F325: Module 3: Redox Reactions and Titration Calculations

Question 1

I) When transition metals form ions, which sub shell looses its electrons first? 4s or 3d

Answer 1

The 4s electrons are added and removed before the 3d as it has a lower energy state.

Question 2

I) What happens to the relative stability of +2 state with respect to +3 state as you go across the period?

Answer 2

The relative stability increases.

Question 3

I) If a compound has a high oxidation state is it likely to be a reducing agent or an oxidising agent?

Answer 3

An oxidising agent.

Question 4

I) If a compound has a low oxidation state is it likely to be a reducing agent or an oxidising agent?

Answer 4

A reducing agent.

Question 5

I) What are the half equations for Fe, MnO4- and Cr2O7 2- (redox half equations with e-)

Answer 5

Fe3+ + e- ==> Fe2+
MnO4 - + 8H+ +5e- ==> Mn2+ + 4H2O
Cr2O7 2- + 14H+ + 6e- ==> 2Cr3+ + 7H2O

Question 6

I) NOTE: In acidic conditions use H+

Answer 6

No answer

Question 7

I) Go through how thiosulphate ions can be used to determine the concentration of Cu2+ ions in a solution.
What is the indicator used?

Answer 7

Firstly, excess iodide ions are added to the solution of Copper(II) ions:
2Cu2+ +4I- ==> 2CuI + I2

The iodine liberated is then titrated against standard sodium thiosulfate solution:
I2 + 2S2O3 2- ==> S4O6 2- + 2I-

From these equations 2Cu2+ = 2S2O3 2-
Therefore the umber of thiosulfate ions = the number of copper(II) ions.

The indicator used is starch which goes blue/black in the presence of iodide ions.

Question 8

How could you determine the end point of a titration which involves iodine?

Answer 8

Add starch which will be decolourised when the iodine is converted into iodide ions (NOT COLOURLESS- MUST SAY DECOLOURISED)

Question 9

An unusual compound of iron has been detected on the surface of the planet Mars. This compound contains the ferrate(VI) ion.
Aqueous sodium ferrate(VI) is a very powerful oxidising agent.
Predict what you would see when aqueous sodium ferrate(VI) is added to aqueous potassium iodide.
Explain your answer

Answer 9

Oxidised to) iodine so a brown (solution) formed /

Fe3+ formed which is yellow or orange / Fe2+ formed which is green

Question 10

Aqueous sodium ferrate(VI) will oxidise ammonia into substance X.
Suggest an identity for X.

Answer 10

Nitrogen/ N2