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9701/35

Chemistry 9701/35 — May/June 2010

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

2
questions
40
marks
120
minutes

Topics Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Presentation of Data and Observations · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

You are required to find the molar enthalpy change for the decomposition of sodium hydrogencarbonate, NaHCO3\text{NaHCO}_3.

NaHCO3→12Na2CO3+12H2O+12CO2\text{NaHCO}_3 \rightarrow \frac{1}{2}\text{Na}_2\text{CO}_3 + \frac{1}{2}\text{H}_2\text{O} + \frac{1}{2}\text{CO}_2
  • FA 1 is anhydrous sodium hydrogencarbonate.
  • FA 2 is anhydrous sodium carbonate.
  • FA 3 is 4.0 mol dm−34.0\text{ mol dm}^{-3} hydrochloric acid.

You will determine the value for the molar enthalpy change of neutralisation for each of FA 1 and FA 2, using an excess of FA 3. You will then use these values to calculate the molar enthalpy change for the decomposition of sodium hydrogencarbonate.

Warning: 4.0 mol dm−34.0\text{ mol dm}^{-3} hydrochloric acid is an irritant.
Very rapid effervescence occurs when FA 1 or FA 2 are added to FA 3.
For safe working, and to minimize the formation of 'acid mist', the solid must be added carefully, with stirring, to the acid. See further instructions below.

Before starting any practical work read through the instructions in (a) and draw up two tables to record your results.

(a)

Determining the molar enthalpy change of neutralisation between FA 1 and FA 3

NaHCO3+HCl→NaCl+H2O+CO2\text{NaHCO}_3 + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} + \text{CO}_2

Follow the instructions below to determine the temperature change when a known mass of FA 1, sodium hydrogencarbonate, reacts with an excess of FA 3, hydrochloric acid.

  • Weigh the stoppered tube labelled FA 1 containing sodium hydrogencarbonate.
  • Use a 50 cm350\text{ cm}^3 measuring cylinder to transfer 50 cm350\text{ cm}^3 of FA 3 into a 250 cm3250\text{ cm}^3 beaker.
  • Stir the acid in the beaker with the thermometer. Record the temperature of the acid; this is the temperature at t=0 mint = 0\text{ min}.
  • Record the temperature each minute for 3 minutes.
  • At t=4 mint = 4\text{ min}, start to carefully add the weighed sample of FA 1 to the acid.
  • Add the solid in at least 4 or 5 portions to prevent excessive effervescence.
  • While adding the solid stir the mixture constantly with the thermometer, but do not record the temperature.
  • Continue to stir the mixture throughout the experiment.
  • Record the temperature at t=5 mint = 5\text{ min}, then every 12\frac{1}{2} minute until t=8 mint = 8\text{ min}.
  • Reweigh the tube labelled FA 1, its stopper and any residual sodium hydrogencarbonate.
  • Empty and rinse the beaker and dry it using a paper towel.
  • In one of your tables record the balance readings and the mass of FA 1 added; in the second table record all measurements of time and temperature.

Table of results

3M
(b)

Graph plotting

  1. Plot a graph of temperature (yy-axis) against time (xx-axis) on the grid below.
  2. Draw two straight lines of best fit; one through the points up to t=3 mint = 3\text{ min}; the second through the points from t=5 mint = 5\text{ min} to t=8 mint = 8\text{ min}. Extrapolate both lines to t=4 mint = 4\text{ min}.
  3. From the extrapolated lines read the minimum and the maximum temperatures at t=4 mint = 4\text{ min}. Record these values in the spaces provided below.
  4. Determine the value for the temperature fall at t=4 mint = 4\text{ min}.

  • The minimum temperature at t=4 mint = 4\text{ min} is ............. °C.
  • The maximum temperature at t=4 mint = 4\text{ min} is ............. °C.
  • The temperature fall, ΔT1\Delta T_1, at t=4 mint = 4\text{ min} is ............. °C.
4M
(c)

Calculations

Working should be shown in all calculations.

3M
(i)

Use the equation below and the temperature fall, ΔT1\Delta T_1, from (b) to calculate the change in heat energy during the reaction of FA 1 with FA 3.

change in heat energy=50×4.3×ΔT1\text{change in heat energy} = 50 \times 4.3 \times \Delta T_1

The change in heat energy = ................. J.

(ii)

Calculate how many moles of FA 1, NaHCO3\text{NaHCO}_3, were used in your experiment.
[ArA_r: C,12.0\text{C}, 12.0; H,1.0\text{H}, 1.0; O,16.0\text{O}, 16.0; Na,23.0\text{Na}, 23.0]

.................. mol of FA 1 were used in the experiment.

(iii)

Calculate the enthalpy change when 1 mol1\text{ mol} of FA 1 reacts with an excess of the hydrochloric acid, FA 3.
This is the molar enthalpy change of neutralisation, ΔHneutralisation(NaHCO3)\Delta H_{\text{neutralisation}}(\text{NaHCO}_3).
Make certain that your answer is given in kJ mol−1\text{kJ mol}^{-1} and has the appropriate sign.

ΔHneutralisation(NaHCO3)=sign calculated value kJ mol−1\Delta H_{\text{neutralisation}}(\text{NaHCO}_3) = \text{sign} \text{ } \text{calculated value} \text{ kJ mol}^{-1}
(iv)

Explain the significance of the sign you have given in (iii) and how it is related to your experimental results.

(d)

Determining the molar enthalpy change of neutralisation between FA 2 and FA 3

Na2CO3+2HCl→2NaCl+H2O+CO2\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2

Follow the instructions below to find the temperature change when a known mass of FA 2, sodium carbonate, reacts with an excess of FA 3, hydrochloric acid.

