CHEMISTRY · Multi-part · AP · Chemistry · Chemical Reactions · Chemical Reactions
Problem
In an analytical procedure, iodide is determined indirectly through a three-step aqueous sequence carried out under acidic conditions at 20 °C.
Step 1 0.020 M KMnO₄ is added to an excess of KI(aq), producing a light brown solution.
Step 2 The resulting solution is immediately titrated with standardized Na₂S₂O₃(aq). A colorless solution results.
Step 3 The colorless solution is made alkaline with NaOH(aq), and a brown precipitate forms that is filtered and washed.
Assume MnO₄⁻ is fully reduced to Mn²⁺ in step 1 and that all sulfur-containing products remain in solution.
(a) Balance the net ionic equation for step 1 using the smallest whole-number coefficients.
(b) Write the balanced net ionic equation for step 2.
(c) The brown precipitate in step 3 is identified as MnO₂(s). Write the balanced net ionic equation for step 3.
(d) Derive the overall net ionic equation for the entire sequence by combining your equations from parts (a)–(c) and canceling any species that appear on both sides.
(e) Discuss how the disappearance and reappearance of iodine-containing species across the steps exemplifies the concept of a reaction intermediate in multi-step processes.