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(a) Determining the approximate heat capacity of the 250 cm³ beaker
When samples of hot and cold water are mixed in the 250 cm³ beaker, some heat is lost to the beaker in raising its temperature. To determine the approximate heat capacity of your 250 cm³ beaker, you will determine the maximum temperature rise when a sample of hot water is added to cold water in the beaker.
- Use a 50 cm³ measuring cylinder to transfer 50 cm³ of cold water into the 250 cm³ beaker.
- Use the 50 cm³ measuring cylinder to transfer 50 cm³ of cold water into a 100 cm³ beaker. Note the temperature of the water in this 100 cm³ beaker and heat it carefully and gently until the temperature of the water in it has increased by 45-50 °C then stop heating, e.g. if the water is at 20.0 °C you should warm it to 65-70 °C.
- Stir the cold water in the 250 cm³ beaker with the thermometer.
- Record the temperature of the cold water (this is the temperature at t = 0 min).
- Record the temperature each minute for 3 minutes.
- After you have taken the reading at t = 3 min, use the thermometer to stir the hot water in the 100 cm³ beaker.
- At t = 4 min, measure the temperature of the hot water and record this value in the box below.
- Immediately add the hot water from the 100 cm³ beaker to the cold water in the 250 cm³ beaker. Stir with the thermometer but do not record the temperature.
- Continue to stir the water throughout the experiment.
- Record the temperature at t = 5 min, and then every ½ minute until t = 8 min.
- Empty and rinse the 250 cm³ beaker. Dry it using a paper towel.
- Record all measurements of time and temperature obtained.
The temperature, $$T_{1}$$, of the hot water at t = 4 min is .......... °C.
Table of results
(b) Graph plotting
1. Plot a graph of the temperature of the water in the 250 cm³ beaker (y-axis) against time (x-axis) on the grid below. Do not plot the temperature, $$T_{1}$$, of the hot water at t = 4 min.
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 values for the two temperature changes at t = 4 min.
Minimum temperature, $$T_{2}$$, at t = 4 min is ........... °C.
Maximum temperature, $$T_{3}$$, at t = 4 min is ........... °C.
Temperature rise for 50 cm³ of cold water in the 250 cm³ beaker, $$(T_{3} - T_{2})$$ is ............ °C.
Temperature fall for 50 cm³ of hot water from the 100 cm³ beaker, $$(T_{1} - T_{3})$$ is ............ °C.
(c) Calculations
Working should be shown in all calculations.
[4.2 J are absorbed or released when the temperature of 1.0 cm³ of water changes by 1.0 °C.]
(i) Calculate the heat energy gained by the 50 cm³ of cold water in the 250 cm³ beaker.
The heat energy gained by the cold water = ........... J.
(ii) Calculate the heat energy lost by the 50 cm³ of hot water from the 100 cm³ beaker.
The heat energy lost by the hot water = ........... J.
(iii) The difference between the values calculated in (i) and (ii) is an approximate value for the total heat energy absorbed by the 250 cm³ beaker during the experiment. The heat capacity of the beaker is the amount of heat energy absorbed for a 1 °C change in temperature.
$$\text{approximate heat capacity of the 250 cm}^3 \text{beaker} = \frac{\text{(heat energy lost) – (heat energy gained)}}{(T_{3} – T_{2})} \text{ J}^\circ\text{C}^{-1}$$
Use your answers to (i) and (ii) and the temperature rise from (b) to calculate the approximate heat capacity of the 250 cm³ beaker.
The approximate heat capacity of the 250 cm³ beaker = .......... J °C⁻¹.
(d) Determining the enthalpy change of solution for ammonium chloride
Follow the instructions below to find the temperature change when a known mass of solid ammonium chloride dissolves in water.
You are provided with two samples of ammonium chloride. You should use the sample labelled $$NH_4Cl$$ in experiment 1 and the sample labelled $$FA 1$$ in experiment 2.
Experiment 1
- Enter all results in the table below.
- Weigh the stoppered tube containing ammonium chloride, which is labelled $$NH_4Cl$$.
- Use the 50 cm³ measuring cylinder to transfer 100 cm³ of cold water into the rinsed and dried 250 cm³ beaker used in (a).
- Stir the water in the beaker with the thermometer and record the temperature.
- Add the solid from the weighed tube to the water.
- Stir the mixture constantly with the thermometer.
- Record the minimum temperature obtained in the solution.
- Reweigh the tube labelled $$NH_4Cl$$, its stopper and any residual ammonium chloride.
- 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 9.8 g and 10.2 g of $$FA 1$$, ammonium chloride, into the boiling-tube.
