Structures and Properties of Substances

1. 12 Structures and Properties of Substances 12.1 Classification of Substances According to Structures 12.2 Classification of Substances According to the Nature of Bonding

2. 12.1 Classification of Substances According to Structures

3. forces of attraction between particles weaken liquid Particles (may be molecules, ions or metallic atoms)  gas forces of attraction between particles broken    12.1 Classification of substances according to structures (SB p.302) Some pre-knowledge solid

4. 12.1 Classification of substances according to structures (SB p.302) Molecular structures • Consist of discrete molecules • Atoms in the molecules are held by strong covalent bonds • Molecules are held together by weak intermolecular forces • 2 types: simple molecular structures, macromolecular structures

5. Substances with simple molecular structures may be solids, liquids or gases at room conditions Substances with macromolecular structures are always solids at room conditions 12.1 Classification of substances according to structures (SB p.302)

6. 12.1 Classification of substances according to structures (SB p.302) Giant structures • All the atoms or ions are linked by strong bonding • Classified into 3 types: giant covalent, giant ionic and giant metallic structures

7. Substances with giant covalent (graphite), giant ionic (sodium chloride) and giant metallic (copper) structures 12.1 Classification of substances according to structures (SB p.302)

8. Check Point 12-1 12.1 Classification of substances according to structures (SB p.303) Classification of substances according to their structures

9. 12.2 Classification of Substances According to the Nature of Bonding

10. forces of attraction between particles weaken forces of attraction between particles broken 12.2 Classification of substances according to the nature of bonding (SB p.304) Simple molecular substances Examples liquid iodine (by raising the temp.) solid iodine(at room temp.) Gaseous iodine (by further raising the temp.)

11. 12.2 Classification of substances according to the nature of bonding (SB p.304) Properties of simple molecular substances • Low melting points and boiling points • Soft and easily distorted • Non-conductors of electricity • Usually insoluble in polar solvents but soluble in non-polar solvents Like dissolves like

12. Let's Think 1 Like dissolves like Non-polar solvents dissolve non-polar substances best E.g. hexane dissolves I2 Polar solvents dissolve polar substances best. E.g. water dissolves NH3 / SO2 / HCl / NaCl

13. 12.2 Classification of substances according to the nature of bonding (SB p.305) Giant covalent substances • Non-metallic atoms are joined together by strong covalent bonds • No discrete molecules are present • Examples: diamond, graphite, quartz

14. The giant covalent structure of diamond The giant covalent structure of quartz 12.2 Classification of substances according to the nature of bonding (SB p.305) Giant covalent substances

15. Let's Think 2 12.2 Classification of substances according to the nature of bonding (SB p.306) Properties of giant covalent substances 1. High melting points and boiling points 2. Hard and rigid 3. Poor conductors of heat and electricity 4. Insoluble in any solvents

16. The giant covalent structure of graphite (a layered structure) 12.2 Classification of substances according to the nature of bonding (SB p.306) Unique properties of graphite

17. 12.2 Classification of substances according to the nature of bonding (SB p.306) Unique properties of graphite • Soft and can act as a lubricating agent •  layers of carbon atoms are held together by weak van der Waals’ forces • Good conductor of electricity •  unhybridized  electrons of carbon atoms are delocalized within the layers

18. 12.2 Classification of substances according to the nature of bonding (SB p.306) Macromolecules • Long chains of molecules containing a very large number of atoms • Examples: plastics, proteins, some carbohydrates • Higher m.p./b.p. than simple molecules due to much stronger van der Waals’ forces

19. 12.2 Classification of substances according to the nature of bonding (SB p.307) Ionic substances • Cations and anions are held together by ionic bonds which are strong non-directional electrostatic attraction • Oppositely charged ions are closely packed together to give a 3-dimensional giant lattice

20. The giant ionic structure fractures when a stress is applied 12.2 Classification of substances according to the nature of bonding (SB p.307) Properties of ionic substances • High melting and boiling points • Hard but brittle

21. 12.2 Classification of substances according to the nature of bonding (SB p.307) Properties of ionic substances • 3. Good conductors of electricity in molten or aqueous state • Soluble in polar solvents but insoluble in non-polar solvents • Like dissolves like

22. The formation of aqueous sodium chloride solution, in which the ions are attracted and surrounded by water molecules 12.2 Classification of substances according to the nature of bonding (SB p.307)

23. 12.2 Classification of substances according to the nature of bonding (SB p.308) Metallic substances • Consist of a regular lattice of metallic cations and a “sea” of delocalized electrons • The lattice is held by strong electrostatic attraction between the delocalized electrons and metallic cations known as metallic bonds

24. 12.2 Classification of substances according to the nature of bonding (SB p.308) Properties of metallic substances • High melting and boiling points • Good conductors of heat and electricity • High density

25. Illustration of the malleability and ductility of a metal 12.2 Classification of substances according to the nature of bonding (SB p.308) Properties of metallic substances 4. Malleable and ductile

