Answer to Question #224159 in Inorganic Chemistry for Prince

Question

Write the principles involved in the estimation of fe2+ ion using k2cr2o7

Answer 1

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:

Cr₂O₇^2-+ 14H⁺ + 6 e^- ® 2Cr³⁺ + 7H₂O

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

Fe³⁺ + e^- ® Fe²⁺

Therefore, 1 mole of Cr₂O₇^2-(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:

Cr₂O₇^2-+ 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 Cr₂O₇^2-and Fe²⁺ consumed.