CAIE Chemistry (9701)

Calcium carbonate, CaCO

, decomposes when heated, as shown. CaCO

CaO(s) + CO

, for the thermal decomposition of CaCO

cannot be measured directly . Instead, a procedure involving two experiments is used. In each ex periment, the enthalpy change of a different reaction is determined. experiment 1 CaCO

+ 2HCl (aq) CaCl

+ H O(l) + CO

experiment 2 CaO

+ 2HCl (aq) CaCl

+ H O

Experiment 1 step 1 Weigh a 0.0500 mol sample of powdered CaCO

step 2 Transfer 50.00 cm , an excess, of 2.00 mol dm hydrochloric acid, HCl (aq), into a small glass beaker. step 3 Start a timer and measure the temperature of the HCl (aq) in the beaker every 30 seconds for 2 minutes. step 4 After 3 minutes add the sample of CaCO

to the HCl (aq) in the beaker . Continue measuring the temperature of the reaction mixture every 30 seconds for a further 5 minutes. Experiment 2 Repeat experiment 1 using calcium oxide, CaO

, instead of CaCO

Suggest why the enthalpy change of reaction for the thermal decomposition of calcium carbonate cannot be measured directly .

(i) Calculate the mass, in g, of CaCO

to be weighed using a two-decimal-place balance in step 1. mass of CaCO

= g [1] * 0019654970002 *

Outline how a student should weigh by dif ference using a weighing boat in order to determine the exact mass of CaCO

added to HCl (aq) in the beaker . Draw a results table, with appropriate headings, ready for the student to complete.

Identify which piece of apparatus should be used to measure the volume of HCl (aq) in step 2 and give a reason for your choice.

Without making any changes to the apparatus, suggest an instruction to be added to step 3 and step 4 to make the experiment more accurate.

A student carries out experiment 1 and obtains the results given in Table 1.1. Table 1.1 time / minutes 0.51.01.52.02.53.03.54.04.55.05.56.0 temperature / °C 19.019.019.019.019.0 27.536.034.532.532.031.0 time / minutes 6.57.07.58.0 temperature / °C 29.028.026.025.5

Plot a graph on the grid in Fig. 1.1 to show the relationship between temperature and time. Use a cross (×) to plot each data point. The points and line of best fit for the data before 3 minutes have been drawn for you. Draw a line of best fit for the data after 3 minutes that will enable you to determine the theoretical temperature increase at 3.0 minutes. 0 1 2 3 4 5 6 7 8 temperature / °C time / minutes

Use your graph to determine the theoretical temperature increase at 3.0 minutes. theoretical temperature increase at 3.0 minutes = °C [1] * 0019654970004 *

Suggest why the temperature measured at 3.5 minutes is lower than the temperature measured at 4.0 minutes.

A student carries out experiment 2 and determines a temperature increase of 62.0 °C. The heat released by the reaction, q, is given by: where m is the mass of HCl (aq). Assume that 1.00 cm of HCl (aq) has a mass of 1.00 g and that the specific heat capacity of the solution, c, is 4.18 J g in kJ mol q = J = kJ mol [2] for the thermal decomposition of CaCO = ±84 kJ mol (If you were unable to calculate a final answer in

, assume a value of 179 kJ mol . This is not the correct answer and the sign has been omitted.) CaCO

+ 2HCl (aq) CaO

+ 2HCl (aq) + CO

+ H O(l) + CO

= kJ mol

Identify the main weakness of the experimental procedure and suggest one improvement to overcome this weakness. The main weakness is not the type of thermometer used. main weakness improvement [2] [Total: 15] * 0019654970005 *

This question is about an experiment to investigate the ef fect of temperature on the equilibrium constant, K , of the reaction shown.

+ SCN

FeSCN

The data collected is used to determine the value of the enthalpy change of the reaction. To set up the equilibrium, aqueous iron(III) nitrate, Fe(NO

, is mixed with aqueous potassium thiocyanate, KSCN(aq). Aqueous iron thiocyanate ions, FeSCN

, have a red colour. A colorimeter is used to measure the absorbance of the reaction mixture. A calibration graph can then be used to determine the concentration of FeSCN

in the reaction mixture. Table 2.1 shows the solutions for the experiments. Table 2.1 solution ion concentration / mol dm A SCN

0.00920 B SCN

0.00200 C Fe

0.00200 D Fe

0.500

Describe how you would prepare 100.0 cm of solution B from solution A. Include a calculation of the volume of solution A required for the preparation of solution B. Give the name and capacity of any key apparatus that should be used. W rite your answer as a series of numbered steps.

Before starting the experiment, solutions B and D are used to produce a calibration graph. Known volumes of each solution are added together and the absorbance for each mixture is recorded. The calibration graph is shown in Fig. 2.1. The concentration of solution D is much greater than the concentration of solution B in order that solution D is in excess. Suggest a reason why solution D is in ex cess. [FeSCN ] / 10 mol dm 0.2 0.4 0.6 0.8 1.0 1.2 0 2 4 6 8 10 12 14 relative absorbance Fig. 2.1 * 0019654970107 *

The following experimental procedure is used. step 1 Half-fill a large beaker with water at room temperature (25 °C). step 2 Transfer about 40 cm of solution B into a boiling tube and place the boiling tube in the beaker of water. step 3 Transfer 5.00 cm of solution C into a test-tube and place the test-tube in the beaker of water. step 4 Wait for 10 minutes. step 5 Transfer 5.00 cm of solution B from the boiling tube to the test-tube containing solution C. Stir the mixture in the test-tube and record the temperature of the mixture. step 6 Measure the absorbance of the mixture in the test-tube using the colorimeter . Change the temperature of the water in the beaker and repeat steps 3 to 6 for dif ferent temperatures.

