1. ELECTROCHEMISTRY By Mr. Abdulwakil Daudi (Msc. BSc.Ed) Oxidation Numbers or Oxidation State

3. Oxidation Number Oxidation number or Oxidation state is an apparent charge on an element will have when in a compound. Oxidation number is assigned to an atom to indicate its degree of oxidation or reduction. Example:- Fe 2+ = +2; Al 3+ = +3; NH 4 + = +1 Cl - =-1; NO 3 - = -1; SO 4 2- = -2

4. Rules for Assigning Oxidation Numbers to Elements Rule 1: The oxidation number of an element in its uncombined state is zero. For examples: Simple elements:- Al(s), Zn(s), Ar (g). Ox. No., X=0 Simple Molecules:- Diatomic elements. Like H 2 (g), O 2 (g), N 2 (g), F 2 (g), Br 2 (l), I 2 (s). Ox. No. of X 2 = 0 and other molecules, S 8 =0, P 4 = 0 Rule 2: The oxidation number of a simple charged element is the same is the same as the charge on the ion, for example: Cations: Na + = +1; H + = +1; Ca 2+ = +2; Mg 2+ = +2; Al 3+ = +3; Cr 3+ = +3; Mn 7+ = +7 etc Anions: Cl - = -1; Br - = -1; O 2- = -2; S 2- = -2; N 3- = -3 etc

5. Rules for Assigning Oxidation Numbers to Elements Rule 3: The oxidation number of hydrogen in all compound is always +1 except in a binary metal hydride where the oxidation number of hydrogen is –1. Examples (H= +1): In H 2 O, H 2 S, HCl, NH 3, H 2 SO 4, NH 3 Note- Non-metal with Hydrogen Examples (H=-1): In hydrides like NaH, CaH 2, AlH 3, Note- Metallic hydrides. Rule 4: The oxidation number of oxygen in a compound always –2. Except in peroxides where its –1 and OF 2 where it is +2. Examples: Oxides (-2) NaOH, H 2 O, CO 2, H 2 SO 4, MgO etc Exceptions: Peroxides (-1) Na 2 O 2, H 2 O 2, BaO 2 and (+2) OF 2

6. Rules for Assigning Oxidation Numbers to Elements Rule 5: The sum of all oxidation numbers of all elements in a neutral compound is zero. E.g. NaOH= Na+O+H=(+1)+(-2)+(+1)=0 Rule 6: The sum of all oxidation numbers in a polyatomic (Radical) ion is equal to the overall charge on the ion. E.g. SO 4 2- = S+4O=(+6)+4(-2)=+6-8= -2

7. Rules for Oxidation Number or Oxidation State ElementOxidation Number ExceptionExample Group 1 metals Always +1-Na +, K + Group 2 metals Always +2Ca 2+, Mg 2+ HalogensAlways -1F -, Cl -, Br - OxygenUsually -2Peroxide (-1), OF 2 (+2) Na 2 O (-2); Na 2 O 2 (-1) HydrogenUsually +1Metal hydrides (-1) H 2 O, NH 3 (+1); NaH (-1) CompoundsAlways sum = 0CO, Al 2 (SO 4 ) 3, Polyatomic ion Sum = chargePO 4 3- (-3); Al 3+ (+3)

8. Functions of Oxidation Numbers Naming inorganic compounds Tracking electron movement in redox reaction Calculation of oxidation number of an element in a compound or polyatomic ions.

10. Calculation of oxidation number of elements in Neutral Compounds

11. Calculation of oxidation number of elements Polyatomic Ions

12. Exercises on calculations of oxidation numbers (i)NH 3 (ii)NaNO 2 (iii)Ca(NO 3 ) 2 (iv)MnO 2 (v) CO 2 (vi)CO (vii) NO (viii)NO 2 (ix) N 2 O (x) MgCO 3 (xi) K 2 Cr 2 O 7 (i)PH 4 + (ii)SO 4 2- (iii)Cr 2 O 7 2- (iv)ClO - (v)MnO 4 2- (vi)PO 4 3- (vii)NO 2 - (viii)ClO 3 - (ix)ClO 3 - (x) HCO 3

13. Tracking electron movement in redox reaction

14. Tracking electron movement in redox reaction

15. FURTHER REDOX REACTIONS Prepare a fresh solution of iron(II)sulphate and divided into two test tubes. In the 1 st tt, NaOH confirms presence of Fe 2+ ions. In the 2 nd tt, H 2 O 2 oxidizes Fe 2+ to Fe 3+ ions. In the oxidized 2 nd tt, NaOH confirms presence of Fe 3+ ions. The observations made and inference were recorded in the table below:

16. Results and Discussions

17. Results from Oxidation Experiment Observations and Ionic equation for 1 st test tube: Fe 2+ (aq) + 2OH - ➙ Fe(OH) 2 (s), green precipitate. Half equations on effect of acidified H 2 O 2 in 2 nd test tube. Oxidation- increase in oxidation number 2Fe 2+ (aq) ➙ 2Fe 3+ (aq) + 2e - (l) Reduction – decrease in oxidation number H 2 O 2(aq) + 2H + (aq) + 2e - ➙ 2H 2 O (l) Overall equation from the two halve equations in the above 2Fe 2+ (aq) + H 2 O 2(aq) + 2H + (aq) ➙ 2Fe 3+ (aq) + 2H 2 O (l) light-green Yellow Effect of NaOH on Fe 3+ in excess Fe 3+ (aq) + 3OH - ➙ Fe(OH) 3 (s), brown precipitate.

18. Determining Oxidation (reducing agent) or reduction (oxidizing agent) of substances

19. Redox in Terms of Oxidation Number

20. Exercise for Redox and Oxidation Number