2019年11月IB化学SL真题下载-Paper2
1.The equations show steps in the formation and decomposition of ozone in the stratosphere, some of which absorb ultraviolet light.
Step 1 O2 → 2O•
Step 2 O• + O2 → O3
Step 3 O3 → O• + O2
Step 4 O• + O3 → 2O2
(a)Draw the Lewis structures of oxygen, O2, and ozone, O3.
(b)Outline why both bonds in the ozone molecule are the same length and predict the bond length in the ozone molecule. Refer to section 10 of the data booklet.
Reason:
Length:
(c)Distinguish ultraviolet light from visible light in terms of wavelength and energy.
(d)Discuss how the different bond strengths between the oxygen atoms in O2 and O3 in the ozone layer affect radiation reaching the Earth’s surface.
2.The biochemical oxygen demand of a water sample can be determined by the following series of reactions. The final step is titration of the sample with sodium thiosulfate solution, Na2S2O3 (aq).
2Mn2+ (aq) + O2 (aq) + 4OH− (aq) → 2MnO2 (s) + 2H2O (l)
MnO2 (s) + 2I− (aq) + 4H+ (aq) → Mn2+ (aq) + I2 (aq) + 2H2O (l)
2S2O32− (aq) + I2 (aq) → 2I− (aq) + S4O62− (aq)
A student analysed two 300.0 cm3 samples of water taken from the school pond: one immediately (day 0), and the other after leaving it sealed in a dark cupboard for five days (day 5). The following results were obtained for the titration of the samples with 0.0100 mol dm−3 Na2S2O3 (aq).
(a)(i) Determine the mole ratio of S2O32− to O2, using the balanced equations.
(ii)Calculate the number of moles of oxygen in the day 0 sample.
(iii)The day 5 sample contained 5.03 × 10−5 moles of oxygen.Determine the 5-day biochemical oxygen demand of the pond, in mg dm−3(“parts per million”, ppm).
(b)(i) Calculate the percentage uncertainty of the day 5 titre.
(ii)Suggest a modification to the procedure that would make the results more reliable.
2019年11月IB化学SL真题余下省略！
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