Zadanie 1d
Wzorowane na: Chemistry SL Paper 3, November 2023, TZ1, pytanie 1(d). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 1a-1d.
A food chemistry group investigated how the processing and storage of fruit juice affects its vitamin C (ascorbic acid) content.
Polecenie
The vitamin C content of a juice can be found by titrating a sample against a standard iodine solution. Starch is used as the indicator: it forms a dark blue-black complex with the first excess of iodine once all the vitamin C has reacted.
C6H8O6(aq) + I2(aq) → C6H6O6(aq) + 2 H+(aq) + 2 I−(aq)
(i) Identify the oxidizing agent in this reaction.
(ii) A student noted the burette reading when the blue-black colour first appeared, but the colour disappeared again after the flask had been swirled for a further 30 seconds. Suggest why this occurred.
(iii) A second student kept adding iodine solution until the mixture was very dark blue-black, well past the first appearance of the colour. State the effect of this on the calculated concentration of vitamin C.
(iv) Suggest why this method cannot be used to determine the vitamin C content of blackcurrant juice.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) iodine/I2/I2(aq) ✔
(ii) the end point/equivalence point had not yet been reached «some vitamin C was still present»
OR
the reaction between iodine and vitamin C is not instantaneous/not yet complete
OR
the mixture had not been mixed well enough, so the blue-black colour came from a local excess of iodine ✔
(iii) higher/greater/increased «the larger titre is taken to mean that more vitamin C was present» ✔
(iv) the blue-black end point colour is the same as/would be hidden by the dark colour of the juice ✔
Schemat punktowania
One mark per sub-part. (i) Accept the name or the formula of iodine; do not accept 'iodide' or 'starch'. (ii) Accept 'iodine evaporated'; do not accept 'iodine was oxidized' or 'iodine was reduced to iodide'. (iii) Accept 'as the end point volume increases the calculated concentration increases'. (iv) A reference to colour is required; do not accept 'cannot determine the end point' alone.
Komentarz
The end point is the first persistent colour, so an early reading gives too low a result and a late one too high.