PRACTICAL QUESTIONS

TITRATION AND ENTHALPY

Questions

Dr Chris Clay http://drclays-alevelchemistry.com/

Practical Questions

Q1.Read the following instructions that describe how to make up a standard solution of a solid in a volumetric flask. Answer the questions which follow.

‘Take a clean 250 cm3 volumetric flask. Use the balance provided and a clean, dry container, to weigh out the amount of solid required. Tip the solid into a clean, dry 250 cm3 beaker and add about 100 cm3 of distilled water. Use a stirring rod to help the solid dissolve, carefully breaking up any lumps of solid with the rod. When the solid has dissolved, pour the solution into the flask using a filter funnel. Add water to the flask until the level rises to the graduation mark.’

(a) Suggest three further instructions that would improve the overall technique in this account.

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(b) In a series of titrations using the solution made up in part (a), a student obtained the following titres (all in cm3).

Rough 1 2

25.7 25.20 25.35

State what this student must do in order to obtain an accurate average titre in this experiment.

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(Total 5 marks)

Practical Questions Q2.In a titration experiment, a good technique is essential for an accurate result to be obtained.

(a) Suggest a reason for removing the funnel after it has been used for filling the burette.

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(b) Suggest one other source of error in using the burette to carry out a titration.

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(c) During the titration, the inside of the conical flask is rinsed with distilled water.

Suggest why rinsing improves the accuracy of the titre.

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(d) Explain why adding this extra water does not change the volume of EDTA solution that is required in the titration.

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(Total 4 marks)

Q3.The maximum errors for the pipette and the burette are shown below. These errors take into account multiple measurements.

Pipette ± 0.05 cm3

Burette ± 0.15 cm3

Estimate the maximum percentage error in using each of these pieces of apparatus.

Use an average titre 24.25 cm3 to calculate the percentage error in using the burette.

Show your working.

Pipette ....................................................................................................................

Practical Questions .................................................................................................................................

Burette ....................................................................................................................

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Q4.A value for the enthalpy of combustion of an alcohol can be determined using the apparatus shown in the diagram. The calorimeter is held in position by a clamp.

This experiment can be repeated by using a different volume of water that would result in a more accurate value for the enthalpy of combustion because there would be a reduction in the heat lost.

State a change in the volume of water that would cause a reduction in heat loss and explain your answer.

Change in volume: ..................................................................................................

Explanation: ............................................................................................................

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Q5.The alcohol 2-methylpropan-2-ol, (CH3)3COH, reacts to form esters that are used as flavourings by the food industry. The alcohol can be oxidised to produce carbon dioxide and water.

A student carried out an experiment on a pure sample of 2-methylpropan-2-ol to determine its enthalpy of combustion. A sample of the alcohol was placed into a spirit burner and positioned under a beaker containing 50 cm3 of water. The spirit burner was ignited and allowed to burn for several minutes before it was extinguished.

The results for the experiment are shown in Table 1.

Practical Questions Table 1

Initial temperature of the water / °C 18.1

Final temperature of the water / °C 45.4

Initial mass of spirit burner and alcohol / g 208.80

Final mass of spirit burner and alcohol / g 208.58

(a) Use the results from Table 1 to calculate a value for the heat energy released from the combustion of this sample of 2-methylpropan-2-ol. The specific heat capacity of water is 4.18 J K–1 g–1. Show your working.

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(b) Calculate the amount, in moles, of 2-methylpropan-2-ol burned in the experiment. Hence calculate a value, in kJ mol–1, for the enthalpy of combustion of 2-methylpropan-2-ol. Show your working.

(If you were unable to calculate an answer to part (a), you should assume that the heat energy released was 5580 J. This is not the correct value.)

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(c) An equation for the combustion of 2-methylpropan-2-ol is

(CH3)3COH(I) + 6O2(g) 4CO2(g) + 5H2O(I)

Table 2 contains some standard enthalpy of formation data.

Practical Questions Table 2

(CH3)3COH(I) O2(g) CO2(g) H2O(I)

∆Hf / kJ mol–1 –360 0 –393 –286

Use the data from Table 2 to calculate a value for the standard enthalpy of combustion of 2-methylpropan-2-ol. Show your working.

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(d) An accurate value for the enthalpy of combustion of 2-methylpropan-2-ol in which water is formed as a gas is –2422 kJ mol–1.

