Practical Techniques & Data Analysis (AQA A Level Chemistry): Exam Questions

A student is given an unknown organic liquid, A, which is known to be one of the four compounds shown below.

butan-1-ol

butanal

butanone

butanoic acid

The student carries out a series of tests on sample A. The results are shown in the flow chart in Figure 1.

Figure 1

Use the results in Figure 1 to identify compound A.

The reagent used in Test 1 is acidified potassium dichromate(VI).

Give the formula of the active chromium species in the reagent before and after a positive test.

Describe the preparation of the Tollens' reagent used in Test 2.

A student carried out an experiment to determine the enthalpy of combustion of ethanol (C₂H₅OH). They burned a known mass of ethanol in a spirit burner and used the heat released to raise the temperature of a known volume of water in a copper calorimeter.

The student's results and relevant data are shown below.

Data

• M_r of ethanol = 46.0

• Specific heat capacity of water, c = 4.18 J g^-1 K^-1

• Density of water = 1.00 g cm^-3

• Accepted standard enthalpy of combustion of ethanol, ΔH_c^⦵ = –1367 kJ mol^-1

Use the student's results to calculate an experimental value for the enthalpy of combustion of ethanol, in kJ mol⁻¹.

Use your answer from part (a) and the accepted data book value to calculate the percentage error in the student's experimental result.

The student's calculated value is significantly less exothermic than the accepted data book value.

Suggest two major sources of error in this experimental method that could account for this difference.

A student carried out a series of experiments to determine the relative molecular mass, M_r, of a volatile organic liquid, Y.

In each experiment, a known mass of liquid Y was injected into a gas syringe which was heated to a constant temperature of 90 ^oC in an oven. The liquid vaporised, and the volume of the gas produced was recorded. The pressure in the laboratory was 101 kPa. The student's results are shown in Table 1.

Table 1

Use the results from the first experiment only (mass = 0.18 g) and the ideal gas equation, pV = nRT, to calculate a value for the M_r of liquid Y.

The gas constant R = 8.31 J K^-1 mol^-1.

Show your working.

On the grid provided, plot a graph of volume of gas against mass of liquid. Draw a line of best fit.

Use your line of best fit from part (b) to determine a more accurate value for the M_r of liquid Y. Show your working.

Explain why the value for the M_r calculated from the line of best fit is likely to be more accurate than the value calculated from the single experiment in part a.

Suggest two potential sources of error in this experimental method, other than measurement errors.

A student carried out an experiment to determine the enthalpy of solution (ΔH_sol) for sodium chloride (NaCl).

The student dissolved 2.34 g of NaCl in 50.0 cm³ of deionised water in a polystyrene cup and recorded a temperature change of –0.7 ^oC.

State one key procedural step, other than insulating the cup, that the student should have taken to ensure the measured temperature change was as accurate as possible.

Use the student's results to calculate the enthalpy of solution, ΔH_sol, for sodium chloride.

Give your answer in kJ mol^-1.

(M_r of NaCl = 58.5; specific heat capacity of water = 4.18 J g^-1 K^-1; assume the density of water is 1.00 g cm^-3)

The accepted data book value for the enthalpy of solution of NaCl is +3.9 kJ mol^-1.

Suggest one reason for the difference between the student's experimental value and the accepted value.

The enthalpy of solution can be related to lattice enthalpy and hydration enthalpies using the energy cycle shown in Figure 1.

Figure 1

Use your experimental value for the enthalpy of solution from part (b) and the data below to calculate a value for the lattice enthalpy of dissociation for NaCl.

• Enthalpy of hydration of Na⁺ (g) = –406 kJ mol^-1

• Enthalpy of hydration of Cl^- (g) = –364 kJ mol^-1

Explain why the lattice enthalpy of dissociation for magnesium oxide is significantly more endothermic than that of sodium chloride.

A student carried out a series of experiments to investigate the kinetics of the reaction between hydrogen peroxide and iodide ions in an acidic solution.

H₂O₂ (aq) + 2I^- (aq) + 2H⁺ (aq) → I₂ (aq) + 2H₂O (l)

In each experiment, a small, fixed amount of sodium thiosulfate and starch indicator were added. The initial rate of reaction was determined by measuring the time, t, for the blue-black colour to appear. The initial rate is proportional to 1/t.

The student's results are shown in Table 1.

Table 1

Explain the purpose of the sodium thiosulfate in this experiment.

The student ensured that the concentration of hydrogen peroxide and the total volume of the solution were kept constant in each experiment.

Explain the steps the student would have taken to ensure the concentration of hydrogen peroxide was kept constant.

Use the data in Table 1 to deduce the order of reaction with respect to I^- (aq) and with respect to H⁺ (aq). Explain your reasoning.

Give the rate equation for this reaction.

Use the data from Experiment 4 to calculate a value for the rate constant, k.

Give the units for k.

A chemist analyses a sample of petrol using gas chromatography (GC). The stationary phase is a non-polar liquid adsorbed onto a solid support, and the mobile phase is an inert gas.

The resulting chromatogram is shown in Figure 1. Each peak corresponds to a different hydrocarbon.

Figure 1

In gas chromatography, describe the stationary phase and the mobile phase.

Use the chromatogram in Figure 1 to identify:

• the most abundant hydrocarbon in the mixture.

• the hydrocarbon with the highest boiling point.

Explain your reasoning for the hydrocarbon with the highest boiling point.

Explain why gas chromatography is a suitable technique for analysing a mixture of hydrocarbons found in petrol.

The gas chromatograph can be connected to a mass spectrometer (GC-MS).

State the two pieces of information that could be obtained about a compound by analysing a peak using a mass spectrometer.

A student prepared a sample of aspirin. After filtering, the student obtained impure crystals of aspirin. The student purified the crude product by recrystallisation.

State three properties of a suitable solvent for the recrystallisation of aspirin.

Outline the key steps of the recrystallisation process that the student should carry out to obtain a purified sample of dry aspirin crystals.

State the apparatus used to filter the purified crystals from the solution and explain why this method of filtration is preferred over gravity filtration.

The student then measured the melting point of both the impure and the purified samples of aspirin. The results are shown in Table 1.

Table 1

Explain how the data in Table 1 indicate that the recrystallisation process was successful.

A student investigated the rate of decomposition of hydrogen peroxide, catalysed by manganese(IV) oxide.

2H₂O₂ (aq) → 2H₂O (l) + O₂ (g)

The student monitored the concentration of hydrogen peroxide over time. The results are shown on the graph in Figure 1.

Figure 1

Draw a tangent to the curve at time t = 40 s on Figure 1.

Use your tangent to calculate the rate of reaction at this time.

State the units for the rate of reaction.

Explain why the rate of reaction decreases as the reaction proceeds.

Use the graph to show that the reaction is first order with respect to H₂O₂.
Show your working on the graph.

Use the half-life of the reaction to calculate the value of the rate constant, k. Include units for k.

A student investigated the effect of temperature on the rate constant, k, for the decomposition of an organic compound. The results are shown in Table 1.

Table 1

Complete Table 1.

The Arrhenius equation can be expressed in the form ln(k) = –E_a/RT + ln(A).

Plot a graph of ln(k) on the y-axis against 1/T on the x-axis.

Draw a line of best fit.

Use your graph to determine the gradient of the line of best fit.

Use the gradient to calculate the activation energy, E_a, for this reaction.

Give your answer in kJ mol^-1, to three significant figures.

The gas constant R = 8.31 J K^-1 mol^-1.