AC9: Redox reactions

Question 1

Lay out the steps for writing half equations involving polyatomic ions in neutral/acidic conditions

Answer 1

1- balance atoms undergoing redox
2- Balance O using H2O
3- Balance H using H+
4- Balance charge using e-

Question 2

Lay out the steps for writing half equations involving polyatomic ions in alkaline conditions

Answer 2

1- balance atoms undergoing redox
2- Add H2O to balance O
3- Add H+ to balance H
4- Add OH- to BOTH sides to remove H+
5- Combine H+ and OH- to give H2O
6- Cancel down & simplify
7- Add e- to balance charge

Question 3

What must you make sure of when combining half equations to give the overall equation

Answer 3

that there are no electrons in the overall equation

Question 4

What are the two most common redox titrations

Answer 4

1- Manganate under acidic conditions
2- Thiosulfate for determining [I2]

Question 5

How can you tell when the end point has occurred in a redox titration with manganate under acidic conditions

Answer 5

KMnO4 is self indicating, MnO4- is purple/violet
the end point colour change is colourless (seen in conical flask) to pale pink or purple (if MnO4- is in conical flask at the start) to pale pink

Question 6

What is MnO4-/H+ (acidified manganate) commonly used for with regards to redox titrations

Answer 6

determining Fe2+ in iron tablet
determining ethanedioic acid or ethanedioate ions

Question 7

What is S2O3 2- (thiosulfate) commonly used for with regards to redox titrations and what is this called

Answer 7

to determine [I2(aq)] - concentration of I2
called iodometry

Question 8

What can iodometry be used for

Answer 8

determining % purity of copper in brass/coins
determining the amount of ClO- in a sample of bleach

Question 9

what is the indicator for I2/S2O3 2-
titrations

Answer 9

Starch indicator is added near the end point to give a much more abrupt colour change from blue-black to colourless