Redox Reactions — Practice

Question 1

Assign the oxidation number of Mn in KMnO4 and Cr in Cr2O3.

Accepted Answer

KMnO4: +1 + x + 4(-2) = 0, so x = +7. Cr2O3: 2x + 3(-2) = 0, so x = +3.

Question 2

Balance Fe2+ + Cr2O7 2- -> Fe3+ + Cr3+ in acidic medium.

Accepted Answer

Oxidation: Fe2+ -> Fe3+ + e- (x6). Reduction: Cr2O7 2- + 14H+ + 6e- -> 2Cr3+ + 7H2O. Adding: 6Fe2+ + Cr2O7 2- + 14H+ -> 6Fe3+ + 2Cr3+ + 7H2O.

Question 3

Balance MnO4- + I- -> MnO2 + I2 in basic medium.

Accepted Answer

2MnO4- + 6I- + 4H2O -> 2MnO2 + 3I2 + 8OH-.

Question 4

Calculate E_cell for Zn + Cu2+ -> Zn2+ + Cu (E_Zn = -0.76 V, E_Cu = +0.34 V).

Accepted Answer

Zn is oxidised (anode), Cu2+ reduced (cathode). E_cell = E_cathode - E_anode = 0.34 - (-0.76) = +1.10 V, so the reaction is spontaneous.

Question 5

What is a disproportionation reaction? Give an example.

Accepted Answer

A reaction in which the same element is simultaneously oxidised and reduced. Example: 2H2O2 -> 2H2O + O2, where oxygen (-1 in H2O2) goes to -2 in water and 0 in O2.

Practice — Redox Reactions

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