Use this value and your answer from part (b) to calculate the overall percentage error in the student’s experimental value for the enthalpy of combustion of 2-methylpropan-2-ol.

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(e) Suggest one improvement that would reduce errors due to heat loss in the student’s experiment.

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(f) Suggest one other source of error in the student’s experiment. Do not include heat loss,

Practical Questions apparatus error or student error.

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(Total 11 marks)

Q6.(a) Anhydrous calcium chloride is not used as a commercial de-icer because it reacts with water. The reaction with water is exothermic and causes handling problems.

A student weighed out 1.00 g of anhydrous calcium chloride. Using a pipette, 25.0 cm3 of water were measured out and transferred to a plastic cup. The cup was placed in a beaker to provide insulation. A thermometer was mounted in the cup using a clamp and stand. The bulb of the thermometer was fully immersed in the water.

The student recorded the temperature of the water in the cup every minute, stirring the water before reading the temperature. At the fourth minute the anhydrous calcium chloride was added, but the temperature was not recorded. The mixture was stirred, then the temperature was recorded at the fifth minute. The student continued stirring and recording the temperature at minute intervals for seven more minutes.

The student’s results are shown in the table below.

Time / minutes 0 1 2 3 4

Temperature / °C 19.6 19.5 19.5 19.5

Time / minutes 4 5 6 7 8 9 10 11 12

Temperature / °C 24.6 25.0 25.2 24.7 24.6 23.9 23.4 23.0

Plot a graph of temperature (y-axis) against time on the grid below.

Draw a line of best fit for the points before the fourth minute. Draw a second line of best fit for the appropriate points after the fourth minute. Extrapolate both lines to the fourth minute.

Practical Questions

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(b) Use your graph to determine an accurate value for the temperature of the water at the fourth minute (before mixing).

Practical Questions Temperature before mixing ............................................................................

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(c) Use your graph to determine an accurate value for the temperature of the reaction mixture at the fourth minute (after mixing).

Temperature after mixing ............................................................................. (1)

(d) Use your answers from parts (b) and (c) to determine an accurate value for the temperature rise at the fourth minute. Give your answer to the appropriate precision.

Temperature rise .......................................................................................... (1)

(e) Use your answer from part (d) to calculate the heat given out during this experiment. Assume that the water has a density of 1.00 g cm–3 and a specific heat capacity of 4.18 JK–1 g–1. Assume that all of the heat given out is used to heat the water. Show your working.

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(f) Calculate the amount, in moles, of CaCl2 in 1.00 g of anhydrous calcium chloride (Mr = 111.0).

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(g) Use your answers from parts (e) and (f) to calculate a value for the enthalpy change, in kJ mol–

1, for the reaction that occurs when anhydrous calcium chloride dissolves in water.

CaCl2(s) + aq CaCl2(aq)

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(h) Explain why it is important that the reaction mixture is stirred before recording each temperature.

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(i) Anhydrous calcium chloride can be prepared by passing chlorine over heated calcium. To prevent unreacted chlorine escaping into the atmosphere, a student suggested the diagram of the apparatus for this experiment shown below.

(i) Suggest one reason why the student wished to prevent unreacted chlorine escaping into the atmosphere.

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(ii) Suggest one hazard of using the apparatus as suggested by the student for this experiment.

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(Total 16 marks)

Q7.A teacher noticed that a student had not cleared a large air bubble from below the burette tap in preparing the burette for use before starting the titration. This air bubble was ejected during the first titration of the volumetric flask mixture.

(a) State the effect that this mistake would have on the value of the first titre.

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(b) State and explain the effect, if any, that this mistake would have on the average titre for this experiment.

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(Total 3 marks)

Q8.Alcohols such as methanol (CH3OH), ethanol (CH3CH2OH) and propan-1-ol (CH3CH2CH2OH) are good fuels.

(a) A student carried out an experiment to determine the enthalpy of combustion of methanol.

Methanol was placed in a spirit burner and the mass of the spirit burner measured. The student placed 100 g of water in a copper calorimeter and clamped it above the spirit burner. The burner was lit and allowed to burn for a few minutes. The flame was then extinguished and the new mass of the spirit burner found.

The measured temperature rise was 38.0 °C. The specific heat capacity of water is 4.18 J K−1 g−1.

