Zadanie 1c, Paper 1B, maj 2026, TZ1, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2026, TZ1, 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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The Salton Sea is a large saline lake in the Colorado Desert, California, USA, whose surface lies below sea level. Its water level has been falling markedly over the last few decades.
Polecenie
(c) A simple set-up was used to compare the conductivity of water samples obtained from the Salton Sea at different depths.
PowiększThe following data were obtained:
| Depth water sample was obtained from / m (±0.05 m) | Electric current recorded by the ammeter / A (±0.01 A) |
|---|---|
| 4.00 | 2.65 |
| 8.00 | 2.72 |
| 12.00 | 3.06 |
| 16.00 | 3.11 |
| 20.00 | 3.15 |
(i) State the independent and the dependent variable in this experiment.
(ii) Suggest two variables, besides temperature, that must be controlled to make a fair comparison of the conductivity of the samples.
(iii) Calculate the fractional uncertainty in the electric current at a depth of 4.00 m. State your answer to the appropriate number of significant figures.
(iv) Suggest a conclusion for the experiment, with an explanation.
(v) Deduce the half-equations for the electrolysis reactions taking place at the cathode and at the anode while the current is flowing. Use section 19 of the data booklet.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) independent variable: the depth from which the water sample was obtained ✔
dependent variable: the electric current recorded by the ammeter/the conductivity of the sample ✔
(ii) Any two of:
the potential difference (voltage) of the DC power source ✔
the distance between the two graphite electrodes ✔
the surface area of the electrodes in contact with the sample ✔
the depth to which the electrodes are immersed in the water ✔
(iii) 0.004 «1 significant figure, matching the absolute uncertainty» ✔
(iv) conclusion: as the depth increases, the electric current/conductivity increases ✔
explanation: the water deeper in the lake contains a higher concentration of dissolved ions/salts «the denser, saltier brine settles towards the bottom» ✔
(v) cathode: 2 H2O(l) + 2 e− → H2(g) + 2 OH−(aq) ✔
OR cathode: 2 H+(aq) + 2 e− → H2(g) ✔
anode: 2 Br−(aq) → Br2(aq) + 2 e− ✔
Schemat punktowania
(i) [1] for each variable; accept 'conductivity' or 'reading on the ammeter' for the dependent variable. (ii) [1] each for two different variables from the list; apply the list principle. Accept volume of water/solution, resistance, diameter or length of the wires, cleanliness of the electrode surfaces. 'The water sample' alone is too vague to credit. (iii) [1] for 0.004 «1 significant figure». (iv) M1: the conclusion that the current/conductivity increases with depth; M2: an explanation in terms of a higher concentration of dissolved ions deeper in the lake, consistent with the direction of the trend. (v) [1] for the cathode half-equation (either form) and [1] for the anode half-equation; ignore state symbols. Award [1] if the two equations are correct but assigned to the wrong electrodes. Using section 19: bromide is oxidized in preference to chloride (E for Br2/Br- is less positive than for Cl2/Cl-), and water is reduced in preference to the metal cations present.
Komentarz. Section 19 of the data booklet gives E⊖(Br2/Br−) = +1.09 V against E⊖(Cl2/Cl−) = +1.36 V, so bromide, not chloride, is discharged at the anode even though it is present at a much lower concentration (compare the ion table in 1b).