Zadanie 2b
Wzorowane na: Chemistry SL Paper 1B, May 2026, TZ2, pytanie 2(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 2a-2f.
Specific energy is the heat energy released per unit mass of fuel burnt. A student determined the specific energy of ethanol by burning it in a spirit burner under a copper can containing 120.0 g of water. The diagram shows the apparatus used.
The burner was weighed before and after each of five trials, and the temperature of the water was measured before and after heating. Fresh water was used for every trial.
| Spirit burner with ethanol | Trial 1 | Trial 2 | Trial 3 | Trial 4 | Trial 5 |
|---|---|---|---|---|---|
| Initial mass / g (± 0.001 g) | 148.236 | 147.801 | 148.115 | 147.652 | 148.021 |
| Final mass / g (± 0.001 g) | 147.374 | 146.870 | 147.370 | 146.774 | 147.121 |
| Water in copper can | Trial 1 | Trial 2 | Trial 3 | Trial 4 | Trial 5 |
|---|---|---|---|---|---|
| Initial temperature / °C (± 0.05 °C) | 20.4 | 19.8 | 22.0 | 21.2 | 21.3 |
| Final temperature / °C (± 0.05 °C) | 34.0 | 35.3 | 29.0 | 34.7 | 35.8 |
Polecenie
The student decided to ignore the uncertainty in the mass of ethanol burnt when calculating the specific energy.
Discuss whether this decision is valid, using the percentage uncertainties for trial 5.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
percentage uncertainty in mass «= 100 × 0.002/0.900 = ±» 0.22 «%» AND percentage uncertainty in temperature change «= 100 × 0.1/14.5 = ±» 0.69/0.7 «%» ✔
OR
0.49/0.5 «%» for the temperature if ±0.07 °C is used
not valid/the uncertainty in mass should not be ignored, because it is about one third of the uncertainty in temperature «a quarter of the total ±0.91 %» ✔
Schemat punktowania
M1: both percentage uncertainties; the mass uncertainty is ±0.002 g, the sum of two readings of ±0.001 g. M2: a decision supported by comparing them. A statement that the mass uncertainty is negligible because it is the smaller one scores 0.
Komentarz
A percentage uncertainty can be ignored only when it is very small compared with the others; here it adds about a quarter to the total.