A2 redox

Question 1

Define oxidising agent

Answer 1

Takes electrons from the species being oxidised (is reduced)

Question 2

Define reducing agent

Answer 2

Adds electrons to the species being reduced (is oxidised)

Question 3

Manganate (VII) Redox titration procedure

Answer 3

1) Add standard solution of KMnO4 to a burette
2) Using a pipette, add a measured volume of solution to a conical flask
3) Add dilute excess sulfuric acid (H+ ions will reduce MnO4)
4) End point = first permanent pink colour (excess of MnO4- ions)
5) repeat until concordant, read from the top of the meniscus as it is too dark

Question 4

How to calculate the percentage purity of an iron (II) compound from a MnO4- titration

Answer 4

1) Calculate the moles of MnO4- that reacted
2) Use the equation ratio (1:5 in this case) to work out moles of Fe
3) Scale up to original amount
4) x Mr = mass
5) mass percentage

Question 5

Analysing % purity of an iron (II) compound - procedure

Answer 5

1) 250cm3 solution of impure FeSO4.7H20 in a volumetric flask
2) Use a pipette to get 25cm3 into a conical flask
3) Using a burette, titrate using 0.2moldm-3 KMnO4(aq)

Question 6

Manganate and Iron reduction equation

Answer 6

MnO4-(aq) + 8H+(aq) + 5e- —> Mn2+(aq) +4H20(l)

Question 7

Manganate and Iron oxidation reaction

Answer 7

Fe2+(aq) —> Fe3+(aq) + e-

Question 8

Manganate and Iron overall equation

Answer 8

MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) –> Mn2+(aq) + 5Fe3+(aq) +4H2O(l)

Question 9

What are iodine/thiosulphate titrations used for?

Answer 9

To determine the The ClO- content in bleach, or the Cu2+ content in copper (II) alloys

Question 10

Equations for Copper and iodine

Answer 10

2Cu2+ +4I- –> “CuI +i2
The iodine produced in equation one is then titrated with a known solution of sodium thiosulfate :
S2O3 2- + I2 –> 2I- + S4O6 2-

Question 11

Procedure for an iodine/ thiosulfate titration

Answer 11

1) Add standard solution of Na2S2O3 to a burette
2) Prep the solution of oxidising agent and add to a conical flask with a pipette
3) Add an excess of potassium iodine (the oxidising agent will react with the iodide ions to produce iodine, making it go brown)
4) Titrate with Na2S2O3, when it turns a pale straw colour, add starch solution and wait until decolourises