2012年11月IB化学SL真题下载-Paper2

2012年11月IB化学SL真题下载-Paper2

1. Two groups of students (Group A and Group B) carried out a project* on the chemistry of some group 7 elements (the halogens) and their compounds.

(a) In the first part of the project, the two groups had a sample of iodine monochloride (a corrosive brown liquid) prepared for them by their teacher using the following reaction.

(i) State the number of significant figures for the masses of I2(s) and ICl (l).

I2(s):

ICl (l):

(ii) The iodine used in the reaction was in excess. Determine the theoretical yield, in g,of ICl (l).

(iii) Calculate the percentage yield of ICl (l).

(iv) Using a digital thermometer, the students discovered that the reaction was exothermic. State the sign of the enthalpy change of the reaction, ΔH.

(b) Although the molar masses of ICl and Br2 are very similar, the boiling point of ICl is 97.4 °C and that of Br2 is 58.8 °C. Explain the difference in these boiling points in terms of the intermolecular forces present in each liquid.

(c) The students reacted ICl (l) with CsBr (s) to form a yellow solid, CsICl2 (s), as one of the products. CsICl2 (s) has been found to produce very pure CsCl (s) which is used in cancer treatment.

To confirm the composition of the yellow solid, Group A determined the amount of iodine in 0.2015 g of CsICl2 (s) by titrating it with 0.0500 mol dm–3 Na2S2O3 (aq). The following data were recorded for the titration.

(i) Calculate the percentage of iodine by mass in CsICl2(s), correct to three significant figures.

(ii) State the volume, in cm3, of 0.0500 mol dm–3 Na2S2O3 (aq) used in the titration.

(iii) Determine the amount, in mol, of 0.0500 mol dm–3 Na2S2O3 (aq) added in the titration.

(iv) The overall reaction taking place during the titration is:

Calculate the amount, in mol, of iodine atoms, I, present in the sample of CsICl2 (s).

(v) Calculate the mass of iodine, in g, present in the sample of CsICl2 (s).

(vi) Determine the percentage by mass of iodine in the sample of CsICl2 (s), correct to three significant figures, using your answer from (v).

(d) Group B heated the yellow solid, CsICl2 (s), which turned white and released a brown gas which condensed into a brown liquid.

Group B identified the white solid as CsCl (s). Suggest the identity of the brown liquid.

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