You are provided with two samples of sodium carbonate. You should use the sample labelled Na2CO3\text{Na}_2\text{CO}_3 in experiment 1 and the sample labelled FA 2 in experiment 2.

Experiment 1

  • Enter all results in the table below.
  • Weigh the stoppered tube containing sodium carbonate, which is labelled Na2CO3\text{Na}_2\text{CO}_3.
  • Use a 50 cm350\text{ cm}^3 measuring cylinder to transfer 50 cm350\text{ cm}^3 of FA 3 into the rinsed and dried 250 cm3250\text{ cm}^3 beaker used in (a).
  • Stir the acid in the beaker with the thermometer and record the temperature.
  • Start to add carefully the weighed sample of Na2CO3\text{Na}_2\text{CO}_3 to the acid.
  • As in the experiment in (a), add the solid in at least 4 or 5 portions to prevent excessive effervescence.
  • While adding the solid stir the mixture constantly with the thermometer.
  • Record the maximum temperature obtained during the reaction.
  • Reweigh the tube labelled Na2CO3\text{Na}_2\text{CO}_3, its stopper and any residual sodium carbonate.
  • Empty and rinse the beaker and dry it using a paper towel.

Experiment 2

  • Enter all results in the table below.
  • Weigh a clean, dry, boiling-tube.
  • Weigh between 7.0 g7.0\text{ g} and 7.5 g7.5\text{ g} of FA 2, sodium carbonate, into the boiling-tube.
  • Repeat the procedure in experiment 1 and record the maximum temperature obtained when this mass of FA 2 reacts with 50 cm350\text{ cm}^3 of FA 3.
  • Reweigh the boiling-tube and any residual sodium carbonate.

Results

6M
(e)

Calculations

Working should be shown in all calculations.

5M
(i)

Use the equation below and the temperature rise, ΔT2\Delta T_2, from (d) for experiment 1 to calculate the change in heat energy during the reaction of sodium carbonate with FA 3.

change in heat energy=50×4.3×ΔT2\text{change in heat energy} = 50 \times 4.3 \times \Delta T_2

Change in heat energy = ................. J.

(ii)

Calculate how many moles of sodium carbonate were used in your experiment 1.
[ArA_r: C,12.0\text{C}, 12.0; O,16.0\text{O}, 16.0; Na,23.0\text{Na}, 23.0]

.................. mol of sodium carbonate were used in experiment 1.

(iii)

Calculate the enthalpy change when 1 mol1\text{ mol} of sodium carbonate reacts with an excess of the hydrochloric acid, FA 3.
This is the molar enthalpy change of reaction, ΔHneutralisation(Na2CO3)\Delta H_{\text{neutralisation}}(\text{Na}_2\text{CO}_3).
Make certain that your answer is given in kJ mol−1\text{kJ mol}^{-1} and has the appropriate sign.

ΔHneutralisation(Na2CO3)=sign calculated value kJ mol−1\Delta H_{\text{neutralisation}}(\text{Na}_2\text{CO}_3) = \text{sign} \text{ } \text{calculated value} \text{ kJ mol}^{-1}
(iv)

Explain the significance of the sign you have given in (iii) and how it is related to your experimental results.

(f)

Calculating the enthalpy change of decomposition of FA 1, ΔHdecomposition(NaHCO3)\Delta H_{\text{decomposition}}(\text{NaHCO}_3)

Use your values for ΔHneutralisation(NaHCO3)\Delta H_{\text{neutralisation}}(\text{NaHCO}_3) and ΔHneutralisation(Na2CO3)\Delta H_{\text{neutralisation}}(\text{Na}_2\text{CO}_3) in the expression below to calculate a value for the molar enthalpy change of decomposition, ΔHdecomposition(NaHCO3)\Delta H_{\text{decomposition}}(\text{NaHCO}_3), for FA 1.

ΔHdecomposition(NaHCO3)=ΔHneutralisation(NaHCO3)−12ΔHneutralisation(Na2CO3)\Delta H_{\text{decomposition}}(\text{NaHCO}_3) = \Delta H_{\text{neutralisation}}(\text{NaHCO}_3) - \frac{1}{2}\Delta H_{\text{neutralisation}}(\text{Na}_2\text{CO}_3) ΔHdecomposition(NaHCO3)=sign calculated value kJ mol−1\Delta H_{\text{decomposition}}(\text{NaHCO}_3) = \text{sign} \text{ } \text{calculated value} \text{ kJ mol}^{-1}
1M
(g)

Evaluation

An alternative method for the determination of ΔHdecomposition(NaHCO3)\Delta H_{\text{decomposition}}(\text{NaHCO}_3) gave a value of +42.5 kJ mol−1+42.5\text{ kJ mol}^{-1}.
The value you have obtained may be significantly different from this value.

Calculate the difference between your value of ΔHdecomposition(NaHCO3)\Delta H_{\text{decomposition}}(\text{NaHCO}_3) and that obtained by the alternative method.

Record this difference below. Express this difference as a percentage of the alternative value.

  • difference = ................. kJ mol−1\text{kJ mol}^{-1}
  • percentage difference = ................. %
1M
(h)

Sources of error

Describe one major source of error in this experiment. Suggest an improvement which would significantly increase the accuracy of the experiment. Explain why your suggestion would produce a more accurate value.

  • description of major source of error
    .........................................................................................................................................
  • suggested improvement
    .........................................................................................................................................
  • explanation of why suggestion would increase experimental accuracy
    .........................................................................................................................................
2M

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