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You are to determine the enthalpy change of reaction, $\Delta H$, for the reaction shown below.
$$\text{Cu(s) + H}_2\text{SO}_4\text{(aq) } \rightarrow \text{CuSO}_4\text{(aq) + H}_2\text{(g)}$$
Since copper is an unreactive metal it does not react directly with dilute acids. You will therefore need to find the enthalpy change of reaction for two reactions that do occur. The equations for these two reactions are below.
$$\text{Mg(s) + H}_2\text{SO}_4\text{(aq) } \rightarrow \text{MgSO}_4\text{(aq) + H}_2\text{(g)}$$ Reaction 1
$$\text{Mg(s) + CuSO}_4\text{(aq) } \rightarrow \text{MgSO}_4\text{(aq) + Cu(s)}$$ Reaction 2
You will carry out experiments to find the enthalpy changes for each of Reaction 1 and Reaction 2 and use these values to calculate the enthalpy change for the reaction of copper with sulfuric acid.
(a) Method
FA 1 is 1.00 mol dm^{-3} sulfuric acid, H_2SO_4.
FA 2 is magnesium powder, Mg.
Read through the method before you start any practical work and prepare a suitable table for your results.
• Weigh the stoppered tube containing FA 2. Record the mass.
• Support the plastic cup in the 250 cm^3 beaker.
• Use the measuring cylinder to transfer 25 cm^3 of FA 1 into the plastic cup.
• Measure the temperature of FA 1 in the plastic cup and start the stop clock. Record this temperature as being the temperature at time = 0.
• Measure, and record, the temperature of this FA 1 every half minute for 2 minutes.
• At time = 2\frac{1}{2} minutes add the FA 2 to the acid and stir carefully to reduce acid spray.
• Measure the temperature of the mixture in the cup at time = 3 minutes and then every half minute up to time = 7 minutes.
• Continue stirring occasionally throughout this time.
• Weigh the stoppered tube that had contained FA 2. Record the mass.
• Calculate and record the mass of FA 2 added to the sulfuric acid.
• Rinse the plastic cup with water and shake to dry.
(b) (i) On the grid below plot a graph of temperature (y-axis) against time (x-axis).
(ii) Complete the graph by inserting two, straight lines of best fit:
• one to show the temperature up to time = 2\frac{1}{2} minutes,
• one to show the temperature after time = 2\frac{1}{2} minutes.
[4]
(iii) From your graph, use the two straight lines of best fit to calculate the change in temperature at time = 2\frac{1}{2} minutes.
temperature change = .............. °C
[4]
(c) Calculations
(i) In the reaction in (a), the sulfuric acid was in excess. Without carrying out any additional tests, what observation could you have made during your experiment to confirm this?
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(ii) Calculate the energy change that occurred during the reaction in (a). [Assume that 4.2 J is needed to raise the temperature of 1.0 cm^3 of solution by 1.0 °C.]
energy change = .............. J
[2]
(iii) Use your answer to (ii) to calculate the enthalpy change, in kJ mol^{-1}, for the reaction between sulfuric acid and magnesium. [$A_r$: Mg, 24.3]
$$\text{Mg(s) + H}_2\text{SO}_4\text{(aq) } \rightarrow \text{MgSO}_4\text{(aq) + H}_2\text{(g)}$$ Reaction 1
enthalpy change for Reaction 1 = .......... ............. kJ mol^{-1}
$sign$ $value$
[4]
Determining the enthalpy change for Reaction 2
$$\text{Mg(s) + CuSO}_4\text{(aq) } \rightarrow \text{MgSO}_4\text{(aq) + Cu(s)}$$ Reaction 2
(d) Method
FA 3 is 1.00 mol dm^{-3} copper(II) sulfate, CuSO_4.
FA 4 and FA 5 are magnesium powder, Mg.
Read through the method before you start any practical work and prepare a suitable table for your results.
• Weigh the stoppered tube containing FA 4. Record the mass.
• Support the plastic cup in the 250 cm^3 beaker.
• Use the measuring cylinder to transfer 25 cm^3 of FA 3 into the plastic cup.
• Measure the temperature of FA 3 in the plastic cup and record the temperature.
• Add the FA 4 to the FA 3 in the cup and stir the mixture constantly.
• Measure and record the maximum temperature reached during the reaction.
• Calculate and record the maximum temperature change that occurred during the reaction.
• Weigh the stoppered tube that had contained FA 4. Record the mass.
• Calculate and record the mass of FA 4 added to the copper(II) sulfate.
• Empty the contents of the plastic cup into the 100 cm^3 beaker labelled waste.
