5.2 - Redox And Titrations

Question 1

Oxidation def

Answer 1

Species that loses electrons

- increases in oxidation state

Question 2

Reduction def

Answer 2

Species gains electrons

- decreases in oxidation state

Question 3

Oxidising agent def

Answer 3

Oxidises another species in a reaction, an it itself becomes reduced
- gains electrons from the oxidised species

Question 4

Reducing agent def

Answer 4

Reduces another species in a reaction, but itself becomes oxidised
- donates electrons to the reduced species

Question 5

Two redox titration mechanisms to know

Answer 5

Potassium manganate and iron
- analysing purity of iron sulfate or amount of iron in ferrous oxide
Sodium thiosulphate and iodine

Question 6

Potassium manganate and iron redox titration

- ionic equations, half equations

Answer 6

Half equations:

Reduction: MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O
Oxidation: Fe2+ —> Fe3+ + e-

Overall equation: 5Fe2+ + MnO4- + 8H+ —> Mn2+ + 4H2O + 5Fe3+

Question 7

Manganate titration mechanism

Answer 7

Add a standard solution of potassium manganate to a burette
Use a pipette to measure to measure volume of solution being analysed to a conical flask
Add an excess of dilute sulfuric acid to provide H+ ions for reaction to occur
No indicator needed, as reaction is self-indicating
During titration, manganate solution reacts and is decolourised as is being added to solution in conical flask
End point of titration when first permanent pink colour visible
- can use a spotting tile to see colour change more clearly or quicker
This colour change indicates when there is an excess of MnO4- ions present
Repeat titration until two concordant titres are obtained - within 0.10cm3 of each other

Question 8

Analysing percentage purity of an iron(II) compound using manganate ions mechanism

Answer 8

Reduction equation: MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O
Oxidation: Fe2+ —> Fe3+ + e-

Overall: 5Fe2+ + MnO4- + 8H+ —> 5Fe3+ + Mn2+ + 4H2O

Procedure:

Prepare a solution of impure iron compound - 250cm3 in a volumetric flask
Use a pipette to measure 25.0cm3 of this solution into a conical flask
Then add 10cm3 of 1moldm-3 H2SO4 - an excess - to provide H+ ions for reaction to occur
Use a burette to titrate a solution of 0.0200 moldm-3 solution of potassium manganate(VII), KMnO4(aq), into the conical flask with your 25cm3 of standard solution of iron compound
Carry out titrations until a colour change from colourless to pink is seen
Use a spotting tile
Repeat titrations until two concordant titres (0.10cm3) are achieved - calculate mean titre

Question 9

Oxidation of a (COOH)2 group - ionic equation

Answer 9

(COOH)2 —> 2CO2 + 2H+ + 2e-

Question 10

Iodine -thiosulphate redox equations

Answer 10

S2O32- is being oxidised to S4O62-
I2 is being reduced to 2I-

Oxidation: 2S2O32- —> S4O62- + 2e-
Reduction: I2 + 2e- —> 2I-
Overall: 2SO32- + I2 —> S4O62- + 2I-

Question 11

What can iodine-thiosulphate titrations be used to determine

Answer 11

ClO- content in household bleach
Cu2+ content in copper compounds
Cu content in copper alloys

Question 12

Iodine-thiosulfate redox titration mechanism

Answer 12

Add a standard solution of Na2S2O3 to a burette
Prepare a solution of the oxidising agent to be analysed
Using a pipette, add the ox. agent solution to a conical flask - 25cm3 e.g.
Then add an excess of potassium iodide
The oxidising agent reacts with iodide ions to produce iodine
This turns solution a yellow brown colour
Titrate the solution with the Na2S2O3(aq)
During titration, iodine is reduced back to I- ions
Brown colour fades gradually, making it difficult to decide on an end point
This problem is solved using starch indicator
When end point is being reached, iodine colour has faded enough to become a pale straw colour

Question 13

Colour change in iodine-thiosulphate titration

Answer 13

Iodine reduced to I- ions
Yellow-brown colour fades to a pale yellow colour when reaction nearing endpoint
When starch is added, a blue-black solution is formed
However, as iodine is being reduced by thiosulphate ions, this colour fades to a pale-straw colour

Question 14

Copper content analysis using iodine-thiosulphate titrations mechanism

Answer 14

Redox equations:

Copper-iodine redox:

Oxidised: Cu is reduced
Reduction: I- is oxidised
Overall: 2Cu2+ + 4I- —> 2CuI + I2

• Cu2+ ions react with I- to form a solution of iodine, I2, and a white ppt., CuI(s)
• the mixture appears as a brown colour

The iodine, I2(aq), in the brown mixture is then titrate with a standard solution of sodium thiosulphate, equation as standard:

Overall: 2S2O32- + I2 —> 2I- + S4O62-

2mol Cu2+ produces 1mol I2 which reacts with 2mol S2O3
So 1mol Cu2+ is equivalent to 1mol S2O32-