1. CHEMISTRY 161 Chapter 4 www.chem.hawaii.edu/Bil301/welcome.html ‘4’

2. 1.redox reactions 2. oxidation versus reduction 3. oxidation numbers versus charges 4. calculation of oxidation numbers REVISION

3. TYPES OF REDOX REACTIONS 1.combination reactions A + B → C 2. decomposition reactions C → A + B 3. displacement reactions A + BC → AC + B 4. disproportionation reactions

4. 1.combination reactions A + B → C two or more compounds combine to form a single product S(s) + O 2 (g) → SO 2 (g) 1.oxidation numbers 2. balancing charges

5. MENUE 1.oxidation states of group I – III metals 2.oxidation state of hydrogen (+1, -1) 3. oxidation states of oxygen (-2, -1, -1/2, +1) 4.oxidation state of halogens 5.remaining atoms

6. S(s) + O 2 (g) → SO 2 (g) 00+4 -2 oxidation (1) S → S +4 + 4e - reduction (2) O 2 + 4 e - → 2 O -2 (1)+(2) S + O 2 + 4e - → S +4 + 2 O -2 + 4e - (1)+(2) S + O 2 → S +4 + 2 O -2 S(s) + O 2 (g) → SO 2 (g)

7. Mg(s) + N 2 (g) → Mg 3 N 2 (s) 00+2 -3 oxidation (1) Mg → Mg +2 + 2e - reduction (2) N 2 + 6 e - → 2 N -3 (1)+(2) 3 Mg + N 2 + 6e - → 3 Mg +2 + 2 N -3 + 6e - (1)+(2) 3 Mg + N 2 → 3 Mg +2 + 2 N -3 / × 3 3 Mg(s) + N 2 (g) → Mg 3 N 2 (s)

8. 2. decomposition reactions C → A + B breakdown of one compound into two or more compounds 2 HgO(s) → 2 Hg(l) + O 2 (g) 1.oxidation numbers 2. balancing charges

9. HgO(s) → Hg(l) + O 2 (g) 00-2 +2 oxidation (1) O -2 → ½ O 2 + 2e - reduction (2) Hg +2 + 2 e - → Hg (1)+(2) O -2 + Hg +2 + 2e - → Hg + 1/2 O 2 + 2e - / × 2 (1)+(2) 2 O -2 + 2 Hg +2 → 2 Hg + O 2 2 HgO(s) → 2 Hg(l) + O 2 (g)

10. 0 -2+5 KClO 3 (s) → KCl(s) + O 2 (g) +1 oxidation (1) O -2 → ½ O 2 + 2e - reduction (2) Cl +5 + 6 e - → Cl - / × 3 (1)+(2) 3 O -2 + Cl +5 + 6e - → 3/2 O 2 + Cl - + 6e - (1)+(2) 3 O -2 + Cl +5 → 3/2 O 2 + Cl - (1)+(2) ClO 3 - → 3/2 O 2 + Cl - (1)+(2) KClO 3 → 3/2 O 2 + KCl / × 2 2 KClO 3 (s) → 3 O 2 (g) + 2 KCl(s)

11. 3. displacement reactions A + BC → AC + B an ion or atom in a compound is replaced by an ion or atom of another element 3.1. Hydrogen displacement 3.2. Metal displacement 3.3. Halogen displacement

12. 3.1. Hydrogen displacement group I and some group II metals (Ca, Sr, Ba) react with water to form hydrogen Na(s) + H 2 O(l) → NaOH + H 2 (g) 0 0 +1 -2 +1 oxidation (1) Na → Na + + e - reduction (2) H +1 + e - → ½ H 2 (1)+(2) Na + H + + e - → Na + + ½ H 2 + e - / × 2 (1)+(2) 2 Na + 2 H + → 2 Na + + H 2 2 Na(s) + 2 H 2 O(l) → 2 NaOH + H 2 (g)

13. less reactive metals form hydrogen and the oxide in water (group III, transition metals) Al(s) + H 2 O(l) → Al 2 O 3 (s) + H 2 (g) 0 0 +1 -2 +3 oxidation (1) Al → Al +3 + 3 e - reduction(2) H +1 + e - → ½ H 2 / × 3 (1)+(2) Al + 3 H + + 3 e - → Al +3 + 3/2 H 2 + 3 e - / × 2 (1)+(2) 2 Al + 6 H + → 2 Al +3 + 3 H 2 2 Al(s) + 3 H 2 O(l) → Al 2 O 3 (s) + 3 H 2 (g)

14. even less reactive metals form hydrogen in acids Zn(s) + HCl(aq) → ZnCl 2 (aq) + H 2 (g) 00+1 +2 oxidation(1) Zn → Zn +2 + 2 e - reduction(2) H +1 + e - → ½ H 2 / × 2 (1)+(2) Zn + 2 H + + 2 e - → Zn +2 + H 2 + 2 e - (1)+(2) Zn + 2 H + → Zn +2 + H 2 Zn(s) + 2 HCl(aq) → ZnCl 2 (aq) + H 2 (g)

15. Li K Ba Ca Na Mg Al Zn Cr Fe Cd Co Ni Sn Pb H Cu Hg Ag Pt Au activity series of metals displace H from water displace H from steam displace H from acids

16. 3.2. Metal displacement V 2 O 5 (s) + 5 Ca(s) → 2 V(s) + 5 CaO(s) TiCl 4 (g) + 2 Mg (l) → Ti(s) + 2 MgCl 2 (l)

17. 3.3. Halogen displacement F 2 > Cl 2 > Br 2 > I 2 reactivity (‘likes’ electrons) Cl 2 (g) + 2 KBr(aq) → 2 KCl(aq) + Br 2 (l) 00+1 Br 2 (g) + 2 KI(aq) → 2 KBr(aq) + I 2 (s)

18. 4. disproportionation reactions an element in one oxidation state is oxidized and reduced at the same time H 2 O 2 (aq) → 2 H 2 O(l) + O 2 (g) +1 +1-20 Cl 2 (g) + 2 OH - (aq) → ClO - (aq) + Cl - (aq) + H 2 O(l) 0 -2+1 -2+1-2+1

19. SUMMARY 1.combination reactions A + B → C 2. decomposition reactions C → A + B 3. displacement reactions A + BC → AC + B 4. disproportionation reactions

20. Homework Chapter 4, p. 121-129 problems