- Repeat the procedure in experiment 1 and record the minimum temperature obtained when this mass of $$FA 1$$ dissolves in 100 cm³ of water.
- Reweigh the boiling-tube and any residual ammonium chloride.
Results
[Table of results]
mass of tube + ammonium chloride / g
mass of empty tube / g
mass of tube + residual ammonium chloride / g
mass of ammonium chloride / g
initial temperature of water / °C
minimum temperature obtained / °C
temperature fall, $$\Delta T$$ / °C.
(e) Calculations
Working should be shown in all calculations.
(i) Use the temperature fall from (d), experiment 1, to calculate the change in heat energy of the solution.
[4.3 J are absorbed or released when the temperature of 1.0 cm³ of solution changes by 1.0 °C.]
The change in heat energy of the solution = ................. J.
(ii) To calculate the total change in heat energy as ammonium chloride dissolves in water, the change in heat energy of the 250 cm³ beaker has to be added to the change in heat energy of the solution. Explain why these two changes in heat energy have to be added together.
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(iii) Use your answer in (i) above and the approximate heat capacity of the 250 cm³ beaker calculated in (c)(iii) to calculate the combined change in heat energy of the beaker and solution.
The combined change in heat energy of the beaker and solution = ................ J.
(iv) Calculate how many moles of $$FA 1$$, $$NH_4Cl$$, were used in (d), experiment 1.
[ $$A_r$$; Cl, 35.5; H, 1.0; N, 14.0]
........... mol of $$FA 1$$ were used in experiment 1.
(v) Calculate the enthalpy change when 1 mol of $$FA 1$$ dissolves in an excess of water. This is the molar enthalpy change of solution, $$\Delta H_{solution}$$ ($$NH_4Cl$$). Make certain that your answer is given in kJ mol⁻¹ and has the appropriate sign.
$$\Delta H_{solution} (NH_4Cl) = .............. \text{ } .................... \text{ kJ mol}^{-1}$$
sign calculated value
(vi) Explain the significance of the sign you have given in (v) and how it is related to your experimental results.
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(f) Evaluation
A data book value for the molar enthalpy change of solution, $$\Delta H_{solution}$$ ($$NH_4Cl$$), is +15.2 kJ mol⁻¹.
The value you have obtained may be significantly different from this value.
Calculate the difference between your value of $$\Delta H_{solution}$$ ($$NH_4Cl$$) and the data book value. Record this difference below. Express this difference as a percentage of the data book value.
difference = ................ kJ mol⁻¹
percentage difference = .................... %
(g) 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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547
537
519
(a) You are provided with three solutions, FA 2, FA 3 and FA 4. The only anions that may be present in these solutions are sulfate and carbonate. One or more of the solutions may contain both anions.
Identification of the anions in FA 2, FA 3 and FA 4.
Most metal carbonates are insoluble, most metal sulfates are soluble and all metal nitrates are soluble in water.
(i) Use this information and the Qualitative Analysis Notes on page 16 to select
- reagent 1, to identify any carbonate ion present,
- reagent 2, to identify any sulfate 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 2, FA 3 and FA 4.
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(iii) Use the reagents selected in (a)(i) to test each of the solutions FA 2, FA 3 and FA 4. Record your observations in the table below.
[Table_1: reagents, observations for FA 2, FA 3, FA 4]
[2]
(iv) From your observations, identify the anion or anions present in each of the solutions.
[Table_2: solution, carbonate, sulfate, evidence for FA 2, FA 3, FA 4]
[2]
(b) You are provided with four solutions, FA 5, FA 6, FA 7 and FA 8. Perform the test-tube experiments described below and record your observations in the table. Where gases are released they should be identified by a test, described in the appropriate place in your observations.
[Table_3: tests, observations for FA 5, FA 6, FA 7, FA 8]
(i) To about 1 cm depth of solution in a test-tube, add 10 drops of aqueous sodium hydroxide. Shake the mixture,
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then add a further 2 cm depth of aqueous sodium hydroxide. Shake the mixture again,
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then leave the tube to stand for 2–3 minutes.
(ii) If no precipitate has formed in test (i) above; transfer the solution to a boiling-tube and warm gently with a Bunsen burner.
Care: heating aqueous sodium hydroxide in a tube may cause the solution to be ejected from the tube.
(iii) To about 1 cm depth of solution in a test-tube, add 10 drops of aqueous ammonia.
Shake the mixture,
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then add a further 2 cm depth of aqueous ammonia. Shake the mixture again,
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then leave the tube to stand for 2–3 minutes.
[7]
(c) From your observations in (b), identify the cation present in each of the following solutions.
[Table_4: solution, cation, evidence for FA 5, FA 6, FA 8]
[2]
563
501
583
