2018年5月IB化学SL真题下载-Paper3
1. Palmitic acid has a molar mass of 256.5 g mol−1.
(a) (i) Part of this molecule is hydrophilic (bonds readily to water) and part hydrophobic (does not bond readily to water). Draw a circle around all of the hydrophilic part of the molecule.
(ii) When a small amount of palmitic acid is placed in water it disperses to form a layer on the surface that is only one molecule thick. Explain, in terms of intermolecular forces, why this occurs.
(b) The apparatus in the diagram measures the surface pressure created by palmitic acid molecules on the surface of water. This pressure is caused by palmitic acid molecules colliding with the fixed barrier. The pressure increases as the area, A, available to the palmitic acid is reduced by the movable barrier.
(i) Suggest why there is a small increase in the surface pressure as the area is reduced to about 240 cm2, but a much faster increase when it is further reduced.
Above about 240 cm2:
At less than about 240 cm2:
(ii) The solution of palmitic acid had a concentration of 0.0034 mol dm−3. Calculate the number of molecules of palmitic acid present in the 0.050 cm3 drop, using section 2 of the data booklet.
(iii) Assuming the sudden change in gradient occurs at 240 cm2, calculate the area, in cm2, that a single molecule of palmitic acid occupies on surface of the water. If you did not obtain an answer for (b)(ii) use a value of 8.2 × 1016, but this is not the correct answer.
2. Students were asked to investigate how a change in concentration of hydrochloric acid, HCl,affects the initial rate of its reaction with marble chips, CaCO3.
They decided to measure how long the reaction took to complete when similar chips were added to 50.0 cm3 of 1.00 mol dm−3 acid and 50.0 cm3 of 2.00 mol dm−3 acid.
Two methods were proposed:
(1) using small chips, keeping the acid in excess, and recording the time taken for the solid to disappear
(2) using large chips, keeping the marble in excess, and recording the time taken for bubbles to stop forming.
(a) Annotate the balanced equation below with state symbols.
CaCO3 (__) + 2HCl (__) → CaCl2 (__) + CO2 (__) + H2O (__)
(b) Neither method actually gives the initial rate. Outline a method that would allow the initial rate to be determined.
(c) (i) Deduce, giving a reason, which of the two methods would be least affected by the chips not having exactly the same mass when used with the different concentrations of acid.
(ii) State a factor, that has a significant effect on reaction rate, which could vary between marble chips of exactly the same mass.
(d) A group recorded the following results with 1.00 mol dm−3 hydrochloric acid:
(i) Justify why it is inappropriate to record the uncertainty of the mean as ±0.01 s.
(ii) If doubling the concentration doubles the reaction rate, suggest the mean time you would expect for the reaction with 2.00 mol dm−3 hydrochloric acid.
(iii) Another student, working alone, always dropped the marble chips into the acid and then picked up the stopwatch to start it. State, giving a reason, whether this introduced a random or systematic error.
2018年5月IB化学SL真题余下省略！
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