Types of Chemical Reactions

1. Types of Chemical Reactions

2. Chemical Reactions • Neutralisation reaction • Precipitation reaction • Synthesis reaction • Displacement reaction • Decomposition reaction

3. Oxidation and Reduction

4. Rusting

5. Combustion

6. Batteries

8. Restoring Old Paintings • Done by oxidation process. Most old paintings are lead-based paints. • Become discoloured due to reactions with pollutant gases in the air, esp. with hydrogen sulphide gas, H2S. • This reacts with the lead ions to form black lead sulphide: Pb2+ (aq) + S2- (aq) PbS (s)

9. The original white colour can be restored by repainting with oxidising agent such as hydrogen peroxide, H2O2. • PbS(s) + 4H2O2 (aq) PbSO4(aq) + 4H2O (l)

10. Topics to be learnt: • Oxidation and Reduction as Gain or Loss of Oxygen • Oxidation and Reduction as Loss or Gain of Hydrogen • Oxidation and Reduction as Loss or Gain of Electrons • Oxidation State • Oxidation and Reduction as Change of Oxidation State • Oxidising and reducing agents

11. Oxidation and Reduction as Gain or Loss of Oxygen • Oxidation involves the gain of oxygen • A substance is oxidised if it gains oxygen • An oxidising agent is a substance which causes oxidation and itself is reducedin the process

12. Oxidation and Reduction as Gain or Loss of Oxygen • Reduction involves the loss of oxygen • A substance is reduced if it loses oxygen • A reducing agent is a substance which causes reduction and itself is oxidised in the process

13. gain of oxygen reducing agent loss of oxygen oxidising agent Oxidation and Reduction as Gain or Loss of Oxygen For example Zn(s) + CuO(s)  ZnO(s) + Cu(s) Zn oxidised CuO reduced

14. Oxidation and Reduction as Gain or Loss of Oxygen Combustion Combustion reactions are oxidation reactions - gaining of oxygen e.g.1 burning of C C + O2 CO2 e.g.2 burning of Mg 2Mg + O2 2MgO

15. Oxidation and Reduction as Gain or Loss of Oxygen Extraction of metals metals can extracted from their ores by heating with oxides of C e.g. 1 Fe2O3 + 3CO  2Fe + 3CO2 e.g.2 ZnO + CO  Zn + CO2 Which are the oxidised and reduced species?

16. reduction oxidation Oxidation and Reduction as Gain or Loss of Oxygen Oxidation and Reduction usually takes place together in the same reaction,hence such reactions are also known as REDOX

17. Oxidation and Reduction as Gain or Loss of Hydrogen • Oxidation involves the loss of hydrogen • A substance is oxidised if it loses hydrogen • Reduction involves the gain of hydrogen • A substance is reduced if it gains hydrogen

18. loss of hydrogen gain of hydrogen reducing agent oxidising agent Oxidation and Reduction as Gain or Loss of Hydrogen H2S oxidised For example H2S(g) + Cl2(g) -> 2HCl(g) + S(s) Cl2 reduced

19. Oxidation and Reduction as Gain or Loss of Electrons • Oxidation involves the loss of electrons • A substance is oxidised if it loses electrons • Reduction involves the gain of electrons • A substance is reduced if it gains electrons

20. electrons Oxidation and Reduction as Gain or Loss of Electrons For example, 2Na + Cl2 -> 2Na+Cl-

21. e e e e e e e e e e e e e e e e Cl e Cl e e Na Na e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e Oxidation and Reduction as Gain or Loss of Electrons

22. e e e e e e e e e e e e e e e Na e e e e e e e e e e e Cl e Cl e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e Na e e e e e e e e e e e e Oxidation and Reduction as Gain or Loss of Electrons

23. e e e e e Na e e e e e e e e e Cl e e e e e e e e e e e e e e e e e e Na e e e e e e e Oxidation and Reduction as Gain or Loss of Electrons e e e e e Cl e e e e e e e e e e e e

24. + e e - - e e e Na e e e e e e e e Cl e e e e e e e e e e e e + e e e e e e e Na oxidised Na e e loss of electrons e e e Cl2 reduced e e gain of electrons Oxidation and Reduction as Gain or Loss of Electrons e e e e e Cl e e e e e e e e e e e e

25. + e e - - e e e Na e e e e e e e e Cl e e e e e e e e e e e e + e e e e e e e Na oxidised Na e e Reducing agent e e e Cl2 reduced e e Oxidising agent Oxidation and Reduction as Gain or Loss of Electrons e e e e e Cl e e e e e e e e e e e e

26. electrons Oxidation and Reduction as Gain or Loss of Electrons Another example, Mg + 2H+ -> Mg2+ + H2

