AG Oxidation Reduction Revision

AG Definitions Oxidation is addition of loss of increase in oxygen electrons oxidation number Exam Q (Hons) ‘13/Q10(b) ’09/Q10(b) ‘08/Q10(b) ’06/Q10(b) ’05/Q11

AG Reduction is loss of oxygen gain of electrons decrease in oxidation number

AG More… An oxidising agent causes(allows) oxidation and is itself reduced. A reducing agent causes causes(allows) reduction and is itself oxidised. What is a redox reaction?

AG What is oxidised and reduced in each of the following? Br 2 + 2Fe 2+ → 2Br – + 2Fe 3+ Cu 2+ + Zn  Cu + Zn 2+ 2Na + Cl 2  2NaCl

AG Learning Check Can I give definition for a)Oxidation In terms of Electron transfer Oxygen transfer Oxidation number Repeat for reduction NOT The End - click to go on

AG Oxidation Number Oxidation number is defined as  The charge an atom has  Or appears to have  When electrons are distributed  according to certain rules

AG Oxidation Number Rules The oxidation number of an Element is 0 group One elements is +1 group Two elements is +2 in compounds

AG The oxidation number of an ion is equal to the charge on the ion halogens is -1 (in binary compounds) (except ……????)

AG The oxidation number of H in a compound is +1 –except in metal hydrides when it is -1

AG The oxidation number of O in a compound is -2 –except (x2) in peroxides when it is -1 (H 2 O 2 ) in OF 2 when it is +2 (why?)

AG Oxidation numbers add up to zero in a compound add up to the charge of a complex ion

AG What is the oxidation number of each element in :- (write down before you go on) H20H20 MnO 4 ¯ I2I2 KBrO 3 Na 2 S 2 O 3 H2O2H2O2 NaOCl

AG The oxidation number of each element is :- H20H20 MnO 4 ¯ I2I2 KBrO 3 Na 2 S 2 O 3 H2O2H2O2 NaOCl

AG Learning Check Can I give the oxidation number RULE for a)Oxygen b)Hydrogen c)free element d)Neutral atom (sum) e)Ion (simple and complex) f)Group 1 element g)Group 2 element h)HALOGEN STILL NOT The End - click to go on

AG Balancing Equations with oxidation numbers STEPS 1.Assign oxidation numbers 2.Identify what is oxidised and reduced 3.Write half equation SIDE by SIDE for each (showing number of electrons on the move for one atom of each) 4.Rewrite for the number of atoms given e.g. Cr 2 5.Balance the electrons 6.REWRITE the original equation using these “prefixes” 7.Balance remainder by inspection 8.CHECK – do the charges on each side cancel out??

AG Example Assign & Identify Cr 2 O Fe 2+ + H +  Cr 3+ + Fe 3+ + H Reduced x3 Oxidised x1

AG 2 Identify oxidised or reduced & number of electrons lost or gained per atom & as given ReducedOxidised Cr + 3e -  CrFe – e -  Fe ATOM Cr 2 + 6e -  2CrFe – e -  Fe GIVEN Balance Electrons Cr 2 + 6e -  2CrFe – e -  Fe Rewrite and sub back Cr 2 O Fe 2+ + H+  2 Cr Fe 3+ + H 2 0 Balance remainder by inspection Cr 2 O Fe 2+ + H+  2 Cr Fe 3+ + H

AG Check Charges on each side should balance Cr 2 O Fe H +  2 Cr Fe H 2 0 LEFT RIGHT ___24+

AG KMnO 4 oxidising agent purple read top of meniscus is reduced from Mn (VII)  Mn (II) in presence of H + purple  colourless own indicator (end point = first permanent pink)

AG KMnO 4 get brown Mn (IV) if H + absent (which acid MUST be used – why x2)  Not HCl – it reacts with KMnO 4 /Cl - oxidised to Cl 2  Not HNO 3 – is oxidising agent too – would react with the Fe 2+ not primary standard (x2) »Not available pure »Decomposes in sunlight standardised by titrating against standard solution of acidified Fe 2+

AG H 2 SO 4 added during KMnO 4 titrations to provide H + and ensure the complete reduction of Mn (VII)  Mn (II) and prevent formation of Mn (IV) (brown) added during prep. of Fe (II) solutions to prevent oxidation of Fe 2+ to Fe 3+ by oxygen in the air ( why does this matter?)

AG Na 2 S 2 O 3 S 2 O 3 2- ion reducing agent used in photography not primary standard – why ? standardised by titrating against I 2 starch indicator – when added and why colour change at end point ?

AG IodineI 2 Oxidising agent NOT a primary standard (X2) Produced when MnO 4 - oxidises I - to I 2 »(known concentration)(in excess) Starch indicator – when added? why then? Colour change at end point Blue/black to colourless

AG Bleach sodium hypochlorite Na + ClO - bleach diluted x10 with distilled water not de-ionised water (why? ) ClO - oxidises I - to I 2 I 2 v. thiosulfate starch indicator as before NB remember dilution factor in calculations

AG Learning Check Can I name 2 oxidising agents Name 2 reducing agents say why KMnO 4 isn’t primary standard say why Na 2 S 2 O 3 isn’t primary standard say why I 2 isn’t primary standard say why bleach isn’t primary standard Give details of titrations + indicators for each FINALLY The End