Redox titrations that show their own end

Science · Chemistry · ages 17-18

Name ______________________ Date ____________

Manganate(VII) is run from a burette into acidified iron(II) solution in a flask. What does the end point look like?
- The pink colour vanishes for good
- A precipitate falls to the bottom
- A first faint pink that stays
Why is no separate indicator added in a manganate(VII) titration?
- Manganate acts as its own indicator
- Indicators do not work in acid solution
- The end point needs no colour change
In an iodine and thiosulfate titration, starch is added only when the iodine colour has faded to pale yellow.
Circle one: True False
What must you build from the half equations before any titration maths?
- The full ionic equation for the reaction
- The pH curve for the mixture
- The rate equation for the reaction
A titration uses 22.50 cm3 of 0.0200 mol/dm3 manganate(VII). How many moles of manganate is that? Give the answer as a decimal.
Answer: ______________
Why is starch held back until near the end of an iodine titration?
- Starch reacts with thiosulfate and uses some of it up in the flask
- Early starch binds plenty of iodine, so the end point drags
- Starch stops iodine forming, so there is nothing left to titrate
An iron tablet is dissolved in acid so that all of its iron is present as Fe2+. The solution needs 15.00 cm3 of 0.0200 mol/dm3 manganate(VII). Iron is 55.8 g/mol. What mass of iron, in grams, was in the tablet?
Answer: ______________
A student assumes 1 mol of manganate reacts with 1 mol of iron(II). What is the error?
- 1 manganate takes 5 electrons, so it reacts with 5 iron(II)
- Each iron(II) gives 5 electrons, so 1 iron(II) needs 5 manganate
- The 1 to 1 ratio is right, since both ions carry a positive charge

Answer key

For grown-ups. Fold this page away before handing over the rest.

Redox titrations that show their own end W1-mt_tKZTSfo0Ra-s1

A first faint pink that stays · The lasting pink is the first extra manganate with nothing left to react with.
Manganate acts as its own indicator · Excess manganate tints the flask pink, so it shows its own end point.
True · Starch goes in near the end, when the iodine is pale yellow. It turns the last iodine blue-black, and the end point is when that colour vanishes. The sentence is true.
The full ionic equation for the reaction · The full ionic equation fixes the mole ratio that every later step needs.
0.00045 · 22.50 cm3 is 0.02250 dm3, and 0.02250 x 0.0200 = 4.50 x 10-4 mol.
Early starch binds plenty of iodine, so the end point drags · Starch added early binds plenty of iodine in a complex that breaks up slowly, so the colour change drags. Adding it at pale yellow keeps the end point sharp.
0.0837 · Manganate: 0.01500 x 0.0200 = 3.00 x 10-4 mol. Iron(II): 5 x 3.00 x 10-4 = 1.50 x 10-3 mol. Mass: 1.50 x 10-3 x 55.8 = 0.0837 g.
1 manganate takes 5 electrons, so it reacts with 5 iron(II) · Manganate gains 5 electrons and each iron(II) loses 1, so 1 mol of manganate reacts with 5 mol of iron(II). Using 1 to 1 makes the iron(II) moles 5 times too small.