3.2 Redox Reactions

Question 1

How to construct a half equation if compound ions are involved

Answer 1

1. Write reagent and product
2. Balance atoms
   - add water if oxygen numbers are imbalanced
   - add H+ if hydrogen numbers are imbalanced
3. Add electrons to balance overall charges of the LHS and the RHS

Question 2

Half equation of MnO4- to Mn2+

Answer 2

MnO4- + 8H+ + 5e- —> Mn2+ + 4H2O

Question 3

Half equation of Cr2O72- to Cr3+

Answer 3

Cr2O72- + 14H+ + 6e- —> 2Cr3+ + 7H2O

Question 4

Half equation of S2O32- to S4O62-

Answer 4

2S2O32- —> S4O62- + 2e-

Question 5

How to combine half equations

Answer 5

1. Ensure each half equation is balanced
2. Ensure number of electrons in each is balanced
3. Add equations together - cancel out electrons and anything that is present on both sides

Question 6

Can you directly analyse a solution for Cu2+ ions

Answer 6

No
You must use an indirect route

Question 7

What is done to react Cu2+ with I-

Answer 7

Colourless solution containing iodide ions are added to a blue solution containing Cu2+ ions

Question 8

What is observed when iodide solution is added yo Cu2+ solution

Answer 8

A cloudy brown solution
Cloudy - ppt of CuI
Brown - iodide formed

Question 9

What is done with the iodine formed from mixing Cu2+ ions and I- ions to analyse how many Cu2+ ions were there initially ?

Answer 9

• They’re titrated with sodium thiosulphate to calculate how much copper was present originally
• The solution becomes pale until it is straw coloured
• starch indicator is then added which is blue/black in the presence of iodine
• end point is when the solution turns from blue/black to colourless