2016年11月IB化学SL真题下载-Paper3
1. In order to provide safe drinking water, a water supply is often treated with disinfectants, which aim to inactivate disease-causing bacteria in the water.
To compare the effectiveness of different disinfectants, a CT value is used as a measure of the dosage of disinfectant needed to achieve a certain level of inactivation of specific bacteria.
(i) Deduce the oxidation state of chlorine in the following disinfectants.
HOCl:
ClO2:
(ii) From the data on CT values, justify the statement that bacterium B is generally more resistant to disinfection than bacterium A.
(iii) CT values can be used to determine whether a particular treatment process is adequate. Calculate the CT value, in mg min dm−3, when 1.50 × 10−5 g dm−3 of chlorine dioxide is added to a water supply with a contact time of 9.82 minutes.
(iv) From your answer to (a) (iii) and the data in the table, comment on whether this treatment will be sufficient to inactivate 99 % of bacterium A.
(b) CT values are influenced by temperature and by pH. The table below shows the CT values for chlorine needed to achieve 99 % inactivation of a specific bacterium at stated values of pH and temperature.
(i) With reference to the temperature data in the table, suggest why it may be more difficult to treat water effectively with chlorine in cold climates.
(ii) Sketch a graph on the axes below to show how the CT value (at any temperature) varies with pH.
(iii) Comment on the relative CT values at pH 6.0 and pH 9.0 at each temperature.
(iv) Chlorine reacts with water as follows:
Cl2 (g) + H2O (l) ⇔ HOCl (aq) + HCl (aq)
HOCl (aq) ⇔ OCl− (aq) + H+ (aq)
Predict how the concentrations of each of the species HOCl (aq) and OCl− (aq) will change if the pH of the disinfected water increases.
HOCl (aq):
OCl− (aq):
(c) Despite widespread improvements in the provision of safe drinking water, the sale of bottled water has increased dramatically in recent years. State one problem caused by this trend.
2. In a class experiment, students were asked to determine the value of x in the formula of a hydrated salt, BaCl2･ xH2O. They followed these instructions:
1. Measure the mass of an empty crucible and lid.
2. Add approximately 2 g sample of hydrated barium chloride to the crucible and record the mass.
3. Heat the crucible using a Bunsen burner for five minutes, holding the lid at an angle so gas can escape.
4. After cooling, reweigh the crucible, lid and contents.
5. Repeat steps 3 and 4.
Their results in three trials were as follows:
(a) State and explain the further work students need to carry out in trial 2 before they can process the results alongside trial 1.
(b) In trial 3, the students noticed that after heating, the crucible had turned black on the outside. Suggest what may have caused this, and how this might affect the calculated value for x in the hydrated salt.
(c) List two assumptions made in this experiment.
2016年11月IB化学SL真题余下省略！
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