Zadanie 1d
Wzorowane na: Chemistry HL Paper 3, November 2023, TZ2, 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.
Grapefruit juice is a rich source of vitamin C (ascorbic acid). To destroy pathogenic bacteria, producers often heat-treat the juice by filling it into bottles at 85 °C. A food laboratory monitored the concentration of vitamin C in heat-treated juice and in untreated, cold-pressed juice. Both were stored in partly filled clear glass bottles at 10 °C for 56 days.
Polecenie
The concentration of vitamin C in juice can be measured by a redox titration. A sample of acidified juice containing excess iodide ions and starch solution is titrated with potassium iodate solution. Iodate ions react with iodide ions to produce iodine (reaction 1). The iodine is immediately used up by vitamin C (reaction 2). When all the vitamin C has reacted, the next drop of iodate solution leaves excess iodine, which forms a dark blue complex with starch.
Reaction 1: IO3−(aq) + 5 I−(aq) + 6 H+(aq) → 3 I2(aq) + 3 H2O(l)
Reaction 2: C6H8O6(aq) + I2(aq) → C6H6O6(aq) + 2 I−(aq) + 2 H+(aq)
(i) Identify the reducing agent in reaction 1.
(ii) The student recorded the end point, then swirled the flask for a further minute and saw that the dark blue colour slowly faded. Suggest why this happened.
(iii) The end point volume recorded by the student was smaller than the true volume needed to reach the equivalence point. State the effect of this on the calculated concentration of vitamin C.
(iv) Suggest why this method cannot be used to measure the concentration of vitamin C in blackcurrant juice.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) iodide/I− «ions» ✔
(ii) the equivalence point had not been reached «vitamin C was still present, so the iodine reacted further»
OR
reaction between iodine and vitamin C is slow/incomplete/stirring was insufficient «locally excess iodine»
OR
iodine was lost by evaporation ✔
(iii) lower/decrease «calculated concentration of vitamin C» ✔
(iv) the juice is already dark purple/the juice colour would hide the dark blue end point colour
OR
starch–iodine complex has a similar colour to the juice ✔
Schemat punktowania
One mark for each of (i)-(iv). (i) Do not accept IO3−. (ii) Do not accept "iodine was oxidized" or "I2 turned into I−". (iii) Accept "a smaller titre gives a smaller concentration"; do not accept a general definition of end point. (iv) Reject "the end point cannot be found" unless colour is mentioned.
Komentarz
Faded blue colour after the end point is a sign that the end point was recorded too early, which links (ii) to (iii).