
The diagrams below show the hybridised orbitals in ethane and ethene. What words best represent the letters A to D? A bond
Overlap of C hybrid orbitals
B bond
A bond
Overlap of three D hybrid orbitals
ii Refer to the hybridised structure of ethane to explain why the H−C−H bond angles are 109.5º.
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