d - and f -Block Elements

Important Compounds of Transition Elements

  • Preparation of K2Cr2O7:
  • Na2CrO4 is extracted with water as yellow solution and acidified by H2SO4 till pH is close to 4
    (ii) 2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O
    After removal of Na2SO· 10H2O, the solution is boiled with KCl.
    (iii) Na2Cr2O7 + 2 KCl → 2 NaCl + K2Cr2O7
    Orange coloured crystals of K2Cr2O7 (m.p. 398° C) are collected.
    (iv) Na2Cr2O7 is hygroscopic and is therefore, converted to stable K2Cr2O7.
  • Properties of K2Cr2O7:
    (a) Effect of heat : 4K2Cr2O7 → 4 K2CrO4 + 2Cr2O3 + 3O2
    (b) Change of pH :
  • (c) Action of H2SO4:
  • (f) Chromyl chloride test:

  • (ii) Orange-red fumes of chromyl chloride are dissolved in NaOH to obtain Na2CrO4, yellow solution which gives yellow precipitates of PbCrO4 an reacting with (CH3COO)2 Pb
    (iii) CrO2Cl2 + 2NaOH → Na2CrO4 +2HCl
          Na2CrO4 + (CH3COO)2 Pb → PbCrO4 +2 CH3COONa
    (iv) This test is not given by HgCl2 and other covalent chlorides.
  • (g) Oxidizing properties: Solution turns green due to the formation of Cr3+ [Cr2(SO4)3]
    \tt Cr_{2}O_7^{2-}+14 H^{+}+6e^{-}\rightarrow 2Cr^{3+}+7H_{2}O
    \tt Eq.wt = \frac{Mol.wt\ of\ M}{6}
    (i) Molecular equation K2Cr2O7 + 4 H2SO4 → K2SO4 + Cr2(SO4)3 + 4 H2O + 3[O]
    (ii) Oxidation of HCl : \tt Cr_{2}O_7^{2-}+8H^{+}+6HCl \rightarrow 2Cr^{3+}+7H_{2}O+3Cl_2
    (iii) Oxidation of I: \tt Cr_{2}O_7^{2-}+14H^{+}+6I^{-} \rightarrow 2Cr^{3+}+7H_{2}O+3I_2
    (iv) Oxidation of Fe2+: \tt Cr_{2}O_7^{2-}+14H^{+}+3H_2S \rightarrow 2Cr^{3+}+7H_{2}O+3S
    (v) \tt SO_3^{2-} \ to \ SO_4^{2-}:Cr_{2}O_7^{2-}+8H^{+}+3SO_3^{2-} \rightarrow 2Cr^{3+}+3SO_4^{2-}+4H_{2}O
    (vi) SO2 to H2SO4: \tt Cr_{2}O_7^{2-}+3SO_{2}+2H^{+} \rightarrow 2Cr^{3+}+H_{2}O+3SO_4^{2-}
    (vii) Chrome alum [K2SO4. Cr2(SO4)3. 24H2O] used for tanning of leather is collected from this solution by crystallization.
    (viii) Sn2+ to Sn4+ : \tt Cr_{2}O_7^{2-}+3Sn^{2+}+14H^{+} \rightarrow 2Cr^{3+}+3Sn^{4+}+7H_{2}O
    (ix) Oxidation of ethanol: \tt C_{2}H_{5}OH\xrightarrow[-H_{2}O]{{[O]}}CH_{3}CHO\xrightarrow[]{{[O]}}CH_{3}COOH.
  • Preparation of KMnO4:

  • \tt MnO_{2}+2KOH+\frac{1}{2}O_{2}\rightarrow K_{2}MnO_{4}+H_{2}O
    (b) 2K2MnO4 + Cl2 → 2KMnO4 + 2KCl
         2K2MnO4 + H2O + O3→ 2KMnO4 + 2KOH + O2
         3K2MnO4 + 2CO2 → 2KMnO4 + MnO2 + 2K2CO3
    (c) Electrolytic oxidation : \tt MnO_4^{2-}\rightarrow MnO_4^{^-}+e^{-}
  • Properties of KMnO4 :
    (a) Effect of heat: \tt 2KMnO_4 \xrightarrow[]{{473^{o}C}} K_{2}MnO_4+MnO_{2}+O_{2}

