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(a) Method 1
• Weigh the container with FA 1. Record this mass.
mass of container with FA 1 = ................................ g
• Support one of the plastic cups in the 250 cm³ beaker.
• Use the measuring cylinder to place 25 cm³ of FA 2 into the cup.
• Measure the temperature of the FA 2 in the cup. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. Record this temperature at time t = 0.
• Start the stopclock and leave it running for the whole experiment.
• Measure and record the temperature of FA 2 in the cup every half minute for 2 minutes.
• At t = 2$$\frac{1}{2}$$ minutes tip all the FA 1 into the cup. Stir the contents of the cup.
• Measure and record the temperature of the contents of the cup at t = 3 minutes and then every half minute up to t = 9 minutes.
• Weigh the container with any residual FA 1. Record this mass.
mass of container with residual FA 1 = ................................ g
(b) (i) On the grid on page 3, plot a graph of temperature (y-axis) against time (x-axis). You should choose a scale that allows you to plot 2 °C above the maximum temperature reached.
On your graph, draw two straight lines of best fit. One line is for the temperature before adding FA 1 and the other line for the cooling of the solution once reaction is complete.
Extrapolate these two lines to t = 2$$\frac{1}{2}$$ minutes. [4]
(ii) From your graph, find the theoretical temperature rise at t = 2$$\frac{1}{2}$$ minutes.
theoretical temperature rise = ............................... °C [1]
(c) (i) Calculate the energy released in the reaction.
(Assume 4.2 J of heat energy changes the temperature of 1.0 cm³ of solution by 1.0 °C.)
energy released = ............................... J [1]
(ii) The equation for the reaction between anhydrous sodium carbonate and hydrochloric acid is shown.
Na$_2$CO$_3$(s) + 2HCl(aq) → 2NaCl(aq) + CO$_2$(g) + H$_2$O(l)
The literature value for the enthalpy change of this reaction is -27.0 kJ mol$^{-1}$.
Use this figure, and the value that you found in (i), to find the mass of anhydrous sodium carbonate you used in (a). You should assume that no energy was lost to the surroundings in your experiment.
mass Na$_2$CO$_3$ = ............................... g [2]
(iii) Calculate the percentage of anhydrous sodium carbonate present in FA 1.
percentage Na$_2$CO$_3$ in FA 1 = ............................... % [1]
(d) In your calculation in (c), what assumption have you made about the impurity present in FA 1?
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............................................................................................................................ [1]
(e) Method 2
• Weigh a clean, dry plastic cup and record the mass.
• Add between 1.70 g and 1.90 g of FA 3 to the plastic cup and record the mass.
• Support the plastic cup in the 250 cm³ beaker.
• Pour 25 cm³ of FA 2 into the measuring cylinder.
• Measure and record the initial temperature of FA 2 in the measuring cylinder.
• Pour the 25 cm³ of FA 2 into the plastic cup.
• Stir the contents of the cup and record the maximum temperature. Tilt the cup if necessary so that the bulb of the thermometer is fully covered.
• Calculate and record the mass of FA 3 used and the change in temperature. [2]
(f) Use the temperature rise in (e), and the fact that the enthalpy change for the reaction between anhydrous sodium carbonate and hydrochloric acid is -27.0 kJ mol$^{-1}$, to calculate the percentage of anhydrous sodium carbonate in FA 3.
percentage Na$_2$CO$_3$ in FA 3 = ............................... % [2]
(g) FA 1 and FA 3 are both samples of the same impure anhydrous sodium carbonate and so the percentage of anhydrous sodium carbonate found using Method 1 and Method 2 should be the same. In practice the percentages are sometimes different from each other.
In both methods, percentage errors occur due to measuring the mass of solid and the temperature rise.
Ignoring these errors, which method is more accurate? Tick the correct box and explain your answer.
Method 1 more accurate [ ]
Method 2 more accurate [ ]
Method 1 and Method 2 equally accurate [ ]
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............................................................................................................................ [1]
(h) A student decided to confirm by experiment the literature value for the enthalpy change of the reaction between anhydrous sodium carbonate and hydrochloric acid. By mistake the student weighed a sample of hydrated sodium carbonate, Na$_2$CO$_3$·10H$_2$O, instead of anhydrous sodium carbonate, Na$_2$CO$_3$.
State what effect this would have on the calculated value of the enthalpy change for the reaction. Explain your answer.
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(i) A student used 3.00 g of anhydrous sodium carbonate that was 80.0% pure by mass.
Calculate the minimum volume of 2.00 mol dm$^{-3}$ hydrochloric acid that would be needed to react completely with this sample of impure anhydrous sodium carbonate.
volume of HCl = ............................... cm³ [3]
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(a) (i) FA 4 is a sodium compound that was the impurity in the FA 1 and FA 3 that you used in Question 1. The anion in FA 4 is one of those listed in the Qualitative Analysis Notes.
Carry out appropriate tests to allow you to positively identify the anion in FA 4.
For the test that gives a positive result, record the test and the results of it. State the name of the anion in FA 4.
anion in FA 4 = ...................................................... [2]
(ii) Write the ionic equation for the reaction that you have used to identify the anion in FA 4. Include state symbols.
................................................................................................................ [1]
(b) FA 5 is a mixture that contains two cations and three anions from those listed in the Qualitative Analysis Notes.
A sample of FA 5 was added to water and the water stirred. The mixture produced was filtered to give a solid residue, FA 6, and a filtrate, FA 7.
(i) Carry out the following tests on FA 6 and record your observations.
[Table with columns "test" and "observations"]
test: To a small spatula measure of FA 6 in a test tube add dilute hydrochloric acid, then add aqueous ammonia.
observations: .................................................................................................................
test: Place a small spatula measure of FA 6 in a hard-glass test-tube and heat gently.
observations: .................................................................................................................
[4]
(ii) Carry out the following tests on FA 7 and record your observations.
[Table with columns "test" and "observations"]
test: To a 1 cm depth of FA 7 in a test-tube add aqueous sodium hydroxide.
observations: .................................................................................................................
test: To a 1 cm depth of FA 7 in a test-tube add aqueous ammonia.
observations: .................................................................................................................
test: To a 1 cm depth of FA 7 in a test-tube add a few drops of aqueous silver nitrate.
observations: .................................................................................................................
test: To a 1 cm depth of FA 7 in a test-tube add a few drops of aqueous barium nitrate or aqueous barium chloride, then add dilute nitric acid.
observations: .................................................................................................................
test: To a 0.5 cm depth of FA 7 in a boiling tube add a 2 cm depth of aqueous sodium hydroxide and warm, then add a small piece of aluminium foil.
observations: .................................................................................................................
[5]
(iii) From your observations, identify the two cations present in FA 5. cations................................................... and ......................................................... [1]
(iv) From your observations, identify two anions present in FA 5. ................................................................................ [1]
(v) From your observations, identify two anions that could be present in FA 5. ................................................................................ [1]
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