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The pain of muscle strains and swellings can be eased by using heat packs. As a source of heat, some heat packs use the energy released when anhydrous calcium chloride dissolves in water.
$$\text{CaCl}_2(s) \xrightarrow{\text{water}} \text{Ca}^{2+}(aq) + 2\text{Cl}^{-}(aq)$$
A heat pack consists of a bag of water, inside which a smaller bag contains anhydrous calcium chloride. When pressure is applied to the heat pack, the smaller bag bursts releasing the anhydrous calcium chloride into the water. The heat pack is shaken to speed up dissolving. Energy is released which warms the heat pack.
A student carried out an experiment to determine the enthalpy change when anhydrous calcium chloride dissolves in distilled water. The results the student obtained are plotted on the graph on page 4.
(a) By considering the graph of results, draw a labelled diagram of the experimental set-up that the student could have used to produce the graph shown.
Label the apparatus and chemicals required to measure the two variables. [2]
(b) Explain why the student took readings between 0.0 minutes and 2.5 minutes. [1]
(c) Explain why the student did not take a reading at 3.0 minutes. [1]
(d) Explain why the temperature continued to increase between 3.5 minutes and 4.5 minutes. [1]
(e) Draw two straight lines of best fit on the grid. Extrapolate these lines to estimate the theoretical temperature rise at 3.0 minutes. Give your answer to one decimal place.
theoretical temperature rise at 3.0 minutes = ......................... °C [2]
(f) One of the results is anomalous. This occurred because the student took the thermometer out of the solution and then replaced it just before the reading was taken.
The time at which the anomalous reading was taken was .................. minutes.
Explain why these actions led to the anomalous point. [1]
(g) Explain why stirring the mixture would make this experiment more reliable. [1]
(h) Anhydrous calcium chloride is classified as a moderate health hazard. It is an irritant.
Apart from wearing eye protection, state one other relevant safety precaution the student should have taken. [1]
(i) The student found the value for the enthalpy change of solution of anhydrous calcium chloride to be –82.5 kJ mol$^{-1}$.
A manufacturer produces a heat pack that contains 75.0 g of water.
Calculate the mass of anhydrous calcium chloride the manufacturer must use in the inner bag to produce a rise in temperature of 30.0 °C.
The specific heat capacity of water, c = 4.18 J g$^{-1}$ K$^{-1}$.
[A$_r$: Ca, 40.1; Cl, 35.5]
mass of anhydrous CaCl$_2$ = ................................ g [2]
500
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(a) (i) Plot a graph on the grid on page 9 to show the relationship between concentration of sucrose, c, and observed angle of rotation, $\alpha_{obs}$. Use a cross (×) to plot each data point. Draw a line of best fit. [2]
(ii) Circle the most anomalous point on your graph. [1]
(iii) Use the graph to determine the specific rotation, [$\alpha$], of sucrose. Give this value to two decimal places. State the co-ordinates of both points you used in your calculation.
co-ordinates 1 ................................. co-ordinates 2 .................................
specific rotation of sucrose, [$\alpha$] = ................................. [2]
(b) You are asked to write instructions for another student to follow so they can prepare a standard solution of 250 cm³ 0.0750 g cm⁻³ sucrose. The student is provided with solid sucrose and a 250 cm³ volumetric flask.
(i) Calculate the mass, in g, of sucrose the student would need to use.
mass of sucrose = ........................... g [1]
(ii) Describe how the student should accurately prepare the standard solution using a sample of sucrose of mass calculated in (i). [2]
(c) (i) The student used the standard solution prepared in (b) to prepare the solutions in the table on page 8. Calculate the volume of standard solution of concentration 0.0750 g cm⁻³ and the volume of distilled water needed to prepare 15.00 cm³ of sucrose solution of concentration 0.0350 g cm⁻³. Give your answers to two decimal places.
volume of standard solution = ......................... cm³
volume of distilled water = ......................... cm³
The volumes of the two solutions given in (c)(i) could be measured using the same type of apparatus.
(ii) Name a suitable piece of apparatus which could be used to measure these volumes. [1]
(iii) In (a)(ii) you circled an anomalous point. This was caused by the student incorrectly making one of the sucrose solutions. Suggest the error made by the student that caused this anomaly. [1]
(d) The student recorded the observed angle of rotation, $\alpha_{obs}$, for a sucrose solution of unknown concentration as +3.75. Determine the concentration of this sucrose solution in mol dm⁻³.
$\text{[M, sucrose: 342]}$
concentration of sucrose = ......................... mol dm⁻³ [3]
(e) The glass cell of 10 cm length is expensive, so one cell is used for all the solutions that are placed in the polarimeter. Suggest how you would ensure that the concentration of solution in the cell is accurate each time the cell is used for the different sucrose solutions. [1]
(f) Concentration of sucrose is the independent variable in this polarimeter experiment. The glass cell of 10 cm length is replaced by a glass cell of 20 cm length. The 20 cm glass cell is filled with 0.0750 g cm⁻³ sucrose solution. Predict the value for the observed angle of rotation, $\alpha_{obs}$, for the sucrose solution of concentration 0.0750 g cm⁻³ when the 20 cm cell is used. Explain your answer.
predicted value = .........................
explanation ...............................................................
.................................................................................. [2]
(g) Before the angles of rotation of the sucrose solutions are measured, the glass cell is first filled with distilled water and the angle of rotation measured. Explain why this measurement is taken. [1]
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