AS & A Level Chemistry - 9701 Paper 3 2010 Summer Zone 5

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

$$\text{NaHCO}_{3} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_{2}\text{O} + \text{CO}_{2}$$

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 cm³ measuring cylinder to transfer 50 cm³ of FA 3 into a 250 cm³ beaker.
• Stir the acid in the beaker with the thermometer. Record the temperature of the acid; this is the temperature at t = 0 min.
• Record the temperature each minute for 3 minutes.
• At t = 4 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 min, then every \( \frac{1}{2} \) minute until t = 8 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

(b) Graph plotting

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

[Image]

The minimum temperature at t = 4 min is ............ °C.
The maximum temperature at t = 4 min is ............ °C.
The temperature fall, \( \Delta T_{1} \), at t = 4 min is ............ °C.

(c) Calculations
Working should be shown in all calculations.
(i) Use the equation below and the temperature fall, \(\Delta T_{1}\), from (b) to calculate the change in heat energy during the reaction of FA 1 with FA 3.
change in heat energy = 50 \times 4.3 \times \Delta T_{1}

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

(ii) Calculate how many moles of FA 1, NaHCO₃, were used in your experiment.
[\(A_{r}\): C, 12.0; H, 1.0; O, 16.0; Na, 23.0]
................... mol of FA 1 were used in the experiment.

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

$$\Delta H_{\text{neutralisation}}(\text{NaHCO}_{3}) = \quad \text{sign}\quad \text{calculated value} \quad \text{kJ mol}^{-1}.$$

(iv) Explain the significance of the sign you have given in (iii) and how it is related to your experimental results.
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(d) Determining the molar enthalpy change of neutralisation between FA 2 and FA 3

$$\text{Na}_{2}\text{CO}_{3} + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_{2}\text{O} + \text{CO}_{2}$$

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 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.
• Use a 50 cm³ measuring cylinder to transfer 50 cm³ of FA 3 into the rinsed and dried 250 cm³ 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 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, 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 g and 7.5 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 cm³ of FA 3.
• Reweigh the boiling-tube and any residual sodium carbonate.

Results
[Table]
experiment 1 | experiment 2
mass of tube + anhydrous sodium carbonate / g
mass of empty tube / g
mass of tube + residual sodium carbonate / g
mass of anhydrous sodium carbonate / g
initial temperature of acid / °C
maximum temperature obtained / °C
temperature rise, \( \Delta T_{2} / °C \)

(e) Calculations
Working should be shown in all calculations.
(i) Use the equation below and the temperature rise, \(\Delta T_{2}\), 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 \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.
[\(A_{r}\): C, 12.0; O, 16.0; Na, 23.0]
................... mol of sodium carbonate were used in experiment 1.

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

$$\Delta H_{\text{neutralisation}}(\text{Na}_{2}\text{CO}_{3}) = \quad \text{sign}\quad \text{calculated value} \quad \text{kJ mol}^{-1}.$$

(iv) Explain the significance of the sign you have given in (iii) and how it is related to your experimental results.
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(f) Calculating the enthalpy change of decomposition of FA 1, \(\Delta H_{\text{decomposition}}(\text{NaHCO}_{3})\)

Use your values for \(\Delta H_{\text{neutralisation}}(\text{NaHCO}_{3})\) and \(\Delta H_{\text{neutralisation}}(\text{Na}_{2}\text{CO}_{3})\) in the expression below to calculate a value for the molar enthalpy change of decomposition, \(\Delta H_{\text{decomposition}}(\text{NaHCO}_{3})\), for FA 1.

$$\Delta H_{\text{decomposition}}(\text{NaHCO}_{3}) = \Delta H_{\text{neutralisation}}(\text{NaHCO}_{3}) - \frac{1}{2}\Delta H_{\text{neutralisation}}(\text{Na}_{2}\text{CO}_{3})$$

$$\Delta H_{\text{decomposition}}(\text{NaHCO}_{3}) = \quad \text{sign}\quad \text{calculated value} \quad \text{kJ mol}^{-1}.$$

(g) Evaluation
An alternative method for the determination of \(\Delta H_{\text{decomposition}}(\text{NaHCO}_{3})\) gave a value of +42.5 kJ mol⁻¹.
The value you have obtained may be significantly different from this value.

Calculate the difference between your value of \(\Delta H_{\text{decomposition}}(\text{NaHCO}_{3})\) 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
percentage difference = .................. %

(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
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suggested improvement
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explanation of why suggestion would increase experimental accuracy
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(a) You are provided with three solutions, FA 4, FA 5 and FA 6. The only anions that may be present in these solutions are carbonate and chloride. One or more of the solutions may contain both anions.

Identification of the anions in FA 4, FA 5 and FA 6

Most metal carbonates are insoluble, most metal chlorides are soluble and all metal nitrates are soluble in water.

(i) Use this information and the Qualitative Analysis Notes on page 15 to select
• reagent 1, to identify any carbonate ion that is present,
• reagent 2, to identify any chloride ion present.

reagent 1 ................................................ reagent 2 ................................................ [1]

(ii) Explain the order in which you will add your chosen reagents to determine the anion or anions present in each of FA 4, FA 5 and FA 6.

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(iii) Use the reagents selected in (a)(i) to test each of the solutions FA 4, FA 5 and FA 6. Record your observations in the table below.

reagent  observations
[Table_1]

[2]

(iv) From your observations, identify the anion or anions present in each of the solutions.
[Table_2]
[2]

(b) You are provided with four solutions, FA 7, FA 8, FA 9 and FA 10. Perform the test-tube experiments described below and record your observations in the table.

observations
[Table_3]

(i) To about 1 cm depth of solution in a test-tube, add 5 drops of aqueous sodium hydroxide, shake the mixture then,
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add a further 2 cm depth of aqueous sodium hydroxide.

(ii) To about 1 cm depth of solution in a test-tube, add 10 drops of aqueous ammonia, shake the mixture then,
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add a further 2 cm depth of aqueous ammonia.

(iii) Select a reagent that will confirm the identity of any ions producing white precipitates in both of the tests above. To about 1 cm depth of solution in a test-tube add the selected reagent. Record the appropriate observations below.

The reagent is ................................................................................................................................................
[7]

(c) From your observations, identify the cation present in each of the following solutions.
[Table_4]
[2]