Edexcel Jun 2013 Paper 1 Q21 (with explained solutions) |

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Edexcel Jun 2013 Paper 1 Q21

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21 Sodium hydrogencarbonate decomposes on heating to form sodium carbonate. It is difficult to measure the enthalpy change of this reaction directly. 2NaHCO3(s) o Na2CO3(s) + CO2(g) + H2O(l) One method of determining this enthalpy change is to react known amounts of sodium hydrogencarbonate and sodium carbonate, separately, with excess dilute hydrochloric acid. (a) 0.010 mol of solid sodium hydrogencarbonate was added to 25 cm3 of dilute hydrochloric acid. A temperature rise of 11 C was measured using a thermometer graduated at 1 C intervals. (i) Calculate the heat energy produced by this reaction using the equation: Energy transferred in joules = mass4.18change in temperature (ii) Calculate the standard enthalpy change for the reaction when one mole of sodium hydrogencarbonate reacts with hydrochloric acid. Remember to include a sign and units with your answer which should be given to three significant figures. (1) (2) *P41649A01724* 17 Turn over<br />
*(b) The standard enthalpy change for the reaction between sodium carbonate and dilute hydrochloric acid is found by a similar method to be H 9 = 321.6 kJ mol1 Complete the Hess energy cycle below by adding the missing arrow and entities. Use it to calculate the standard enthalpy change for the decomposition of two moles of sodium hydrogencarbonate as in the equation below. Remember to show your reasoning clearly. (5) 2NaHCO3(s) o Na2CO3(s) + CO2(g) + H2O(l) 2HCl(aq)( ) +( ) +( ) 18 *P41649A01824*<br />
(c) The uncertainty for each thermometer reading is0.5 C. Calculate the percentage error in the temperature rise of 11 C. (d) Sodium hydrogencarbonate is used in cooking. Suggest what it is used for and how it works. (1) (2)(Total for Question 21 = 11 marks) *P41649A01924* 19 Turn over<br />
BLANK PAGE 20 *P41649A02024*<br />

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Question Number 21(a)(i) 25 x 4.18 x 11 = 1149.5 (J) Acceptable Answers ALLOW 1.1495 kJ Otherwise ignore units even if incorrect IGNORE sign IGNORE SF except one or two SF Acceptable Answers -115 kJ mol-1 ALLOW -115000 J mol-1 Question Number 21(a)(ii) Sign with correct value (1) Units and three significant figures (1) Mark independently ALLOW TE from (i) -114 kJ mol-1 (rounding error) scores 1 -115.0 kJ mol-1 scores 1 Values of -4600 and -3.86 are quite common ALLOW K and j in any case in units Mark Mark Reject 1149.5 kJ Reject J or kJ alone<br />
Question Number 21(b) Acceptable Answers 2NaHCO3(s)(cid:31)Na2CO3(s)+CO2(g)+H2O(l) 2HCl(aq) (2HCl(aq)) 2NaCl(aq) + 2CO2(g) + 2H2O(l) First mark Arrow from products in top line to lower line and correct entities (1) NaCl + CO2 + H2O Second mark 2NaCl(aq) + 2CO2(g) + 2H2O(l) Correct state symbols and balancing (1) Ho = +91.6 OR +91.7 (kJ mol-1) ALLOW no positive sign only if correct Working with correct signs given (3) OR Third mark Correct use of Hesss Law (in numbers or symbols) consistent with arrow direction (1) Fourth mark 2x(-115) = Ho - 321.6 Correct multiples and numbers (1) ALLOW 2 x any number (including -4600 and -3.86) except 2 x +/- 321.6 Notice Third and Fourth marks can be scored by Ho = 2(115)(-321.6) Reject Mark<br />
Fifth mark Ho = 2(115)(-321.6) = +91.6 (kJ mol-1) OR Ho = 2(114.95)(-321.6) = +91.7 (kJ mol-1) Correct value for their calculation with correct sign IGNORE SF except 1 ALLOW no positive sign only if correct working with correct signs given (1) Omitting 2x gives +206.6 (could get 4 marks) -4600 gives -598.4 -3.86 gives +313.88 Acceptable Answers (() 0.5 x 2 x 100 /11 ) = ()9.09 (%) ALLOW at 9.0909/9.091/9.1 and 9 Reject 9.10/9.0 Mark Question Number 21(c)<br />
Mark Acceptable Answers Reject First mark It is used as a raising agent / self raising flour / baking soda / baking powder OR Causes cakes / (soda) bread to rise / expand. (1) Second mark Carbon dioxide (released on heating causes cakes / bread to rise) OR It reacts with acid to form carbon dioxide (in baking powder) providing bread /cake etc is mentioned (1) ALLOW Used in cooking green vegetables To keep green colour To make pastry rise Bicarbonate of soda Gas Air Neutralizing acid foods Total for Question 21 = 11 Marks Question Number 21(d)<br />

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