A diagram of the apparatus is shown alongside a table which shows the measurements the student recorded.

Use the student’s data to calculate an experimental value for the enthalpy of combustion of methanol in kJ mol−1.

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(b) Suggest one reason, other than incomplete combustion or heat transfer to the atmosphere, why the student’s value for the enthalpy of combustion of methanol is different from that in a Data Book.

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(c) The uncertainty in each of the temperature readings from the thermometer in this experiment was ±0.25 °C. This gave an overall uncertainty in the temperature rise of ±0.5 °C.

Calculate the percentage uncertainty for the use of the thermometer in this experiment.

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(d) The student said correctly that using a thermometer with an overall uncertainty for the rise in temperature of ±0.5 °C was adequate for this experiment.

Explain why this thermometer was adequate for this experiment.

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Practical Questions

(e) The enthalpy of combustion of ethanol is −1371 kJ mol−1. The density of ethanol is 0.789 g cm−3.

Calculate the heat energy released in kJ when 0.500 dm3 of ethanol is burned. Give your answer to an appropriate number of significant figures.

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(Total 10 marks)

Q9.This question is about a white solid, MHCO3, that dissolves in water and reacts with hydrochloric acid to give a salt.

MHCO3 + HCl → MCl + H2O + CO2

A student was asked to design an experiment to determine a value for the Mr of MHCO3. The student dissolved 1464 mg of MHCO3 in water and made the solution up to 250 cm3. 25.0 cm3 samples of the solution were titrated with 0.102 mol dm−3 hydrochloric acid. The results are shown in the table.

Rough 1 2 3

Initial burette reading / cm3 0.00 10.00 19.50 29.25

Final burette reading / cm3 10.00 19.50 29.25 38.90

Titre / cm3 10.00 9.50 9.75 9.65

(a) Calculate the mean titre and use this to determine the amount, in moles, of HCl that reacted with 25.0 cm3 of the MHCO3 solution.

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Practical Questions (b) Calculate the amount, in moles, of MHCO3 in 250 cm3 of the solution.

Then calculate the experimental value for the Mr of MHCO3. Give your answer to the appropriate number of significant figures.

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(c) The student identified use of the burette as the largest source of uncertainty in the experiment.

Using the same apparatus, suggest how the procedure could be improved to reduce the percentage uncertainty in using the burette.

Justify your suggested improvement.

Suggestion ..........................................................................................................

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Justification .........................................................................................................

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(d) Another student is required to make up 250 cm3 of an aqueous solution that contains a known mass of MHCO3. The student is provided with a sample bottle containing the MHCO3.

Describe the method, including apparatus and practical details, that the student should use to prepare the solution.

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(Total 14 marks)

Q10.Magnesium carbonate, MgCO3, can occur as the anhydrous compound, or as hydrates with 2, 3 or 5 molecules of water of crystallisation. All types of magnesium carbonate can be decomposed to form magnesium oxide, an important starting material for many processes. This decomposition reaction can be used to identify the type of magnesium carbonate present in a mineral.

A chemist was asked to identify the type of magnesium carbonate present in a mineral imported from France. The chemist weighed a clean dry crucible, and transferred 0.25 g of the magnesium carbonate mineral to the crucible. The crucible was then heated for a few minutes. The crucible was then allowed to cool, and the crucible and its contents were reweighed. This process was repeated until the crucible and its contents had reached constant mass. The mass of the crucible and its contents was then recorded.

The experiment was repeated using different masses of the magnesium carbonate mineral.

For each experiment the chemist recorded the original mass of the mineral and the mass of magnesium oxide left after heating to constant mass. The chemist’s results are shown in the table below.

Experiment 1 2 3 4 5 6

Mass of mineral / g 1.60 1.17 0.74 1.31 1.80 1.34

Mass of magnesium oxide / g 0.54 0.39 0.24 0.44 0.61 0.49

(a) Plot a graph of the mass of the mineral (x-axis) against the mass of magnesium oxide on the

grid below.

Draw a straight line of best fit on your graph.

Practical Questions

(4)

(b) Use the graph to determine the mass of the mineral which would have formed 0.50 g of magnesium oxide.

Mass of the mineral ....................................................................................... (1)

Practical Questions (c) Calculate the amount, in moles, of MgO present in 0.50 g of magnesium oxide.

