Question

this experiment was performed with excess of Fe 3+ over SCN- in most of the solutions. if a large excess of SCN- had beem used instead, which reaction might have predominated?

Answer 1

When Fe3+ is used in excess, then the reaction equilibrium shifts to the right ,the product side,so the product yield increases.According to Le chatlier’s principle if more Fe3+ is added then the reaction tends to move forward to reduce the production of Fe3+,

Fe3+ +SCN-↔Fe(SCN)²⁺(blood red colour)

If SCN- is used in large excess, then a side reaction takes place,

Fe3+ +3SCN-↔[Fe(SCN)3(H2O)3] (red colored precipitate).

For colorimetric determination the intensity of red color is the measure of Fe3+ concentration ,so the precipitate formation is avoided .