Identify the dependent variable.

Describe how you would adjust the temperature of the water in the large beaker to obtain a temperature of 10 °C.

A student obtains the results given in Table 2.2. Table 2.2 1 2 3 4 temperature / °C relative absorbance[FeSCN ] / 10 mol dm 25 0.60 55 0.42 The value of the equilibrium constant, K , can be determined using equation 1. equation 1 K (0.0010 ± x) x is the value of [FeSCN ] in mol dm

Use the calibration graph in Fig. 2.1 to complete column 3 in Table 2.2. Record values to one decimal place.

Use equation 1 to complete column 4 in Table 2.2. Record values to the nearest whole number. [1] * 0019654970108 *, ,

Another student does the same experiment for seven dif ferent temperatures, plots a graph and draws the line of best fit, as shown in Fig. 2.2. 0.0028 1.7 1.8 1.9 2.0 2.1 2.2 0.0030 0.0032 0.0034 0.0036 0.0038 log K/ K Theory predicts that the relationship between K and T is given by equation 2. equation 2 log K 2.303RT + constant

Explain why the graph supports the relationship between K and T given in equation 2.

Circle the point on the graph in Fig. 2.2 that you consider to be most anomalous. There were no errors in the measurements in the experiment. A student correctly suggests that the anomaly was caused because the absorbance was lower than expected by the line of best fit. Suggest why the absorbance was lower than expected.

Determine the gradient of the line of best fit in Fig. 2.2. State the coordinates of both points you use in your calculation. These must be selected from the line of best fit. Give the gradient to three significant figures. coordinates 1 coordinates 2 gradient = K

Use the gradient calculated in

, then use the value 635 K. This is not the correct answer.) [2] [Total: 15] * 0019654970110 * Important values, constants and standards molar gas constant R = 8.31 J Kmol Faraday constant F = 9.65 × 10 C mol Avogadro constant L = 6.022 × 10mol electronic charge e = ±1.60 × 10±19 molar volume of gas V = 22.4 dmmol at s.t.p. (101 kPa and 273 K) = 24.0 dmmol at room conditions ionic product of water K = 1.00 × 10±14mol (at 298 K (25 °C)) speci®c heat capacity of waterc = 4.18 kJ kg (4.18 J g ) * 0019654970111 * Group The Periodic Ta ble of Elements hydrogen 1.0 helium 4.0 12 13 14 15 16 17 18 3456789 10 11 12 lithium 6.9 beryllium 9.0 atomic number atomic symbolKey name relative atomic mass sodium 23.0 magnesium 24.3 potassium 39.1 calcium 40.1 rubidium 85.5 strontium 87.6 caesium 132.9 barium 137.3 francium radium boron 10.8 aluminium 27.0 gallium 69.7 indium 114.8 thallium 204.4 carbon 12.0 silicon 28.1 germanium 72.6tin 118.7 lead 207.2 titanium 47.9 zirconium 91.2 hafnium 178.5 rutherfordium vanadium 50.9 niobium 92.9 tantalum 180.9 dubnium chromium 52.0 molybdenum 95.9 tungsten 183.8 seaborgium manganese 54.9 technetium rhenium 186.2 bohrium iron 55.8 ruthenium 101.1 osmium 190.2 hassium cobalt 58.9 rhodium 102.9 iridium 192.2 meitnerium nickel 58.7 palladium 106.4 platinum 195.1 darmstadtium copper 63.5 silver 107.9 gold 197.0 roentgenium zinc 65.4 cadmium 112.4 mercury 200.6 copernicium flerovium livermorium nitrogen 14.0 phosphorus 31.0 arsenic 74.9 antimony 121.8 bismuth 209.0 oxygen 16.0 sulfur 32.1 selenium 79.0 tellurium 127.6 polonium fluorine 19.0 chlorine 35.5 bromine 79.9 iodine 126.9 neon 20.2 argon 39.9 krypton 83.8 xenon 131.3 radon nihonium moscovium tennessine oganesson scandium 45.0 yttrium 88.9 57±71 lanthanoids 89±103 actinoids lanthanum 138.9 lanthanoids actinoids actinium cerium 140.1 thorium 232.0 praseodymium 140.9 protactinium 231.0 neodymium 144.2 uranium 238.0 promethium neptunium samarium 150.4 plutonium europium 152.0 americium gadolinium 157.3 curium terbium 158.9 berkelium dysprosium 162.5 californium holmium 164.9 einsteinium erbium 167.3 fermium thulium 168.9 mendelevium ytterbium 173.1 nobelium lutetium 175.0 lawrencium To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge Assessment International Education Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cambridgeinternational.org after the live examination series. * 0019654970112 *