Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryMARCH 20242 Marks
Question
Define Lanthanide contraction. Write the balanced chemical equations when acidified $K_2Cr_2O_7$ reacts with $H_2S.$
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Answer
With an increasing atomic number in the lanthanoid series, there is a progressive decrease in the atomic as well as ionic radii of trivalent ions from $La3+$ to $Lu3+.$
This regular decrease $($contraction$)$ in the size of the atoms and ions with increasing atomic number is known as lanthanoid contraction.
When acidified $K_2Cr_2O_7$ reacts with $H_2S,$ the reaction leads to the oxidation of $H_2S$ to sulfur, and the reduction of $K_2Cr_2O_7$ to $Cr_2(SO_4)_3,$ with the solution changing color from orange to green due to the formation of chromic sulfate. The balanced chemical equation for this reaction is as follows:
$K _2 Cr _2 O _7+4 H _2 SO _4+3 H _2 S \longrightarrow K _2 SO _4+ Cr _2\left( SO _4\right)_3+7 H _2 O +3 S$
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