Edexcel Jun 2013 (R) Paper 2 Q21 (with explained solutions) |

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Edexcel Jun 2013 (R) Paper 2 Q21

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 This is a question about Group 2 compounds. Limewater is a solution of calcium hydroxide, commonly used in the identification of carbon dioxide gas. Since calcium hydroxide is only sparingly soluble in water, technicians often make the solution by adding an excess of the solid calcium hydroxide to the required volume of deionised water, shaking the container and then leaving the mixture to settle. In this way, a saturated solution is produced but it can be of variable concentration. Two students were each given a sample of limewater, from the same batch, in order to determine its concentration. Using 50.0 cm3 portions of the limewater, they carried out titrations using 0.100 mol dm3 hydrochloric acid. One of the students obtained the following results: Titration Final Volume /cm3 Initial Volume /cm3 Volume Added /cm3 Trial 14.50 0.00 14.50 28.60 14.50 14.10 42.70 28.60 14.10 The student decided that the mean titre was 14.10 cm3 The equation for the reaction is: Ca(OH)2(aq) + 2HCl(aq) o CaCl2(aq) + 2H2O(l) (a) (i) Calculate the number of moles of hydrochloric acid that reacted. (ii) Calculate the number of moles of calcium hydroxide, Ca(OH)2, that reacted with the acid. (1) (1) *P42983A0820*<br />
(iii) Calculate the concentration of Ca(OH)2, in mol dm3, in this sample of limewater. (iv) Calculate the concentration of Ca(OH)2, in g dm3, in this sample of limewater. Use the Periodic Table as a source of data. (v) This student did not include the trial value when calculating the mean titre. Explain why. (1) (2) (1)(vi) The second student obtained a different mean titre value for the experiment and thought that this difference may be due to the use of a faulty pipette. Suggest a simple method, involving distilled water and a balance, by which the accuracy of the pipette in measuring out exactly 50.0 cm3 could be checked. (2)*P42983A0920* Turn over<br />
(b) Complete the missing details from the reaction flowchart shown below, giving the condition for A and using chemical formulae for answers B, C and D. State symbols are not required. (4) CaCO3 Condition: A O2(g) D B +CaO HCl(aq) H2O(l) C (c) In certain areas of the UK, calcium and magnesium carbonates tend to be deposited as an off-white solid on the inside surface of pipes and the surface of heating elements in kettles. These deposits can be removed by treatment with a weak acid. An equation for this is shown below. CaCO3(s) + 2HA(aq) o CaA2(aq) + H2O(l) + CO2(g) State one observation, other than the solid disappearing, that would be made when the above reaction is carried out. (1)(d) The thermal stability of these carbonates depends on a combination of factors, including the size of their lattice energies. Explain why the lattice energy of calcium carbonate is less exothermic than that of magnesium carbonate. (2)10 *P42983A01020*<br />
(e) Calcium and magnesium ions can be distinguished by the use of a flame test. State the difference in the flame colour and explain how colours in a flame are produced in terms of electronic transitions. (3) CalciumMagnesium Colour produced by(Total for Question 21 = 18 marks) *P42983A01120* 11 Turn over<br />

