Q4. How does acidified permanganate react with (i) iron(II) ions and (ii) oxalic acid? Write the ionic equations. (NCERT Exercise 4.16, parts (i) and (iii).)
Permanganate is reduced to Mn2+, taking five electrons. (i) 5Fe2+ + MnO4− + 8H+ → Mn2+ + 4H2O + 5Fe3+: green iron(II) becomes yellow iron(III). (ii) At 333 K: 5C2O42− + 2MnO4− + 16H+ → 2Mn2+ + 8H2O + 10CO2 (NCERT pages 107–108).
Q5. Indicate the steps in the preparation of (i) K2Cr2O7 from chromite ore and (ii) KMnO4 from pyrolusite ore. (NCERT Exercise 4.26.)
(i) Chromite + Na2CO3, fused in air → Na2CrO4; acidify with H2SO4 → Na2Cr2O7; add KCl → K2Cr2O7. (ii) Pyrolusite, MnO2, fused with KOH and an oxidant (air or KNO3) → green K2MnO4; then either acidify, so manganate disproportionates to MnO4− and MnO2, or oxidise it electrolytically in alkaline solution → KMnO4 (NCERT pages 105–106).
Q6. What mass of KMnO4 (158.04 g mol−1) is needed to oxidise 0.0300 mol of Fe2+ in acid?
One MnO4− takes five Fe2+ (NCERT page 107), so 0.0300 ÷ 5 = 6.00 × 10−3 mol KMnO4, which is 6.00 × 10−3 × 158.04 = 0.948 g.
Q7. Potassium permanganate is added to potassium iodide (a) in acid and (b) in neutral or faintly alkaline solution. What do iodide and manganese become in each?
(a) In acid, iodide becomes iodine and manganese Mn2+: 10I− + 2MnO4− + 16H+ → 2Mn2+ + 8H2O + 5I2. (b) In neutral or faintly alkaline solution, iodide becomes iodate, IO3−, and manganese MnO2: 2MnO4− + H2O + I− → 2MnO2 + 2OH− + IO3− (NCERT pages 107–108).
Q8. The lowest oxide of a transition metal is basic and the highest is amphoteric or acidic. Give examples and a reason. (NCERT Exercise 4.25 (i).)
CrO is basic, Cr2O3 amphoteric and CrO3 acidic; V2O3 is basic and V2O5 amphoteric but mainly acidic; Mn2O7 gives the acid HMnO4. As the metal’s oxidation number rises its oxide becomes less ionic and more covalent, so the acidic character takes over (NCERT page 105).