27. H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ Mg Mg Mg H+ Mg Mg Mg H+ Oxidation and Reduction as Gain or Loss of Electrons

28. Oxidation and Reduction as Gain or Loss of Electrons H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ Mg Mg Mg H+ H+ H+ Mg Mg Mg H+

29. Oxidation and Reduction as Gain or Loss of Electrons H+ H+ H+ Mg Mg Mg

30. e e Oxidation and Reduction as Gain or Loss of Electrons H+ H+ H+ Mg Mg2+ Mg

31. e e Oxidation and Reduction as Gain or Loss of Electrons H+ H+ H+ Mg Mg2+ Mg

32. e e Oxidation and Reduction as Gain or Loss of Electrons H+ H+ H+ Mg Mg2+ Mg

33. e e Oxidation and Reduction as Gain or Loss of Electrons H+ H+ H+ Mg Mg2+ Mg

34. Oxidation and Reduction as Gain or Loss of Electrons H H+ H Mg Mg2+ Mg

35. H H Oxidation and Reduction as Gain or Loss of Electrons H+ Mg Mg2+ Mg

36. gain of electrons H+ reduced H H Mg oxidised loss of electrons Oxidation and Reduction as Gain or Loss of Electrons H+ Mg Mg2+ Mg

37. Oxidising agent H+ reduced H H Mg oxidised Reducing agent Oxidation and Reduction as Gain or Loss of Electrons H+ Mg Mg2+ Mg

38. Historical Development • Ancient times, 7 metals (Au, Ag, Cu, Fe, Pb, Sn, Zn) • Some metals occur free in nature. • Other metals occurred as oxides in ores • Man used charcoal to reduce the ore – lead the metal in the ore back to its original native state. • ‘Re’ = back; ‘ducere’ = to lead

39. Historical Development • Thru centuries, the meaning of the term ‘reduce’ changed into ‘to diminish’ or ‘to decrease’. • Large portions of iron ores, haematite and magnetite, were diminished to smaller lumps of iron. • A reduction took place.

40. Historical Development • Man burn fuels to obtain fire. • Combustion was followed by visible flames and release of energy. • 1779 CW Scheele called the newly discovered air component that sustained combustion, ildluft (fire air). • Antoine Lavoisier, let different substances burn in air and discovered that the products gave acidic solutions with water.

41. Historical Development • He suggested that the substances had reacted with something in the air that was acidic. – principe oxygene (the acidifying principle, from oxys (Greek) = sharp). • Lavoisier coined the term oxidation to any reaction with oxygen and reserved the expression reduction to the removal of oxygen.

42. Historical Development • In 1837, J Liebig proposed another definition. • He had noticed that hydrogen gas was effective in removing oxygen from compounds. • Organic compounds were oxidised by releasing hydrogen (atoms) in reactions as when alcohols were dehydrogenated.

43. Historical Development • After the discovery of the electron in 1897, GA Lewis explained the formation of sodium chloride by the complete transfer of electrons from sodium atoms to chlorine atoms. • His electronic theory of valence of 1916 lay the ground for defining redox reactions as reactions with transfer of electrons.

44. Historical Development • The terms oxidation and reduction are today connected to changes in oxidation numbers, a term introduced by W Latimer. • Different redox models have evolved through years and four models are still in use in schools. • Incompatibility of various models when applied to the same type of reaction and to the overlaps and anomalies of models.

45. Oxidation State Definition: The charge an atom would have if it existed as an ion.

46. Oxidation States As a rule of thumb: 1. Oxidation state of hydrogen is always +1, except - in metal hydrides where it is -1 e.g. in LiH ( lithium hydride)

47. Oxidation States As a rule of thumb: 2. Oxidation state of oxygen is always -2 except- in peroxides, where it is -1, e.g. in hydrogen peroxide, H2O2 - when H or F are present, the oxidation state of H or F must remain the same , while oxidation state of oxygen is adjusted e.g. in OF2 , oxidation state of oxygen is +2

48. Oxidation States Basic rules to writing oxidation state: 1. A free, uncombined element has 0 oxidation state E.g. Carbon Magnesium Iron o.s. 0

49. Oxidation States Basic rules to writing oxidation state: 2. The sum of the oxidation numbers of the atoms in a molecule or ions in an ionic compound is equal to 0 E.g. Hydrogen gas Hydrogen sulphate Water (H2O) Sodium chloride o.s. 0

50. o.s. +2 o.s. -1 o.s. +1 Oxidation States Basic rules to writing oxidation state: 3. Charge on a simple ion is its oxidation state E.g. Zinc ion, Zn2+ Chloride ion, Cl- Copper (I) ion , Cu+