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Question
In the form of dichromate, \[\ce{Cr (VI)}\] is a strong oxidising agent in acidic medium but \[\ce{Mo (VI)}\] in \[\ce{MoO3}\] \[\ce{and W (VI)}\] in \[\ce{WO3}\] are not because:
(i) \[\ce{Cr(VI)}\] is more stable than \[\ce{Mo(VI)}\] and \[\ce{and W(VI)}\].
(ii) \[\ce{Mo(VI)}\] and \[\ce{and W(VI)}\] are more stable than \[\ce{Cr(VI)}\].
(iii) Higher oxidation states of heavier members of group-6 of transition series are more stable.
(iv) Lower oxidation states of heavier members of group-6 of transition series are more stable.
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Solution
(ii) \[\ce{Mo(VI)}\] and \[\ce{and W(VI)}\] are more stable than \[\ce{Cr(VI)}\].
(iii) Higher oxidation states of heavier members of group-6 of transition series are more stable.
Explanation:
In d-block elements, for heavier elements, the higher oxidation states are more stable. Hence, \[\ce{Mo(VI)}\] and \[\ce{W(VI)}\] are more stable than \[\ce{Cr(VI)}\]. That’s why, \[\ce{Cr(VI)}\] in the form of dichromate is a stronger oxidizing agent in acidic medium whereas \[\ce{MoO3}\] and \[\ce{WO3}\] are not.
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