26. Check Point 12-2 Example 12-2 12.2 Classification of substances according to the nature of bonding (SB p.309)

27. The END

28. 12.1 Classification of substances according to structures (SB p.303) Back Check Point 12-1 • Classify the following elements or compounds into different types of molecular or giant structures: • Lithium fluoride • Argon • Uranium • Polyester • Glucose • Giant ionic structure • Simple molecular structure • Giant metallic structure • Macromolecular structure • Simple molecular structure Answer

29. 12.2 Classification of substances according to the nature of bonding (SB p.305) Let's Think 1 Explain the ability of buckminsterfullerence to partially conduct electricity in relation to its structure. Answer It is because each carbon atom in buckminsterfullerene forms three  bonds only and delocalized  electrons are present which conduct electricity. Back

30. 12.2 Classification of substances according to the nature of bonding (SB p.306) Answer Let's Think 2 Unlike diamond and graphite, fullerenes can be dissolved in various solvents. Why? Diamond and graphite have giant covalent structure. They do not dissolve in any solvents because of the presence of strong covalent bonds between all carbon atoms. However, fullerenes have simple molecular structure. Only weak van der Waals’ forces have to be overcome in the process of dissolution. Back

31. 12.2 Classification of substances according to the nature of bonding (SB p.309) Example 12-2 What are the types of attractive forces present in each of the following substances? Show how the physical properties are related to their structure and bonding. (a) Dry ice (b) Sodium chloride Answer

32. 12.2 Classification of substances according to the nature of bonding (SB p.309) Example 12-2 (a) In dry ice, the atoms are joined together within the molecules by strong covalent bonds while only weak van der Waals’ forces exist between molecules. The melting and boiling points of dry ice are low since only weak van der Waals’ forces are needed to be overcome during the processes of melting and boiling. Besides, dry ice consists of simple molecules with no mobile electrons. Thus, it does not conduct electricity. Carbon dioxide is only slightly soluble in water because it is non-polar in nature.

33. 12.2 Classification of substances according to the nature of bonding (SB p.309) Example 12-2 (b) Strong ionic bonding exists between oppositely charged ions throughout the whole lattice of a sodium chloride crystal. Thus, the melting and boiling points of sodium chloride are very high. Moreover, sodium chloride is hard since the ions are closely packed and strong ionic bonds hold the ions together. It is soluble in water due to the solvation of ions (i.e. the attraction between the ions and water molecules which are polar) and is insoluble in non-polar solvents. Sodium chloride conducts electricity in the molten or aqueous state due to the presence of mobile ions in these states. Back

34. 12.2 Classification of substances according to the nature of bonding (SB p.309) Check Point 12-2 • Explain the following characteristic properties of metals in terms of their structure and bonding. • (i) Malleable • (ii) High density • (iii) High melting point Answer

35. 12.2 Classification of substances according to the nature of bonding (SB p.309) Check Point 12-2 • (i) When stress is applied on a piece of metal, one plane of metal ions slides over another. The delocalized electrons take up new positions and the metallic bonds can be maintained. Therefore, metals are malleable. • (ii) Metals have high densities as metal ions are closely packed together in the metallic lattice. • (iii) Metals have high densities as metal ions are held tightly in the giant lattice. A large amount of energy is required to overcome the strong metallic bonds between the cationic lattice and the delocalized electrons.

36. 12.2 Classification of substances according to the nature of bonding (SB p.309) Check Point 12-2 • Describe briefly how the structures of the following substances are related to their physical properties. • (i) Quartz • (ii) Magnesium Answer

37. 12.2 Classification of substances according to the nature of bonding (SB p.309) Check Point 12-2 (b) (i) Quartz is a substance with a giant covalent structure. In the quartz lattice, each silicon atom is bonded tetrahedrally to four neighbouring oxygen atoms whereas each oxygen atom is bonded to two neighbouring silicon atoms. This gives rise to a tetrahedral diamond-like structure. As the atoms are held together by strong covalent bonds, a large amount of energy is required to overcome the strong covalent bonds in the processes of melting and boiling. Thus, quartz has high melting and boiling points. Besides, quartz is hard and rigid as the atoms are fixed in their positions by strong covalent bonds. Since there are no mobile electrons, quartz is a poor conductor of heat and electricity. Quartz is insoluble in both polar and non-polar solvents

38. 12.2 Classification of substances according to the nature of bonding (SB p.309) Back Check Point 12-2 (b) (ii) Magnesium is a substance with a giant metallic structure. It has high melting and boiling points as the magnesium ions are held tightly in the giant lattice. A large amount of energy is required to overcome the strong metallic bonds between the cationic lattice and the delocalized electrons. Due to the presence of mobile electrons, magnesium is a good conductor of heat and electricity. It also has a high density as the magnesium ions are closely packed together in the metallic lattice. Moreover, magnesium is malleable and ductile. It is because when stress is applied, one plane of metal ions will slide over another, and the delocalized electrons will take up new positions. As a result, the metallic bonds can be maintained.