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प्रश्न
Explain why does colour of KMnO4 disappear when oxalic acid is added to its solution in acidic medium.
Why does pink colour of aqueous KMnO4 solution disappear when warmed with oxalic acid solution in acidic medium?
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उत्तर
The pink colour disappears because the permanganate ion \[\ce{(MnO^-_4)}\], which is purple/pink, is reduced to the manganese(II) ion (Mn2+), which is colourless.
\[\ce{5C2O^{2-}4 + \underset{(Coloured)}{2MnO^-4} + 16H+ -> \underset{(Coloured)}{2Mn^2+} + 8H2O + 10CO2}\]
The reaction is naturally very slow at room temperature. Warming the solution provides the activation energy needed to start the reaction. Once the reaction begins, the Mn2+ ions produced act as an autocatalyst, further accelerating the decolourisation.
संबंधित प्रश्न
Account for the following:
Cu+2 salts are coloured, while Zn2+ salts are white.
Give reasons:
Transition metals show variable oxidation states.
Out of Mn3+ and Cr3+, which is more paramagnetic and why ?
(Atomic nos. : Mn = 25, Cr = 24)
How would you account for the irregular variation of ionization enthalpies (first and second) in the first series of the transition elements?
For M2+/M and M3+/M2+ systems, the E° values for some metals are as follows:
| Cr2+/Cr | −0.9 V |
| Mn2+/Mn | −1.2 V |
| Fe2+/Fe | −0.4 V |
| Cr3/Cr2+ | −0.4 V |
| Mn3+/Mn2+ | +1.5 V |
| Fe3+/Fe2+ | +0.8 V |
Use this data to comment upon:
- The stability of Fe3+ in acid solution as compared to that of Cr3+ or Mn3+ and
- the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal.
Which metal in the first series of transition metals exhibits +1 oxidation state most frequently and why?
NF3 is possible, but NF5 is not. Why?
Give reasons for the following:
The transition metals generally form coloured compounds.
Transition metals with lowest melting point is ______.
Although Zirconium belongs to 4d transition series and Hafnium to 5d transition series even then they show similar physical and chemical properties because ______.
Assertion: \[\ce{Cu^2+}\] iodide is not known.
Reason: \[\ce{Cu^2+}\] oxidises \[\ce{I^-}\] to iodine.
Identify A to E and also explain the reactions involved.
The spin magnetic moment of cobalt in the compound Hg [Co(SCN)4] is:-
Among the following pairs of ions, the lower oxidation state in aqueous solution is more stable than the other in:-
The basic character of transition metals monoxide follow the order.
On adding NaOH, solution to the aqueous solution of K2CrO7 the colour of the solution changes from
Match List - I with List - II.
| List - I | List - II | ||
| (A) | [Fe(CN)6]3− | (i) | 5.92 BM |
| (B) | [Fe(H2O)6]3+ | (ii) | 0 BM |
| (C) | [Fe(CN)6]4− | (iii) | 4.90 BM |
| (D) | [Fe(H2O)6]2+ | (iv) | 1.73 BM |
Choose the correct answer from the options given below.
Which of the following ions acts as a typical transition metal ion?
Consider the following standard electrode potentials (E° in volts) in aqueous solution:
| Element | M3+/M | M+/M |
| Al | - 1.66 | +0.55 |
| Tl | + 1.26 | -0.34 |
Based on these data, which of the following statements is correct?
Complete the following reaction and justify that it is a disproportionation reaction:
\[\ce{3MnO^{2-}4 + 4H^+ -> \underline{}\underline{}\underline{}\underline{} + \underline{}\underline{}\underline{}\underline{} + 2H2O}\]
