OCR A Jun 2017 Paper 3 Q2 (with explained solutions) |

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OCR A Jun 2017 Paper 3 Q2

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2 A student plans to determine the enthalpy change of reaction 3.1 shown below. Na2O(s) + 2HCl(aq) 2NaCl(aq) + H2O(l) reaction 3.1 This enthalpy change can be determined indirectly using Hess Law from the enthalpy changes of reaction 3.2 and reaction 3.3 shown below. Na2O(s) + H2O(l) HCl(aq) + NaOH(aq) 2NaOH(aq) NaCl(aq) + H2O(l) rH = 57.6 kJ mol1 reaction 3.2 reaction 3.3 The student will determine the enthalpy change of reaction 3.2 as outlined below.Weigh a bottle containing Na2O(s) and weigh a polystyrene cup.Weigh the empty bottle and weigh the polystyrene cup with the final solution. Add about 25 cm3 of water to the polystyrene cup and measure its temperature. Add the Na2O(s), stir the mixture, and measure the maximum temperature reached. Mass readings Mass of bottle + Na2O(s) Mass of empty bottle Mass of empty polystyrene cup Mass of polystyrene cup + final solution Temperature readings Initial temperature of water Maximum temperature of final solution = 16.58 g = 15.34 g = 21.58 g = 47.33 g = 20.5 C = 55.5 C The density and specific heat capacity, c, of the solution are the same as for water.<br />
(a)* Calculate the enthalpy change of reaction 3.2 and the enthalpy change of reaction 3.1. Show all your working.[6] (b) The uncertainty in each temperature reading is 0.1 C. The uncertainty in each mass reading is 0.005 g. Determine whether the mass of Na2O or the temperature change has the greater percentage uncertainty. Show all your working.[2]<br />
(c) Suggest a modification to this experiment, using the same apparatus, which would reduce the percentage errors in the measurements. Explain your reasoning.[2] (d) Sodium oxide, Na2O, can be prepared by the redox reaction of NaNO2 and sodium metal. Nitrogen gas is also formed. (i) What is the systematic name for NaNO2?[1] (ii) Using oxidation numbers, with signs, show the element that is oxidised and the element that is reduced in this reaction. Element oxidisedOxidation number change fromtoElement reducedOxidation number change fromto(iii) Construct the equation for this reaction. [2] Equation[1]<br />

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H432/03 Question (a) Mark Scheme June 2017 Answer Please refer to the marking instructions on page 5 of this mark scheme for guidance on how to mark this question. Level 3 (56 marks) A comprehensive conclusion, using all quantitative data, to calculate the energy change and H values for reactions 3.1 and 3.2 AND linking H data using Hess Law There is a well-developed line of reasoning which is clear and logically structured. The working throughout is clearly shown. All values calculated with reasonable numbers of SF and correct signs mostly shown, allowing for ECF. Level 2 (34 marks) Attempts to describe all three scientific points but explanations may be incomplete. OR Explains two scientific points thoroughly with few omissions. There is a line of reasoning with some logical structure. There may be minor errors in energy change and errors in the calculations of H for reaction 3.1 or reaction 3.2. Level 1 (12 marks) Processes raw mass and temperature data and obtains a calculated value for the energy change using mcT OR attempts to obtain values for two scientific points but explanations may be incomplete There is an attempt at a logical structure with a line of reasoning to obtain a value for energy change. There may be minor errors in calculation of energy change. 0 marksNo response or no response worthy of credit. Marks Guidance Indicative scientific points may include: 1. Masses and T from raw resultsm(Na2O) = 1.24 (g)m(solution) = 25.75 (g)T = 35.0 (C) Energy change from mcTenergy released in J OR kJ = 25.754.1835.0 = 3767 (J) OR 3.767 (kJ) (3.767225 unrounded) ---------------------------------------------- 2. rH for reaction 3.2n(Na2O) = = 0.0200 (mol) 1.24 62.0rH value= 188 (kJ mol1) 3767 0.0200 (188.36125 unrounded) ---------------------------------------------- 3. rH for reaction 3.1H value for reaction 3.1 clearly linked to H for reaction 3.2 and reaction 3.3 in energy cycle or an expression: H(3.1) = H(3.2) + 2H(3.3)H(3.1) = 188 + (257.6) = 188115.2 = 303(.2) (kJ mol1) (303.56125 unrounded) Note Throughout, ALLOW ECF from previous value ALLOW omission of trailing zeroes ------------------------------------------------------------------<br />
H432/03 Question Mark Scheme June 2017 Answer Marks (b) % uncertainties to at least 1 SF, rounded or truncated ------------------------------------------------------------------------------ ONE correct % uncertaintyBOTH correct % uncertainties------------------------------------------------------------------------------ mass:100 = 0.8/0.81 OR 0.80 (truncated) 0.0052 1.24 0.12 35.0 T:100 = 0.6 / 0.57 (%)(c) Calculator values: mass: T: 0.8064516129 0.5714285714 ALLOW uncertainty OR error throughout Greater mass of Na2O OR more Na2OFor mass, ALLOW amount/moles/quantity larger T OR reduces % uncertainty in TGuidance ALLOW error for uncertainty ---------------------------------------------------------------- ALLOW ECF from mass and T in 2(a) IGNORE % uncertainty of mass of solution --------------------------------------------------------------- ALLOW one mark for:2 calculations with both 2 factors missing i.e. mass 0.3% AND T 0.4%Not converting to %s using 2 factors i.e. 0.008 AND 0.006 ALLOW up to 2 marks based on a single mass measurement: one mass measurement OR measure mass directlye.g. tare balance % uncertainty reduced by half------------------------------------------------------------ IGNORErepeat and take average read to more figures (same apparatus) increase volume (reduces mass error but increases T error)use a cooling curveuse a lid 10<br />
Mark Scheme June 2017 Answer Marks ALLOW sodium nitrite OR sodium nitrite(III) Guidance ALLOW 1+ for +1 and 3+ for +3 ALLOW N2 for nitrogen ALLOW 1 mark for elements AND all oxidation numbers correct, but N on oxidised line and Na on reduced line + is required in +3 and +1 oxidation numbers ALLOW multiples, e.g. NaNO2 + 3Na2Na2O + N2 H432/03 Question (d) (i) sodium nitrate(III) (d) (ii) Sodium/Na oxidised from 0 to +1Nitrogen/N reduced from +3 to 0(d) (iii ) 2NaNO2 + 6Na4Na2O + N2IGNORE state symbols Total 14 11<br />

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