Edexcel Jun 2010 Paper 4 Q21 (with explained solutions) |

Skip to content

Edexcel Jun 2010 Paper 4 Q21

Answers available below

PMT (2) (1) SECTION C Answer ALL the questions. Write your answers in the spaces provided. 21 (a) Crystals of hydrated cobalt(II) chloride, CoCl2.6H2O, lose water when they are heated, forming anhydrous cobalt(II) chloride, CoCl2. CoCl2.6H2O(s)CoCl2(s) + 6H2O(l) (i) Calculate the entropy change of the system, Ssystem, at 298 K. Include a sign and units in your answer. You will need to refer to your data booklet. (ii) Explain whether the sign of your answer to (a)(i) is as expected from the equation for the reaction.(iii) The standard enthalpy change for the reaction, H , is +88.1kJ mol1. Calculate the entropy change in the surroundings, Sreaction. Include a sign and units in your answer. surroundings, at 298 K for this (iv) Calculate the total entropy change, Stotal, at 298 K for the reaction. 20 *N36506A02024* (2) (1)<br />
PMT (v) Does your answer to (a)(iv) indicate whether hydrated cobalt(II) chloride can be stored at 298 K without decomposition? Explain your answer. (1)(b) A student attempted to measure the enthalpy change of solution of anhydrous cobalt(II) chloride by adding 2.00 g of cobalt(II) chloride to 50.0 cm3 of water in a well-insulated container. A temperature rise of 1.5 C was recorded. The student used a balance which reads to 0.01g, a 50.0 cm3 pipette, and a thermometer which can be read to 0.25 C. (i) Which measuring instrument should be changed to give a result which is closer to the accepted value? Justify your answer. (2)(ii) Suggest ONE other change the student could make to give a result which is closer to the accepted value. Justify your suggestion. (2)*N36506A02124* 21 Turn over<br />
PMT *(c) The lattice energies of magnesium chloride, MgCl2, calcium chloride, CaCl2, and strontium chloride, SrCl2 are shown in the table below. Chloride Lattice energy/kJ mol1 MgCl2 CaCl2 SrCl2 2526 2258 2156 (i) Use data on ionic radii, from your data booklet, to explain the trend in these values. Estimate a value for the lattice energy of cobalt(II) chloride, giving ONE piece of data to justify your estimate. (4)22 *N36506A02224*<br />
PMT (ii) Explain how lattice energy values, together with other data, can be used to predict the solubility of ionic compounds. (3)*(d) Cobalt forms another chloride, CoCl3, but scientists predict that MgCl3 cannot be made. Suggest a reason for this. You should consider the enthalpy changes in the Born-Haber cycle, which provide evidence about why cobalt(III) chloride is known but magnesium(III) chloride is not. (2)(Total for Question 21 = 20 marks) TOTAL FOR SECTION C = 20 MARKS TOTAL FOR PAPER = 90 MARKS *N36506A02324* 23<br />

Show answer

Reject Mark 185 Acceptable Answers system = 109.2 + (6x 69.9)343 (1) So =(+)185.6(J mol1 K-1) / (+)186 (J mol1 K-1) (1) OR (+)0.186 (kJ mol1 K-1) (2) IGNORE units even if incorrect correct answer with no working scores 2 Value using 1 for H2O = -163.9 scores 1 Use of value for H2O(g) (188.7) gives 898.4 (J mol1 K-1) (1) correct value with incorrect sign scores 1 Acceptable Answers Reject Mark Yes as (solid and) liquid forms (from solid) / number of moles increases OR If So system in (i) is negative the sign is not as expected as liquid forms from solid / number of moles increases Disorder increases, with no ref to liquid or number of moles Question Number 21 (a)(i) Question Number 21 (a)(ii)<br />
Acceptable Answers Reject Mark surroundings = 88.1 x (1000 ) (1) First mark So 298 Second mark = -295.6375 = -295.6 J mol-1 K-1 (1) correct units must be shown but order not important OR -0.2956 kJ mol-1 K-1 (1) correct units must be shown but order not important correct answer with or without working and correct units scores (2) ignore sf except 1 correct value with positive sign scores 1 Acceptable Answers Reject Mark Answers where values in J are added to kJ (185.6295.6) = -110 (J mol-1 K-1 ) OR -0.110 ( kJ mol-1 K-1) could use 186 or 296 etc TE from (a)(i) and (iii) (+)602.8 (J mol-1 K-1 ) if value for 6H2O(g) was used in (a) (i) -459.5 (J mol-1 K-1 ) if value for one H2O was used in (a) (i) Question Number 21 (a)(iii) Question Number 21 (a)(iv)<br />
Question Number 21 (a)(v) Question Number 21 (b)(i) Acceptable Answers Reject Mark total is negative / Reactions are only Decomposition (at 298 K) will not occur as So spontaneous if total entropy change is positive / decomposition not thermodynamically feasible / (hydrated cobalt chloride) is thermodynamically stable TE if answer to (a)(iv) is positive showing decomposition (at 298 K) may occur OR Positive total entropy change doesnt indicate rate of reaction Acceptable Answers Reject Mark First mark Thermometer (1) Second mark (dependent on first) depends on choosing thermometer as temperature change is small / (%) error in balance smaller than for temperature reading (%) error in pipette smaller than for temperature reading (can be shown by calculation) / as scale with greater degree of precision needed / scale with more graduations needed (1) IGNORE any references to accurate thermometer<br />
Acceptable Answers Reject Mark Use more cobalt chloride / less water (1) To increase temperature rise (1) Mark independently Just use more reactants Use more cobalt chloride and more water repeat expt add a lid or extra insulation to beaker use distilled water Acceptable Answers Reject Mark Radius (of cation) increases (down group) OR any two values of radius: Mg 2+ = 0.072, Ca 2+ = 0.100 / Sr 2+ = 0.113 (nm) data may be shown beside the table (1) Radius Co2+ = 0.065 nm OR Co2+ radius smaller than other ions (1) Data on EITHER Co2+ OR data showing increase in radius down Group II required for BOTH of first two marks Force of attraction between ions decreases (as radius of ions increases) / charge density of ions decreases / negative ion can come closer to nucleus of positive ion (1) ALLOW weaker ionic bonds Predict lattice energy 2550 to -2900 (kJ mol-1) (1) IGNORE sign Atomic radii unless ionic radii also given Radius of cobalt chloride Polarising power decreases Question Number 21 (b)(ii) Question Number 21 (c)(i) QWC<br />
Question Number 21 (c)(ii) QWC Question Number 21 (d) QWC Acceptable Answers Reject Mark First mark Reference to enthalpy of hydration (may be in equation Hsolution = -LE + Hhydration) (1) Second mark Solubility depends on relative size of lattice energy and enthalpy of hydration (1) Third mark EITHER Solubility more likely if Hsolution is negative OR (If Hsolution is positive,) may / will dissolve if Stotal is positive ACCEPT solvation instead of hydration Acceptable Answers Reject Mark First mark Third ionization energy high(er) for Mg / Mg = 7733 kJ mol1, (third ionization energy for Co = 3232 kJ mol-1) (1) Second mark (Third ionization energy for Mg is high) because the electron is being removed from an inner shell / full shell / 2p level / 2p orbital (1) OR Not compensated by higher lattice energy for Mg3+ (and so Hformation of MgCl3 would be highly endothermic) (1)<br />

Question 17 18 19 20 21

About us | Need Help? | Feedback

Proudly powered by WordPress | Theme: Sydney by aThemes.