• Rinse the plastic cup and shake to dry.
• Repeat this experiment using FA 5 in place of FA 4.
[2]
(e) Calculations
(i) Using your results from (d), calculate the mean temperature rise.
mean temperature rise = ................... °C
[1]
(ii) Using your results from (d), calculate the mean mass of magnesium used.
mean mass = ................... g
[1]
(iii) Show, using a suitable calculation, that the copper(II) sulfate was in excess in these reactions.
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(iv) Using your values from (i) and (ii), calculate the enthalpy change, in kJ mol^{-1}, for the reaction between magnesium and copper(II) sulfate. [Assume that 4.2 J is needed to raise the temperature of 1.0 cm^3 of solution by 1.0 °C.]
$$\text{Mg(s) + CuSO}_4\text{(aq) } \rightarrow \text{MgSO}_4\text{(aq) + Cu(s)}$$ Reaction 2
enthalpy change for Reaction 2 = .......... ............. kJ mol^{-1}
$sign$ $value$
[4]
Enthalpy change for Reaction 3
Reaction 3 is shown below.
$$\text{Cu(s) + H}_2\text{SO}_4\text{(aq) } \rightarrow \text{CuSO}_4\text{(aq) + H}_2\text{(g)}$$ Reaction 3
(f) Use your values for the enthalpy changes for Reactions 1 and 2 to calculate the enthalpy change for Reaction 3.
Show clearly how you obtained your answer.
(If you were unable to calculate the enthalpy changes for Reactions 1 and 2, you should assume that the value for Reaction 1 is -444 kJ mol^{-1} and that the value for Reaction 2 is -504 kJ mol^{-1}. Note: these are not the correct values.)
enthalpy change for Reaction 3 = .......... ............. kJ mol^{-1}
$sign$ $value$
[2]
(g) (i) The method you used to determine the enthalpy change for Reaction 1 was more accurate than the method you used to determine the enthalpy change for Reaction 2. Suggest two reasons why the method used for Reaction 2 was less accurate. Explain your answers.
1 .................................................................................
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2 .................................................................................
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[3]
(ii) A student suggested that the accuracy of the method used for Reaction 2 could be improved by using a larger volume of copper(II) sulfate. Is this a correct suggestion? Give a reason for your answer.
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[3]
(a) FA 6 is a solid that contains one cation and one anion. One of the ions present is included in the lists on pages 14 and 15. This ion contains the element nitrogen.
(i) State which nitrogen-containing ions could be present. Select reagents for use in tests that would distinguish between them.
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(ii) Carry out tests on FA 6 using the reagents selected in (i) to identify the nitrogen-containing ion. Record your tests and observations in the space below.
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(iii) Identify the nitrogen-containing ion in FA 6.
Ion present is ................................................
(b) Half fill the 250 cm$^3$ beaker with water and heat the water to about 60 °C. This is the water bath to be used in one of the following tests.
Carry out the following tests on FA 7 and complete the table below.
[Table_1]
test | observations |
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To a 1 cm depth of FA 7 in a test-tube, add a few drops of dilute hydrochloric acid. | ................................................................................................................................. ................................................................................................................................. ................................................................................................................................. |
To a 1 cm depth of FA 7 in a test-tube, add a few drops of aqueous potassium iodide, then | ................................................................................................................................. ................................................................................................................................. ................................................................................................................................. add aqueous ammonia. |
To a 1 cm depth of FA 7 in a test-tube, add a few drops of aqueous sodium hydroxide and then add aqueous ammonia dropwise, with gentle shaking, until the precipitate just dissolves, then | No observation required. |
add one spatula measure of glucose and leave to stand in the hot water bath. When you have completed this test, dispose of the solution and rinse the test-tube. | ................................................................................................................................. ................................................................................................................................. ................................................................................................................................. |
(c) Solid FA 8 contains one cation and one anion from those included in the lists on pages 14 and 15.
Carry out the following tests on FA 8. For each test record your observations.
(i) In a hard-glass test-tube heat approximately half of the FA 8, gently at first and then more strongly. Leave to cool.
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(ii) To a 2 cm depth of dilute nitric acid in a boiling tube, add the remaining FA 8.
Keep the solution obtained for tests (iii) and (iv).
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(iii) To a 1 cm depth of the solution from (ii) in a test-tube, add aqueous sodium hydroxide until no further change occurs.
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(iv) To a 1 cm depth of the solution from (ii) in a test-tube, add aqueous ammonia until no further change occurs.
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(v) Use your observations from (i) to (iv) to identify the ions present in FA 8.
Ions present ................................... and ................................... and ...................................