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?

• Br2 + 2Fe 2+ → 2Br– + 2Fe 3+

• Cu 2+ + Zn Cu + Zn 2+

• 2Na + Cl2 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 (H2O2)

in OF2 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)

H20

MnO4¯

I2

KBrO3

Na2S2O3

H2O2

NaOCl

AG

The oxidation number of each element is :-

H20

MnO4¯

I2

KBrO3

Na2S2O3

H2O2

NaOCl

+1 -2

+7 -2

0

+1 +5 -2

+1 +2

-2

+1

-2 +1

+1

-1

AG

Learning Check

Can I give the oxidation number RULE for a) Oxygenb) Hydrogenc) free elementd) Neutral atom (sum)e) Ion (simple and complex)f) Group 1 elementg) Group 2 elementh) HALOGEN

STILL NOT The End - click to go on

AG

Balancing Equations with oxidation numbers

STEPS1. 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. Cr2

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

ExampleAssign & Identify

Cr2O72- + Fe2+ + H+ Cr3+ + Fe3+ + H20

+6 -2 +2 +1 +3 +3 +1 -2

Reduced x3

Oxidised x1

AG

2 Identify oxidised or reduced &

number of electrons lost or gained per atom & as given

Reduced OxidisedCr + 3e- Cr Fe – e- Fe ATOM

Cr2 + 6e- 2Cr Fe – e- Fe GIVEN

• Balance ElectronsCr2 + 6e- 2Cr Fe – e- Fe

• Rewrite and sub backCr2O7

2- + 6Fe2+ + H+ 2Cr3+ + 6Fe3+ + H20

• Balance remainder by inspectionCr2O7

2- + 6Fe2++ H+ 2Cr3+ + 6Fe3++ H20

66 6

714

AG

Check

• Charges on each side should balance

• Cr2O72- + 6Fe2++ 14 H+ 2Cr3+ + 6Fe3++ 7 H20

• LEFT RIGHT2- 6+

12+ 18+

14+ ___

24+ 24+

AG

KMnO4

• 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

KMnO4

• get brown Mn (IV) if H+ absent

• (which acid MUST be used – why x2) Not HCl – it reacts with KMnO4 /Cl- oxidised to Cl2 Not HNO3 – is oxidising agent too – would react with the Fe2+

• not primary standard (x2)» Not available pure

» Decomposes in sunlight

• standardised by titrating against standard solution of acidified Fe 2+

AG

H2SO4

• added during KMnO4 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

Na2S2O3

• S2O3 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

Iodine I2

• Oxidising agent

• NOT a primary standard (X2)

• Produced when MnO4- oxidises I- to I2

» (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 I2

• I2 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 KMnO4 isn’t primary standard • say why Na2S2O3 isn’t primary standard•say why I2 isn’t primary standard

•say why bleach isn’t primary standard •Give details of titrations + indicators for each

FINALLY The End