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Section B Question Number 21 (a) (i) Question Number 21 (a) (ii) Question Number 21 (a) (iii) Acceptable Answers In (a) any units given must be correct. Penalise once only IGNORE SF except 1SF. Penalise once only If rounding is done then must be correct, penalise once only TE throughout n = (0.100 x 0.0141) = 1.41 x 10-3 / 0.00141 (mol) Reject 1 x 10-3 Acceptable Answers 7.05 x 10-4 / 0.000705 (mol) ALLOW TE = ans to (i)2 1.4 x 10-3 gives 7.0 x 10-4 0.0014 gives 0.00070 Acceptable Answers c = (7.05 x 10-40.05) = 1.41 x 10-2 / 0.0141 (mol dm-3) ALLOW TE = ans to (ii)0.05 OR ALLOW TE = ans to (ii) x 20 Reject 7.10 x 10-4 / 0.000710 Reject Mark Mark Mark 6CH0201R 1306<br />
Mark Mark Reject 1.05 Acceptable Answers Ca(OH)2 Mr = 74.1 (1) ALLOW 74 m = (1.41 x 10-2 x 74.1) = 1.04481 = 1.045 = 1.04 (g dm-3) (1) If Mr = 74 then m= 1.0434 = 1.04 (g dm-3) ALLOW TE = ans to (iii) x 74.1 ALLOW TE for second mark if ans to (iii) x incorrect Mr value OR 7.05 x 10-4 x 74.1 = 0.0522405 = 0.0522 (g) (1) (0.05220.05) = 1.044 (g dm-3) (1) Acceptable Answers Reject Not titrated accurately It is not precise Control Just its a trial Its only a rangefinder / Its a rough OR approximate titration / Its an estimation / More than 0.2 cm3 from other titres / Overshot on first titration / Not concordant ALLOW It is anomalous / It is out of range It differs / is not consistent with titrations 1 and 2 Titrations 1 and 2 are more consistent If a list of suggestions is given, a wrong cancels a right Question Number 21 (a) (iv) Question Number 21 (a) (v) 6CH0201R 1306<br />
Question Number 21 (a) (vi) Question Number 21 (b) Question Number 21 (c) Acceptable Answers Reject Pipette 50.0 cm3 (of distilled water) into weighed beaker and find the mass ALLOW fill the pipette (with water) and transfer into weighed beaker and find the mass / measure the mass of the pipetted distilled water (1) ALLOW alternative containers to beaker. Use the density of water to determine the exact volume / density of water is 1(.00)g cm-3 /check it weighs 50(.0) g (1) Stand-alone marks Transfer 50cm3 water into a beaker without reference to pipette. Approx. 50g Use of lime water Use of solution Acceptable Answers Reject A(Strong) heat / high temperature (1) BCaCl2 + H2O (Both needed) (1) CCa(OH)2 (1) DCa (1) IGNORE state symbols even if wrong IGNORE any number in front of species, e.g.O2 or 2Ca given in D Warm / Gentle heat Reflux Combustion / burnt Answers suggesting reaction with air or oxygen CaCl CaOH Ca2 Acceptable Answers Reject Bubble(s) / Fizz(ing) / Effervescence IGNORE references to colourless solution, solid disappearing and energy / temperature changes and further tests eg effect on limewater Coloured or colourless fumes Cloudy solution Just CO2 forming Just (colourless) gas forming Bubbles of any gas except CO2 Mark Mark Mark 6CH0201R 1306<br />
Question Number 21 (d) Question Number 21 (e) Mark Acceptable Answers Reject Calcium is bigger Any reference to atoms/molecules scores 0 Reference to ionization energy/weaker attraction for own electrons Method 1: Calcium is larger ion / calcium has a bigger ionic radius / or reverse argument for magnesium ion Use of the reverse argument applies throughout (1) (Distance between centres of ions increases so) weaker attraction/weaker bond between (calcium and carbonate) ions OR Shielding is greater in the calcium ion so weaker attraction (of calcium nucleus for carbonate ion) (1) Method 2: Calcium ion has a lower charge density (1) weaker attraction (between ions) (1) IGNORE references to polarization and the breaking of the covalent bonds in the carbonate ion Mark Acceptable Answers Reject Calciums flame is yellow-red /orange-red / red / brick red Magnesium has no colour (Both needed for first mark) (1) Electrons excited / promoted (by heat energy) (1) (Colour produced from) energy / light emitted as electron returns (to ground state) (1) Crimson Magnesium is white / bright Just Mg / Ca decomposes Electrons escape the orbitals Total for Question 21 = 18 Marks 6CH0201R 1306<br />

Question 21 22 23

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