1. Oxidation Numbers

2. Oxidation numbers Know what oxidation numbers are Understand oxidation and reduction in terms of electron transfer and changes in oxidation number Calculate oxidation numbers of elements in compounds and ions Know redox reactions are Use Roman numerals to indicate the oxidation number of an elements in a compound or ion To be able to write ionic half equations Combine ionic half equations to construct full ionic equations

3. Oxidation numbers Also known as oxidation state, we use these numbers to measure the number of electrons that an atom is using to bond to another atom. We use a set of rules to calculate the oxidation numbers of elements in compounds and ions.

4. Oxidation numbers SpeciesOxidation numberExamples Uncombined element0C, Na, O 2 Combined oxygen (except in peroxides O 2 2- and compounds with fluorine, where it is +2) -2H 2 O, CaO Combined hydrogen (except metal hydrides, H - ) +1NH 3, H 3 S Simple ionCharge on ion Na+ = 1 Mg 2 + = 2 Chlorine (except in compounds with oxygen and fluorine) HCl Combined fluorineNaF, CaF, AlF 3

5. Oxidation States Deduce the ox state of nitrogen in HNO 3 HNO 3 +1 x 3(-2) Sum = (+1) + x + 3(-2) = 0 (neutral compound) +1 + x - 6 = 0  x = 5 nitric(V) acid

6. Oxidation numbers The overall charge on a compound such as SO 2 is 0. The sum of the oxidation numbers in the compound must equal the charge. Look at the table from before, which element do we know the O.N. of? Oxygen = -2 There are two of them.

7. Oxidation numbers Oxygen = -2 There are two of them. Total contribution of oxygen is: -4 Overall the O.N. must sum to 0, so what is the O.N. of sulphur? +4

8. Oxidation numbers Molecular ions In an ion such as CO 3 2-, the overall sum of the O.N.s must equal the charge. Work out the O.N.s of oxygen and carbon in CO 3 2- O = -2 C = +4

9. Oxidation States Deduce the ox state of manganese in MnO 4 2– MnO 4 2– x 4(-2) Sum = x + 4(-2) = -2 (overall charge on ion = -2 ) x - 8 = -2  x = + 6 manganate(VI) this is a green ion different from the purple MnO 4 – manganate(VII)

10. Oxidation States Deduce the ox state of chromium in Cr 2 O 7 2– Cr 2 O 7 2– 2 x7(-2) Sum = 2x + 7(-2) = -2 (overall -2 ion) 2x - 14 = -2 2x = +12  x = + 6 dichromate(VI) orange

11. Oxidation numbers Chemical names: In a compound such as Iron(III) chloride, the Roman numeral tells you the oxidation state of the iron. E.g. FeCl 2 Iron(II) chlorideFe: O.N. = +2 FeCl 3 Iron(III) chlorideFe: O.N. = +3

12. Oxidation numbers Oxyanions: These are negative ions containing an element and oxygen: sulphate, nitrate, carbonate. Oxidation numbers are used again to disambiguate similar oxyanions: E.g. NO 2 - Nitrate(III) N: O.N. = +3 NO 3 - Nitrate(V) N: O.N. = +5

13. Now complete the questions Answer on white boards Page 3 redox starters for 10

14. Oxidation numbers Know what oxidation numbers are Understand oxidation and reduction in terms of electron transfer and changes in oxidation number Calculate oxidation numbers of elements in compounds and ions Use Roman numerals to indicate the oxidation number of an elements in a compound or ion To be able to write ionic half equations Combine ionic half equations to construct full ionic equations

15. Definitions OIL RIG O xidation i s l oss of electrons An increase in oxidation number R eduction i s g ain of electrons A decrease in oxidation number

16. Definitions Redox A reaction in which oxidation and reduction both occur is known as a redox reaction.

17. Electron transfer This is a redox equation: Mg + Cl 2  MgCl 2 But this fact is concealed in the equation. Instead we can write two half equations: Mg  Mg 2+ + 2e - Cl 2 + 2e -  2Cl - But which is oxidation and which reduction?

18. Electron transfer Using oxidation numbers: Mg + Cl 2  MgCl 2 Mg:  Cl:  0+2 0 0

19. Using oxidation numbers Work out what has been reduced and what has been oxidised in the following reaction: MnO 2 (s) + 4HCl(aq)  MnCl 2 (aq) + 2H 2 O(l) + Cl 2 (g)

20. Now complete the questions on white boards Page 4 starter for 10

21. Redox eq

22. Combining half equations Mg  Mg 2+ + 2e - Cl 2 + 2e -  2Cl - How would you combine these to write an overall ionic equation?

23. Displacement of halogens If a halogen is added to a solution of a compound containing a less reactive halogen, it will react with the compound and form a new one. sodium chloride sodium fluoride chlorinefluorine ++  F2F2 2NaCl2NaFCl 2 ++  A more reactive halogen will always displace a less reactive halide from its compounds in solution. This is called displacement.

24. Halogen displacement reactions Halogen displacement reactions are redox reactions. Cl 2 + 2KBr  2KCl + Br 2 Write two half equations two show the electron transfer involved Chlorine has gained electrons, so it is reduced to Cl - ions. What has been oxidized and what has been reduced? 2Br -  Br 2 + 2e - Cl 2 + 2e -  2Cl - Bromide ions have lost electrons, so they have been oxidized to bromine.

25. Halogen displacement reactions Halogen displacement reactions are redox reactions. Cl 2 + 2KBr  2KCl + Br 2 Write two half equations two show the electron transfer involved Can you combine these to write an overall ionic equation? 2Br -  Br 2 + 2e - Cl 2 + 2e -  2Cl -

26. Challenge work Redox equations exam questions Complete worksheet Combining half equations to write full equations Page 5 starter for 10

27. Summary Exercise Now, on your own, without notes: 1.Work out the O.N. of: a.Sulphur in SO 3 2- b.Nitrogen in NH 4 + c.Carbon in CO 3 2- 2.Are the underlined atoms oxidised or reduced: a.Mg(s) + Cl 2 (g)  MgCl 2 (s) b.2S 2 O 3 2- + I 2 (g)  2I - + S 4 O 6 2- c.2Fe 3+ + 2I -  2Fe 2+ + I 2

28. Exercise -answers Now, on your own, without notes: 1.Work out the O.N. of: a.Sulphur in SO 3 2- +4 b.Nitrogen in NH 4 + -3 c.Carbon in CO 3 2- +4 2.Are the underlined atoms oxidised or reduced: a.Mg(s) + Cl 2 (g)  MgCl 2 (s)ox (0  +2) b.2S 2 O 3 2- + I 2 (g)  2I - + S 4 O 6 2- ox (+2  +2.5) c.2Fe 3+ + 2I -  2Fe 2+ + I 2 red (+3  +2)

29. Oxidation numbers Know what oxidation numbers are Understand oxidation and reduction in terms of electron transfer and changes in oxidation number Calculate oxidation numbers of elements in compounds and ions Know redox reactions are Use Roman numerals to indicate the oxidation number of an elements in a compound or ion To be able to write ionic half equations Combine ionic half equations to construct full ionic equations

30. Oxidation numbers Know what oxidation numbers are Understand oxidation and reduction in terms of electron transfer and changes in oxidation number Calculate oxidation numbers of elements in compounds and ions Know redox reactions are Use Roman numerals to indicate the oxidation number of an elements in a compound or ion To be able to write ionic half equations Combine ionic half equations to construct full ionic equations