2010年5月IB化学SL真题下载-Paper2
1. Brass is a copper containing alloy with many uses. An analysis is carried out to determine the
percentage of copper present in three identical samples of brass. The reactions involved in this
analysis are shown below.
Step 1: Cu(s) + 2HNO3(aq) + 2H+(aq) → Cu2+(aq) + 2NO2(g) + 2H2O(l)
Step 2: 4I-(aq) + 2Cu2+(aq) → 2CuI(s) + I2(aq)
Step 3: I2(aq) + 2S2O32-(aq) → 2I-(aq) + S4O62-(aq)
(a) (i) Deduce the change in the oxidation numbers of copper and nitrogen in step 1.
Copper:
Nitrogen:
(ii) Identify the oxidizing agent in step 1.
(b) A student carried out this experiment three times, with three identical small brass nails, and obtained the following results.
Mass of brass = 0.456 g ± 0.001 g
(i) Calculate the average amount, in mol, of S2O32– added in step 3.
(ii) Calculate the amount, in mol, of copper present in the brass.
(iii) Calculate the mass of copper in the brass.
(iv) Calculate the percentage by mass of copper in the brass.
(v) The manufacturers claim that the sample of brass contains 44.2 % copper by mass.Determine the percentage error in the result.
(c) With reference to its metallic structure, describe how brass conducts electricity.
2. Alex and Hannah were asked to investigate the kinetics involved in the iodination of propanone.They were given the following equation by their teacher.
Alex’s hypothesis was that the rate will be affected by changing the concentrations of the propanone and the iodine, as the reaction can happen without a catalyst. Hannah’s hypothesis was that as the catalyst is involved in the reaction, the concentrations of the propanone, iodine and the hydrogen ions will all affect the rate.
They carried out several experiments varying the concentration of one of the reactants or the catalyst whilst keeping other concentrations and conditions the same. Their results are shown graphically below.
(a) Discuss whether either Alex’s or Hannah’s hypothesis is correct.
(b) Explain why the reaction rate will increase with increasing temperature.
(c) (i) This reaction uses a catalyst. Sketch and annotate the Maxwell-Boltzmann energy distribution curve for a reaction with and without a catalyst on labelled axes below.
(ii) Describe how a catalyst works.
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