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(a) Reaction 1 Enthalpy change of a weak acid
FB 1 is ethanoic acid, CH$_3$COOH.
FB 2 is magnesium, Mg.
Method 1
• Weigh the strip of magnesium and record the balance reading in the space below.
• Support the plastic cup in the 250 cm$^3$ beaker.
• Coil the magnesium ribbon loosely so that it fits into the bottom of the plastic cup and then remove the ribbon.
• Use the measuring cylinder to transfer 25 cm$^3$ of the acid, FB 1, into the plastic cup.
• Place the thermometer in the acid and read the initial temperature. This is the temperature at time zero ($t = 0$).
• Start timing and do not stop the clock until the whole experiment has been completed.
• Read the temperature of the acid every half minute for two minutes.
• At time $t = 2\frac{1}{2}$ minutes drop the magnesium, FB 2, into the acid and stir the mixture.
• Measure and record, in the table below, the temperature of the mixture at $t = 3$ minutes and then every half minute until $t = 10$ minutes. Stir the mixture continuously between thermometer readings.
• Rinse the plastic cup for use in Method 2. Shake to remove excess water.
Results
Mass of magnesium
Temperature
[Table_1]
(b) Plot a graph of temperature on the y-axis against time on the x-axis on the grid below.
The scale for temperature should extend 10°C above your highest recorded temperature.
You will use this graph to determine the theoretical maximum temperature rise at 2\frac{1}{2} minutes.
[Graph_1]
Draw two lines of best fit through the points on your graph. The first line should be for the temperature before adding FB 2 and the second for the cooling of the mixture once the reaction is complete.
Extrapolate the two lines to 2\frac{1}{2} minutes, draw a vertical line between the two and determine the theoretical rise in temperature at this time.
Theoretical rise in temperature at 2\frac{1}{2} minutes = ............................. °C
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
Magnesium reacts with ethanoic acid according to the equation shown.
Mg(s) + 2CH$_3$COOH(aq) $
ightarrow$ Mg(CH$_3$COO)$_2$(aq) + H$_2$(g)
(i) Use your answer to (b) to calculate the heat energy, in joules, given out when FB 2 is added to the acid.
[Assume 4.2 J of heat energy raises the temperature of 1.0 cm$^3$ of the mixture by 1.0°C.]
Heat energy evolved = ............................. J
(ii) Use the Periodic Table on page 16 and your answer to (i) to calculate the enthalpy change, in kJ mol$^{-1}$, when 1 mole of FB 2, Mg, reacts with ethanoic acid.
Enthalpy change, $\Delta H = ...... ........................ kJ \cdot mol^{-1}$
(d) Reaction 2 Enthalpy change of a strong acid.
FB 3 is hydrochloric acid, HCl.
The tube labelled FB 4 contains two strips of magnesium, Mg. One strip is longer than the other strip.
Method 2
Read the whole method before starting any practical work and prepare a table for your results in the space below.
• Weigh the longer strip of magnesium and record the balance reading.
• Support the plastic cup in the 250 cm$^3$ beaker.
• Coil the magnesium ribbon loosely so that it fits into the bottom of the plastic cup and then remove the ribbon.
• Use the measuring cylinder to transfer 25 cm$^3$ of the acid, FB 3, into the plastic cup.
• Place the thermometer in the acid and measure and record the initial temperature of the acid.
• Add the piece of magnesium into the acid in the cup.
• Stir constantly until the maximum temperature is reached.
• Measure and record the maximum temperature.
• Rinse the plastic cup for use in the next experiment.
• Calculate and record the temperature rise.
• Repeat this experiment using the shorter strip of magnesium and record all results.
(e) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
Use your results from (d) for the longer strip of magnesium and the Periodic Table on page 16 to calculate the enthalpy change, in kJ mol$^{-1}$, when 1 mole of FB 4, Mg, reacts with hydrochloric acid.
[Assume 4.2 J of heat energy changes the temperature of 1.0 cm$^3$ of the mixture by 1.0°C.]
Enthalpy change, $\Delta H = ...... ........................ kJ \cdot mol^{-1}$
(f) (i) A student suggested that the experiment carried out in (d) could be improved by using a catalyst.
Would the use of a catalyst improve the accuracy of the results in this experiment? Give a reason for your answer.
