F. Redox titrations

Question 1

Name an example of autocatalysis

Answer 1

The oxidation of enthandioic acid by manganate (VII) ions ( acidificed potassium maganagte (VII) )

Question 2

Write the overall equation for the oxidation of enthandioic acid by manganate (VII) ions

Answer 2

2MnO4 - (aq) + 16H+ (aq) + 5C2O4 2- (aq) —->

2Mn2+ (aq) + 8H2O (l) + 10CO2 (g)

Question 3

How does the products of the reaction of the oxidation of enthandioic acid by manganate (VII) ions catalyse the reaction

Answer 3

• Mn2+ ions react with the MnO4 - to form Mn3+ (the intermediate species)
• Mn3+ then reacts with C2O4 2- to reform Mn2+

Question 4

Write the two steps where Mn2+ catalyses the reaction for the oxidation of enthandioic acid by manganate (VII) ions

Answer 4

1. 4Mn2+ (aq) + MnO4 - (aq) + 8H+ (aq) —-> 5Mn3+ (aq) 4H2O (l)
   2.
   2Mn3+ (aq) + C2O4 2- (aq) —> 2Mn2+ (aq) + 2CO2 (g)

Question 5

Write the redox equation for the oxidising of Fe2+ to Fe3+ by potassium manganate (VII). During the reaction the oxidation number of manganese falls from 7+ to 2+. First construct the half equations

Answer 5

Reduction :
MnO4- (aq) + 8H+ (aq) + 5e- —-> Mn2+ (aq) +4H2O (l)
Oxidation: Fe2+ (aq) —> Fe3+ (aq) + e-
Overall:
5Fe2+ (aq) + MnO4- (aq) + 8H+ (aq) —-> 5Fe 3+ (aq)
+ Mn2+ (aq) + 4H2O (l)

Question 6

Write the redox equation for the oxidising of Fe2+ to Fe3+ by acidified dichromate (VI). During the reaction the oxidation number of chromium falls from 6+ to 3+. First construct the half equations

Answer 6

Reduction:
Cr2O7 2- (aq) + 14H+ (aq) + 6e- —> 2Cr3+ (aq) + 7H2O (l)
Oxidation: Fe2+ (aq) —> Fe3+ (aq) + e-
Overall:
6Fe2+ (aq) + Cr2O7 2- (aq) + 14H+ (aq) —>
6Fe3+ (aq) 2Cr3+ (aq) + 7H2O (l)