Question

A student adds half as much 1,10 phenathroline as she was supposed to when attempting the red crystal synthesis. How would this effect the actual, theoretical and percent yield? explain

Answer 1

I am not sure what crystal the student was attempting to prepare, but I believe that we are talking about Fe(SCN)₃ sample preparation. The complex is red in color. Even if that’s not the case, we can treat this as an example.

Write the balanced chemical equation for the reaction.

Fe³⁺ (aq) + 3 SCN^- (aq) --------> [Fe(SCN)₃] (aq) -----> purified and dried to give crystals.

As per the stoichiometric equation,

1 mole Fe³⁺ = 3 moles SCN^-

When half the amount of SCN^- is added, the number of moles of SCN^- required for reaction completion is half of the required value. Consequently, SCN^- will be the limiting reagent and the yield of the product will be cut in half. Infact, the actual yield of the product will be less than the theoretical yield (in ideal cases, the yield will be 50% of the actual value). Consequently, the percent yield of the reaction will be 50% of the actual value. The theoretical yield however, remains the same, since the theoretical yield is calculated from the stoichiometric balanced equation as shown above.