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(ii) Another student could not find the hydrochloric acid, FB 3, so used sulfuric acid, H$_2$SO$_4$, instead. He used the same volume and the same concentration as the hydrochloric acid in FB 3.
What effect would this change have on the temperature rise in the experiment? Give a reason for your answer.
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(g) Ethanoic acid is a weak acid. It is partially ionised in aqueous solution.
CH$_3$COOH(aq) $\rightleftharpoons$ CH$_3$COO$^-$(aq) + H$^+$(aq)
You are to determine the energy needed to cause the molecules of ethanoic acid to ionise completely.
CH$_3$COOH(aq) $\rightarrow$ CH$_3$COO$^-$(aq) + H$^+$(aq)
Hydrochloric acid is a strong acid; it is fully ionised in aqueous solution.
The values for the enthalpy changes you obtained in (c)(ii) and (e) could be used to calculate the energy change for the ionisation but more accurate experiments give the results in Table 1.
[Table_2]
(i) Write the ionic equation, including state symbols, for the reaction of magnesium with aqueous hydrochloric acid.
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(ii) Use the data in Table 1 to calculate the enthalpy change for the ionisation of ethanoic acid.
CH$_3$COOH(aq) $\rightarrow$ CH$_3$COO$^-$(aq) + H$^+$(aq)
Show clearly how you obtained your answer.
Enthalpy change, $\Delta H = ...... ........................ kJ \cdot mol^{-1}$
(h) The experiment in (a) was repeated using trichloroethanoic acid instead of ethanoic acid.
Mg(s) + 2CCl$_3$COOH(aq) $\rightarrow$ Mg(CCl$_3$COO)$_2$(aq) + H$_2$(g) reaction 3
Trichloroethanoic acid, CCl$_3$COOH, is a weak acid that is however stronger than ethanoic acid.
The enthalpy change for reaction 3 is between the two values given in Table 1.
[Table_3]
(i) Explain why the enthalpy change for reaction 3 is more exothermic than the enthalpy change for reaction 1.
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(ii) Explain why the enthalpy change for reaction 3 is less exothermic than the enthalpy change for reaction 2.
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(a) (i) FB 5, FB 6 and FB 7 each contain one anion and one cation.
Carry out the following tests and record your observations.
[Table_1]
To a 1 cm depth of solution in a test-tube, add a few drops of aqueous silver nitrate, then
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add aqueous ammonia.
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To a 1 cm depth of solution in a test-tube, add a few drops of aqueous barium nitrate, or barium chloride, then
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add dilute nitric acid.
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To a 1 cm depth of solution in a test-tube, add a spatula measure of solid sodium carbonate.
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(ii) What cation is present in FB 5, FB 6 and FB 7?
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(iii) Suggest another test that you could carry out to confirm the presence of the cation you identified in (ii).
Carry out this test on one of FB 5, FB 6 or FB 7 and record your observation.
test ..................................................................................................
observation ...................................................................................
(iv) Complete the table to identify, as far as possible, the anions present in FB 5, FB 6 and FB 7. If you are not able to identify the anion from the tests you carried out in (i), write 'unknown'.
[Table_2]
(v) For any one anion that you were unable to identify in (iv) you are to devise a test or tests that will enable you to identify it. You can assume that it is one of the anions listed in the Qualitative Analysis Notes.
Carry out the test(s), record the observation(s) you obtained and identify the unknown anion.
test(s) ..................................................................................................
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observation(s) ....................................................................................
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Anion present in ................................... is ................................. .
(b) FB 8 is an aqueous solution of a mixture containing two anions and two cations.
Carry out the following tests and record your observations.
[Table_3]
To a 1 cm depth of FB 8 in a test-tube, add a 1 cm depth of dilute hydrochloric acid, then
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add a few drops of hydrogen peroxide, then
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add a few drops of starch.
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To a 1 cm depth of FB 8 in a test-tube, add aqueous sodium hydroxide.
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To a 1 cm depth of FB 8 in a test-tube, add a 3 cm depth of aqueous copper(II) sulfate, then
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add a 1 cm depth of dilute hydrochloric acid, then
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add aqueous sodium thiosulfate.
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From these observations, identify two ions present in FB 8.
ions present in FB 8 ...................................................... and ......................................................
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