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(d) Use your answers from part (b) and from part (c) to calculate the Mr of the magnesium carbonate present in the mineral.

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(e) Use your answer from part (d) to confirm that this mineral is MgCO3.2H2O

(If you could not complete the calculation in part (d), you should assume that the experimental Mr value is 122.0 This is not the correct answer.)

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(f) Explain why it was not necessary to use a more precise balance in this experiment.

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(g) Consider your graph and comment on the results obtained by the chemist. Identify any anomalous results.

Comment .......................................................................................................

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Anomalous results .........................................................................................

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Practical Questions (h) Explain why it was necessary for the chemist to heat the crucible and its contents to constant

mass.

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(i) Suggest one reason in each case why

(i) small amounts of the mineral, such as 0.10 g, should not be used in this experiment.

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(ii) large amounts of the mineral, such as 50 g, should not be used in this experiment.

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(j) Analysis of a different hydrated magnesium carbonate showed that it contained 39.05% by mass of water. Determine the formula of this hydrated magnesium carbonate.

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(k) Magnesium oxide is produced by the thermal decomposition of magnesium carbonate and by the thermal decomposition of magnesium hydroxide. The equations for the reactions taking place are shown below.

Reaction 1 Reaction 2

MgCO3

Mg(OH)2

→ →

MgO + CO2

MgO + H2O

Show that Reaction 2 has the greater atom economy for the production of magnesium oxide.

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Practical Questions

(l) Apart from cost, suggest one advantage of using magnesium hydroxide rather than magnesium carbonate to reduce acidity in the stomach.

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(Total 19 marks)

Q11.(a) A student investigated the acid content of a different crater-lake solution. The student used a 50.0 cm3 burette to measure out different volumes of this crater-lake solution. Each volume of crater-lake solution was titrated with a 0.100 mol dm–3 sodium hydroxide solution. Each titration was repeated. The results are shown below.

Volume of crater-lake solution / cm3 10.0 20.0 30.0 40.0 50.0

Volume of sodium hydroxide solution / cm3

Experiment 1 5.85 17.00 20.00 26.50 32.45

Experiment 2 6.15 13.00 19.90 26.50 32.55

Average titre / cm3 6.00 15.00 19.95 26.50 32.50

(i) On the graph paper below, plot a graph of average titre (y-axis) against volume of

crater-lake solution. Both axes must start at zero.

Practical Questions

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(ii) Draw a line of best fit on the graph. (1)

(iii) Use the graph to determine the titre that the student would have obtained using a 25.0 cm3 sample of crater-lake solution.

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Practical Questions (iv) Excluding any anomalous points, which average titre value would you expect to be the

least accurate value? Give one reason for your choice.

Least accurate average titre ................................................................

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(b) Another 100 cm3 sample of crater-lake solution was reacted with an excess of powdered limestone. The gas produced was collected in a gas syringe. The equation for the reaction between the sulfuric(IV) acid in the crater-lake solution and the calcium carbonate in the powdered limestone is shown below.

H2SO3 + CaCO3 CaSO3 + H2O + CO2

The volume of gas collected from the reaction of the sulfuric(IV) acid in 100 cm3 of crater-lake solution with an excess of powdered limestone was 81.0 cm3 at 298 K and 1.00 × 105 Pa.

(i) State the ideal gas equation.

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(ii) Use the ideal gas equation to calculate the amount, in moles, of carbon dioxide formed. Show your working. (The gas constant R = 8.31 J K–1 mol–1)

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(iii) Use the equation for the reaction and your answer from part (b)(ii) to calculate the minimum mass of calcium carbonate needed to neutralise the sulfuric(IV) acid in 1.00 dm3 of crater-lake solution. Show your working.

(If you could not complete the calculation in part (b)(ii) assume that the amount of carbon dioxide is 1.25 × 10–2 mol. This is not the correct value.)

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(3)

(iv) The percentage by mass of calcium carbonate in the powdered limestone was 95.0%. Calculate the minimum mass of this powdered limestone needed to neutralise the sulfuric(IV) acid in 1.00 dm3 of this crater-lake solution.

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(v) Give one reason, other than cost, why limestone rather than solid sodium hydroxide is often used to neutralise acidity in lakes.

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(Total 17 marks)