Zadanie 2b, Paper 1B, listopad 2025, TZ3, Chemia IB SL
Wzorowane na: Chemistry SL Paper 1B, November 2025, TZ3, pytanie 2(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
Two students were asked to determine the concentration of a solution of potassium hydroxide, KOH, by thermometric titration with 2.40 mol dm−3 nitric acid, HNO3, using a thermometer and the glassware available in the school laboratory. Both solutions were at 20 °C at the start.
Student A measured out the two solutions with a 50 cm3 measuring cylinder, combining them in a 150 cm3 glass beaker to give eleven mixtures, each with a total volume of 40.0 cm3, and the highest temperature of every mixture was recorded.
| Volume of HNO3 ± 0.5 / cm3 | Volume of KOH ± 0.5 / cm3 | Highest temperature ± 1 / °C |
|---|---|---|
| 0.0 | 40.0 | 20 |
| 4.0 | 36.0 | 23 |
| 8.0 | 32.0 | 27 |
| 12.0 | 28.0 | 30 |
| 16.0 | 24.0 | 32 |
| 20.0 | 20.0 | 32 |
| 24.0 | 16.0 | 30 |
| 28.0 | 12.0 | 27 |
| 32.0 | 8.0 | 25 |
| 36.0 | 4.0 | 22 |
| 40.0 | 0.0 | 20 |
Polecenie
Student B pipetted 30.00 cm3 of the potassium hydroxide solution into a 150 cm3 glass beaker and added the nitric acid from a burette in portions of 4.00 cm3, recording the steady temperature after each addition.
| Volume of HNO3 added ± 0.05 / cm3 | Temperature ± 1 / °C |
|---|---|
| 0.00 | 20 |
| 4.00 | 24 |
| 8.00 | 27 |
| 12.00 | 29 |
| 16.00 | 31 |
| 20.00 | 33 |
| 24.00 | 33 |
| 28.00 | 32 |
| 32.00 | 31 |
| 36.00 | 30 |
| 40.00 | 29 |
(i) From a green chemistry standpoint, suggest, with a justification, whether Student A's or Student B's procedure is the better one.
(ii) State which student's volume measurements are the more precise, and give a reason.
(iii) Deduce whether this gap in precision has a meaningful effect on the uncertainty of the KOH concentration read off where the two best-fit lines cross.
(iv) Comment on the pattern linking the volume of nitric acid added and the temperature recorded, in each of the two procedures.
(v) Looking at both sets of results, deduce which one temperature reading was most reduced by heat loss to the surroundings, and give a reason.
Student: ... Volume reading: ...
Reason: ...
(vi) Suggest how the apparatus might be altered to limit this loss of heat.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) Student B AND uses «much» smaller amounts of the two solutions / produces less waste ✔
(ii) Student B AND a burette and a pipette are more precise than a measuring cylinder / have a smaller uncertainty «±0.05 cm3 instead of ±0.5 cm3» ✔
(iii) no AND the uncertainty in the position of the intersection «arising from the ±1 °C temperature readings and the drawing of the lines» is much greater than the uncertainty in the volumes ✔
(iv) the temperatures are similar for small volumes of acid «both rise by about 12 °C up to 20 cm3» ✔
for large volumes of acid Student A's temperatures fall back to the starting value whereas Student B's fall only slightly, so Student A's temperatures are lower ✔
(v) Student: B AND Volume reading: 40.00 «cm3» ✔
Reason: the solution has been warm «and losing heat to the surroundings» for the longest time «because the heat from every addition accumulates in the same beaker» ✔
(vi) replace the glass beaker with a polystyrene cup ✔
OR
insulate the beaker / add a lid ✔
Schemat punktowania
(i) [1] for Student B with a reason based on smaller amounts of reagents or less waste. (ii) [1] for Student B with a comparison of the apparatus or of the stated uncertainties. (iii) [1] for 'no' with the reason; 'no' alone does not score. (iv) M1: similar behaviour at low volumes (accept a general description such as 'both rise to a maximum'); M2: Student A's temperatures lower at high volumes. (v) M1: Student B and 40.00 cm3, both needed; M2: longest time for heat loss. (vi) [1]; a water bath, 'a closed system' or 'a calorimeter' without any mention of insulation do not score.
Komentarz. Student B's beaker keeps the heat released by every addition, so the last reading has had the longest time to cool; each of Student A's mixtures is measured straight after mixing.