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Why is Cr^(2+) reducing and Mn^(3+) oxid...

Why is `Cr^(2+)` reducing and `Mn^(3+)` oxidising when both have `d^(4)` configuration ?

Text Solution

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`Cr^(2+)` has the configuration `3d^(4)` . It can lose electron to form`Cr^(3+)` which has the stable `3d^(3)` configuration ( as it has half- filled `t_(2g)` level - explained in unit). Hence, it is reducing . On the other hand, `Mn^(3+)` also has `3d^(4)` configuration but it can gain electron to form `Mn^(2+)` which has stable `3d^(5)` configuration ( as it is exactly half- filled ). Hence, it is oxidizing.
Alternatively. `E^(@)` value for `Cr^(3+) // Cr^(2+) ` is negative `( - 0.41V)` whereas `E^(@) `value for `Mn^(3+) // Mn^(3+)` is positive `( + 1.57V)` . Hence, `Cr^(2+)` ion can easily undergo oxidation to give `Cr^(3+)` ion and , therefore , acts as strong reducing agent where as `Mn^(3+)` can easily undergo reduction to give`Mn^(2+)` and hence acts as oxidizing agent.
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(a) What happens when (i) Manganate ions (MnO_(4)^(2-)) undergoes diSQProportionation reaction in acidic medium ? (ii) Lanthanum is heated with Sulphur? (b) Explain the following trends in the properties of the members of the First series of transition elements: (i) E^(@) (M^(2+)//M) value for copper is positive (+ 0.34 V) in contrast to the other members of the series. (ii) Cr^(2+) is reducing while Mn^(3+) is oxidising, though both have d^(4) configuration. (iii) The oxidising power in the series increases in the order VO_(2)^(+) lt Cr_(2)O_(7)^(2-) lt MnO_94)^(-) .

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Knowledge Check

  • Assertion (A) Cr^(2+) is reducing while Mn^(3+) is oxidising when both have d^4 configuration. Reason ( R) Configuration of Cr^2+ changes from d^3 to d^4

    A
    Both Assertion and Reason are correct explanation of the Assertion
    B
    Both Assertion and Reason are correct but Reason is not the correct explanation of Assertion
    C
    Assertion is correct but Reason is incorrect
    D
    Both Assertion and Reason are incorrect

  • Cr^(2+) and Mn^(3+) both have d^(4) configuration. Thus

    A
    both are reducing agents.
    B
    both are oxidizing agents.
    C
    `Cr^(2+)` is an oxidizing agent while `Mn^(3+)` is a reducing agent.
    D
    `Mn^(3+)` is an oxidizing agent while `Cr^(2+)` is a reducing agent.

  • Assertion : Cr^(2+) is reducing and Mn^(3+) is oxidising. Reason : Cr^(2+) and Mn^(3+) have d^4 configuration.

    A
    If both assertion and reason are true and reason is the correct explanation of assertion.
    B
    If both assertion and reason are true and reason not is the correct explanation of assertion.
    C
    If assertion is trun but reason is false.
    D
    If both assertion and reason are false.

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