Zadanie 1c, Paper 1B, maj 2025, TZ2, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2025, TZ2, pytanie 1(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
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Copper(II) ions, Cu2+, in the rinse water from a copper-plating workshop must be monitored before the water is discharged. Their concentration can be determined either by complexometric titration with a standard solution of EDTA or by atomic absorption spectroscopy (AAS).
Polecenie
Portions of the rinse water were then titrated with the standardized EDTA solution in the presence of murexide as indicator. The burette readings are given in the table.
| Trial 1 | Trial 2 | Trial 3 | Trial 4 | |
|---|---|---|---|---|
| Final burette reading / cm3 (± 0.05 cm3) | 24.10 | 23.75 | 25.30 | 22.70 |
| Initial burette reading / cm3 (± 0.05 cm3) | 0.00 | 1.50 | 3.00 | 0.50 |
| Volume of EDTA added / cm3 | 24.10 | 22.25 | 22.30 | 22.20 |
(i) Suggest two reasons why trial 1 required a much larger volume of EDTA than the other three trials.
(ii) State the term used for a measured value that lies far from the other values in a set of repeated measurements.
(iii) Suggest why trial 1 should be excluded from the calculation of the mean titre.
(iv) Calculate the percentage uncertainty in the titre of trial 2.
(v) The end point is reached when the yellow complex, X, formed between Cu2+ and the indicator changes to the violet form of the free indicator, Y. Explain how these colours show that the radiation absorbed by X has a higher energy than the radiation absorbed by Y.
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Odpowiedź z klucza
(i) Any two of:
trial 1 was a rapid/rough titration «to find the approximate titre», so the end point was overshot ✔
the jet below the tap was not filled/an air bubble was trapped when the initial reading was taken ✔
the tap was leaking/not closed properly ✔
the conical flask was contaminated/rinsed with the sample «instead of with deionized water» ✔
extra EDTA was added «after the colour change» to check that the end point was complete ✔
(ii) outlier ✔
(iii) including it would make the mean titre «and the calculated concentration» too high ✔
OR
it is not concordant with the other three titres «it differs from them by more than 0.10 cm3» ✔
OR
it would introduce a systematic error into the mean ✔
(iv) «» 0.45 «%» ✔
(v) X/yellow complex absorbs violet light/its complementary colour/light of 400-424 nm AND Y/violet ion absorbs yellow light/its complementary colour/light of 575-585 nm ✔
violet light has a shorter wavelength/higher frequency «and therefore higher energy, E = hf» than yellow light ✔
Schemat punktowania
(i) [2 max], list principle: any two distinct reasons. (ii) [1]; accept anomalous result or anomaly. (iii) [1] for any one reason. (iv) [1] for 0.45 % or 0.4 %; the uncertainty of a titre is 0.10 cm3 because two burette readings are subtracted, so 0.22 % from a single reading scores 0. (v) M1: the two complementary colours absorbed, both needed, wavelengths not required; M2: the comparison of wavelength or frequency or energy of violet and yellow light; accept the converse argument written for Y.
Komentarz. The colour observed is the complement of the colour absorbed, so the yellow complex absorbs violet light, which lies at the short-wavelength, high-energy end of the visible spectrum.