AS & A Level Chemistry - 9701 Paper 5 2009 Summer Zone 1

(a) Predict which of the equations below will represent the thermal decomposition of caesium nitrate. Place a tick against the equation of your choice.
[Image_1: Equation Table] Use data provided on page 2 to explain your prediction.
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(b) You are to plan an experiment in which

• caesium nitrate is heated,
• gas is collected,
• the volume of gas collected is measured,
• the experimental results are used in a calculation to confirm or reject your prediction.

(i) Identify the independent variable in the experiment.
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(ii) Identify the dependent variable in the experiment.
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(c) Draw a diagram of the apparatus you would use in this experiment. Your apparatus should use only standard items found in a school or college laboratory. Show clearly how the solid will be heated, the gas collected and its volume measured. Label each piece of apparatus used, indicating its size or capacity, e.g. $250cm^3$ beaker.
Assuming that either equation in (a) might be correct, which gas or gases would you expect to collect in your apparatus. Explain your answer.
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(d) Calculate the volume of gas you would expect to collect in your apparatus if 1 mol of caesium nitrate completely decomposed according to your predicted equation in (a).

(e) Use your answer to (d) and the size of the apparatus selected in (c) to calculate the maximum mass of $CsNO_3$ that can be used in your experiment.

(f) Outline, in a series of numbered steps, the method to be used in the experiment. Make certain that the steps you describe are in the correct order. You need not explain how the apparatus is assembled. Indicate clearly how you will know when decomposition is complete.
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(g) What should be done when decomposition is complete to ensure that the volume of gas measured in the apparatus is the "correct" volume?
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(h) Identify a risk present in the method you have described.
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Describe how you would minimise this risk.
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(a) Process the results in the table to produce values that will enable you to plot a graph to show the solubility of sodium iodide in water at different temperatures.
Record these values in the additional columns of the table. You may use some or all of the columns.
Label the columns you use.
For each column you use include the units and an expression to show how your values are calculated.
You may use the column headings A to H in the expressions e.g. C − B. [3]

(b) Present the values calculated in (a) in graphical form.
Draw two separate lines and extrapolate them to a point of intersection. The line at higher temperatures represents the solubility of NaI. The line at lower temperatures represents the solubility of NaI.2H2O.
Do not start either scale at zero. [4]

(c) Read from the graph the temperature where the two lines intersect. This is the transition temperature.
The transition temperature is ................. °C. [1]

(d) Circle, on the graph, any point(s) you consider to be anomalous.
For any point circled on the graph suggest an error in the conduct of the experiment that might have led to the anomalous result. [4]

(e) Suggest additional measurements that could be made to enable a more precise value of the transition temperature to be determined. [1]

(f) Describe the difference in the variation of solubility with temperature
above the transition temperature,
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below the transition temperature.
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The solubility curve represents equilibrium conditions between solid sodium iodide and dissolved sodium iodide.
The position of equilibrium can be influenced by temperature change and whether a change is exothermic or endothermic.
From the shape of your graph, comment on the likely enthalpy change for solid sodium iodide dissolving under equilibrium conditions. Explain your answer. [2]