What a learner can do afterwards
- Explains why manganate(VII) acts as its own indicator and describes the end point
- Builds the full ionic equation from the two half equations before calculating anything
- Calculates the concentration or the purity of a sample from the titre and the mole ratio
- Explains why the iodine and thiosulfate route needs starch added only near the end
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Acidified potassium manganate(VII) is purple. In the flask it reacts with iron(II) and becomes Mn2+, which is almost colourless, so each drop loses its colour. Once all the iron(II) has reacted, the next drop has nothing to react with. The end point is the first faint pink that stays, so manganate is its own indicator and no separate indicator is added. Both titrations in this topic use reactions that are feasible and fast, so each drop reacts at once.
Before any numbers, build the full ionic equation from the two half equations. Manganate gains 5 electrons: MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O. Each iron(II) loses 1: Fe2+ -> Fe3+ + e-. Multiply the iron half by 5 so the electrons cancel: MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 4H2O + 5Fe3+. So 1 mol of manganate reacts with 5 mol of iron(II).
A dissolved iron tablet needs 20.0 cm3 of 0.0100 mol/dm3 manganate. Manganate: 0.0200 dm3 x 0.0100 = 2.00 x 10-4 mol. Iron(II): 5 x 2.00 x 10-4 = 1.00 x 10-3 mol. Mass of iron: 1.00 x 10-3 x 55.8 = 0.0558 g.
Iodine can be titrated with sodium thiosulfate: I2 + 2S2O3 2- -> 2I- + S4O6 2-, so 1 mol of iodine reacts with 2 mol of thiosulfate. Brown iodine fades to pale yellow as thiosulfate goes in. Add starch only then. It turns the last of the iodine blue-black, and the end point is when that colour vanishes. Added early, starch binds plenty of iodine in a complex that breaks up slowly, so the colour change drags.
Manganate shows its own pink end point, half equations give the 1 to 5 ratio, titres give moles and mass, and starch goes in only when the iodine is pale yellow.
Original chemistry lesson and assessment text written for os-taxonomy.
This page links the Khan Academy video Redox titrations. No text is adapted from it.
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