Answer to Question #224159 in Inorganic Chemistry for Prince

Question #224159
Write the principles involved in the estimation of fe2+ ion using k2cr2o7
1
Expert's answer
2021-08-09T07:48:45-0400


One uses standard solution of potassium dichromate (K2Cr2O7) to determine the percent by weight of iron (as Fe2+) in an unknown solid.

Dichromate ion reduces to two chromium(III) ions. This reaction requires 6 electrons and 14 (!) hydrogen ions:

Cr2O72-+ 14H+ + 6 e- ® 2Cr3+ + 7H2O


Only one electron is necessary to reduce Fe(III) to Fe(II)

Fe3+ + e- ® Fe2+


Therefore, 1 mole of Cr2O72-(the oxidizing agent) reacts with 6 moles of Fe2+ (the reducing agent) to form 6 moles of Fe3+ and 2 moles of Cr3+. Thus, in net ionic form:

Cr2O72-+ 6Fe2+ + 14H+ ® 6Fe3+ + 2Cr3+ + 7H2O

The molecular form ofthe reaction equation can be written as:

K2Cr2O7 + 6Fe(NH4)2(SO4)2 + 7H2SO4 ®

3Fe2(SO4)3 + Cr2(SO4)3 + K2SO4 + 6(NH4)2SO4 + 7H2O


 The 1:6 mole ratio with respect to the amounts of Cr2O72-and Fe2+ consumed.


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