  • (b) Action of H2SO4:

  • (ii) If KMnO4 is in excess (MnO3)2SO4 changes to oily explosive Mn2O7
    (MnO3)2 SO4 + H2O → Mn2O7 + H2SO4:
    (iii) The net reaction in case of excess of KMnO4 solid is 2KMnO4 + 2H2SO4 → 2KHSO4 + Mn2O7 + H2O
    (iv) Mn2O7 explodes if the solution is not ice cold Mn_{2}O_{7}\rightarrow 2MnO_{2}+\frac{3}{2}O_{2}
    (c) Oxidising properties:
    (i) Acidic medium:
    (a) \tt MnO_4^\ominus + 8H^{+}+5e^{-}\rightarrow Mn^{2+}+4H_{2}O
         Equivalent mass = \tt \frac{Mol\ wt \ of \ M}{5}
    (b) Molecular equation 2 KMnO4 + 3H2SO4 (dil) → K2SO4 + 2MnSO4 + 3H2O + 5[O]
    (c) In acidic medium KMnO4
         Oxides I to I2
                    Fe2+ to Fe3+
                    H2S to S; \tt SO_3^{2-} \ to \ SO_4^{2-}
                    SO2 to H2SO4
                    Sn2+ to Sn4+ :
                    \tt NO_2^{^-} \ to \ NO_3^{^-}
                    HCl to Cl2
                    alcohols to acids as K2Cr2O7 / H+ does.
    (d) \tt 2MnO_4^{^-} + 6H^{+}+10 \ HCl \rightarrow 2Mn^{2+}+8H_{2}O+5Cl_{2}
    (e) In Lab KMnO4 / H+ is specifically used to estimate the strength of Mohr's salt (NH4)2 SO4 · FeSO4 · 6H2O and oxalic acid.

  • (f) Oxidation of FeC2O4 : Here, both the ions are oxidised. The same is the case of Fe(NO2)2
    \tt 3MnO_4^{^-} + 24H^{+}+5FeC_{2}O_{4} \rightarrow 3Mn^{2+}+12H_{2}O+5Fe^{3+}+10CO_{2}
    (ii) Neutral medium:
    \tt MnO_4^{^-} + 2H_{2}O+3e^{-}\rightarrow MnO_{2}+4OH^{^-}
    Equivalent mass = \tt \frac{Mol \ wt \ of \ M}{3}=\frac{M}{3}
    (b) Molecular equation 2 KMnO4 + H2O → 2KOH + 2MnO2 + 3[O]
    (c) Oxidation of H2S
                       2KMnO4 + 3H2S → 2KOH + 2MnO2 + 3H2O + 3S
    (d) Oxidation of Na2S2O3
                        2KMnO4 + 3Na2S2O3 + H2O → 2KOH + 2MnO2 + 3Na2SO4 + 3S
    (e) NH3 to N2
                    2NH3 + 2 KMnO4 → 2KOH + 2MnO2 + 2H2O + N2
    (iii) Alkaline medium (1% alk.KMnO4 is called Baeyer's reagent.)
    (a) Purple solution (Mn7+) changes to green (Mn+6) and then to brown due to the formation of Mn4+ as MnO2
                                  \tt MnO_4^{^-}+1e^-\rightarrow MnO_2^{2-}
                                 Equivalent mass = \tt \frac{Mol \ wt \ of \ M}{1}=M.
  • (f) Oxidation of KI
                             2 KMnO4 + H2O + KI → 2KOH + KIO3 + 2MnO2
  • Structures of \tt MnO_4^{^-},CrO_4^{2-} and \tt Cr_{2}O_7^{2-} : In both central atom undergoes d3s hybridization {i.e. 3dxy 3dyz 3dzx 4s}

View the Topic in this Video from 0:10 to 17:37

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