Have you ever heard about oxidation? Where? When? When the iron goes rusty? In a combustion? When we breathe? REDOX

Fe + 1/2 O 2  FeO Fe + HCl  FeCl 2 Do you see similarities in these two reactions? In which valence acts the iron, before and after? The first one is an oxidation, and the second one?

REDOX In the reaction with oxygen the Fe converts in Fe 2+. In the second reaction the Fe converts also in Fe 2+. In both cases Fe has been oxidised. In both cases Fe has lost electrons.

REDOX CONCEPTS OXIDATIONREDUCTION Gain of OxigenLoss of Oxigen Loss of HydrogenGain of Hydrogen Loss of electronsGain of electrons Increase in Decrease in oxidation stateoxidation state

Oxidation state (number) Rules Monoatomic ions: its charge Uncombined elements: 0 Oxigen: -2 excep in peroxides –1 Hydrogen: +1 excep in hidrides –1 Group I ions: +1 (Na + ) Group II ions: +2 (Mg 2+ ) Group III ions: +3 (Al 3+ ) Group VI ions: -2 (O 2-, S 2-,) Group VII ions: -1 (F -, Cl -, Br -, I -, )

REDOX RULES 1. The oxidation level of an atom is 0 when the atom is in its elemental form: Li, C, Na, Mg, etc. H 2, N 2, O 2, F 2, Cl 2, etc. 2. In ionic compounds, the oxidation state of an atom is the same as its charge: NaCl = Na +1 + Cl -1, MgO = Mg +2 + O The sum of the oxidation levels of all the atoms in a compound must equal 0. AlCl 3 = Al Cl -1

REDOX RULES (cont) 4. In covalent compounds, the oxidation level of H is +1, O is -2, F is Rules 3 and 4 allow the calculation of the oxidation levels of other atoms in a molecule: HNO 3 1-H atom = +1, 3-O atoms = 3(-2) = - 6 (+1) + (-6) = -5; N = +5 What is the oxidation level of the P atom in H 3 PO 4, in PH 3 ? What is the oxidation level of S in SO 2, in SF 6 ?

OTHER CONCEPTS oxidizing agent  it becomes reduced  its oxidation state decreases reducing agent  it becomes oxidized  its oxidation state increases spectator ion  don’t change its oxidation state

SUMMARY Something is oxidized if It gains oxygen It loses electrons Its oxidation state increases Something is reduced if It loses oxygen It gains electrons Its oxidation state decreases

EXERCICES Insert electrons (e - ) on the appropiate side of the following half-equations in order to balance and complete them, so that the electrical charges on both sides are equal K  K + H 2  2 H + O  O 2- Cu +  Cu 2+ Cr 3+  Cr 2+

EXERCICES In each case state which element is oxidised or reduced, and give the oxidation states before and after the reaction Cl Br -  2 Cl - + Br 2 2Fe + 3Cl 2  2FeCl 3 H 2 + Cl 2  2HCl 2FeCl 2 + Cl 2  2FeCl 3 2 H 2 O + 2 F 2  4HF + O 2

EXERCICES Indicate whether each element has been oxidised, reduced, both, or has remained unchanged. Cu 2 O + 2H +  Cu 2+ + Cu + H 2 O 3Br 2 + 6OH -  BrO Br - + 3H 2 O 4IO 3 -  3IO I -

BALANCE THE EQUATION BrO Fe 2+  Br - + Fe State the oxidations states 2. Write the half-reactions 3. Balance the number of atoms ox/red 4. Balance O with H 2 O 5. Balance H with H + 6. Balance the charge with electrons (e - )

BALANCING REDOX 7. Make the number of electrons in both half-equations equal 8. Sum the half-equations 9. Transform to molecular compounds

K + + BrO Fe 2+ + SO H +  K + + Br - + Fe 3+ +SO S.Ox. Fe 2+  Fe 3+ 1 e - + S.Re.BrO 3 -  Br H 2 O+6H + +6 e - BrO H + )x6 +6Fe 2+  Br H 2 O+6Fe 3+ KBrO 3 +3H 2 SO 4 +6 FeSO 4  KBr+3H 2 O+3Fe 2 (SO 4 ) 3 Reaction between potassium bromate (KBrO 3 ) and iron (II) sulphate (FeSO 4 ) in presence of sulphuric acid, producing potassium bromide and iron(III) sulphate KBrO 3 + H 2 SO 4 + FeSO 4  KBr+ Fe 2 (SO 4 )