Edexcel Jun 2015 (IAL) Paper 3 Q3 (with explained solutions) |

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Edexcel Jun 2015 (IAL) Paper 3 Q3

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3 This question is about enthalpy changes which occur on dissolving different substances. (a) The enthalpy change which occurs when solid ammonium chloride, NH4Cl, dissolves in water was found using the method below. 25.0 cm3 of water was measured using a burette and put into a small beaker. The temperature of the water was measured. 5.00 g of powdered ammonium chloride was added to the water, the mixture was stirred continuously and the lowest temperature of the resulting solution was recorded. Results: Initial temperature of water = 22.0(cid:113)C Lowest recorded temperature = 11.5(cid:113)C (i) Calculate the energy transferred when 5.00 g of ammonium chloride dissolves in 25.0 cm3 of water. Hence calculate the enthalpy change, (cid:168)Hsolution, which occurs when 1 mol of ammonium chloride dissolves in water. Give your final answer to three significant figures and include a sign and units. Use the equation: Energy transferred (J) = mass of water4.18temperature change. The density of water is 1.00 g cm3 (3) *P44883A0616*<br />
(ii) The thermometer used in this experiment gave a total uncertainty in the temperature measurement of just under 5%. The mass of ammonium chloride was measured using a balance which had an uncertainty of 0.005 g in each reading. Show by calculation that the uncertainty of the result of the experiment would not be improved significantly if a more precise balance was used. (2) *P44883A0716* Turn over<br />
(b) In order to determine a more accurate value for the temperature change, the experiment in (a) was repeated with some modifications to the procedure. The temperature of the water was measured as a stop clock was started, and again after one minute and two minutes. Three minutes after starting the clock, the ammonium chloride was added to the water. The temperature was then read each minute from the fourth to the tenth minute, while stirring the mixture continuously. (i) On the grid below, mark where you would expect to find the points when the temperature measurements are plotted against time. The initial temperature has been plotted for you. You are not expected to plot the position of the points accurately; simply indicate their approximate position relative to the starting point. (2) Temperature / (cid:113)C 5 Time / minutes 10 (ii) On the grid, draw the lines needed to determine the maximum temperature change. Add a label to show the maximum temperature change on the graph. (2) *P44883A0816*<br />
(iii) A student carried out the experiment using water from a tap. What is the purpose of measuring the temperature of the water at 0, 1 and 2 minutes from the time of starting the clock? (1)(c) The equation for the thermal decomposition of ammonium chloride is shown below. NH4Cl(s) (cid:111) NH3(g) + HCl(g) (i) Suggest why the enthalpy change for this reaction, (cid:168)Hreaction, is difficult to determine directly by experiment. (1)*P44883A0916* Turn over<br />
(ii) Some enthalpy changes which can be determined experimentally are listed below. NH4Cl(s) + water (cid:111) NH4Cl(aq) NH3(g) + water (cid:111) NH3(aq) HCl(g) + water (cid:111) HCl(aq) NH3(aq) + HCl(aq) (cid:111) NH4Cl(aq) (cid:507)H1 (cid:507)H2 (cid:507)H3 (cid:507)H4 By adding arrows to the diagram below, construct a Hess cycle which can be used to calculate the enthalpy change, (cid:168)Hreaction, for the thermal decomposition of ammonium chloride. Label each arrow with the appropriate symbol chosen from the list above for the enthalpy change. Assume that water is added where necessary to make a solution. NH4Cl(s) (cid:507)H reaction NH3(g) + HCl(g) NH4Cl(aq) NH3(aq) + HCl(aq) (iii) Give the expression for the enthalpy change, (cid:168)Hreaction, for the thermal decomposition of ammonium chloride, in terms of the other enthalpy changes in the cycle. (cid:168)H reaction = (1) (1) (Total for Question 3 = 13 marks) 10 *P44883A01016*<br />

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Mark Reject Answers not to 3 sf No sign or negative sign Acceptable Answers Correct final answer with + sign, 3 sf and units scores 3 (25 x 4.18 x 10.5) = 1097.25 (J) / 1.097 kJ Ignore sign if given (1) Mol NH4Cl = (5.00/53.5) = 0.09346/ 0.0935 (1) Hsolution = (+1.097/0.09346) (= +11.7376 / +11.7406) = +11.7 kJ mol-1 OR +11700 J mol-1 Sign, unit and sf must be correct for third mark Use of 2sf earlier may lead to an inaccurate answer (1) ALLOW Final answer = +11.8 kJ mol-1 from rounding of MP1 and/or MP2 (3) TE from each step to the next If mass used is 30 g Energy transferred = 1316.7 J Hsolution =+14.1 kJ mol-1 max (2) If mass used is 5 g Energy transferred = 219.45 J Hsolution =+2.35 kJ mol-1 max (2) Question Number 3(a)(i)<br />
Mark Reject Just 0.2% is negligible / very small Acceptable Answers First mark is for calculation of error. Second mark is for comparison of temperature error to mass error. Uncertainty in mass = (0.005 x 100 x 2/5.00) = ()0.2% (1) Uncertainty / error in mass measurement (much) smaller than uncertainty in temperature reading (1) Second mark depends on first being correct, but allow second mark if mass error is 0.1% (as 0.005 not doubled) Question Number 3(a)(ii)<br />
Acceptable Answers Question Number 3(b)(i) Points (close to the) horizontal from starting temperature at 0, 1 and 2 (and 3) minutes (1) Points (on a line) rising from a minimum up to 10 minutes (at least 2 points needed at the warming up stage for extrapolation.) The minimum can be at 4, 5, 6, 7 or 8 minutes. (1) Mark Reject Large change of temperature at 3 minutes Cooling curve instead of warming curve<br />
Reject Mark Question Number 3(b)(ii) Line through temperature points where warming Acceptable Answers occurs extrapolated back to 3 minutes. ALLOW Line at minimum temperature shown as staying horizontal and extrapolated back (1) Max temperature change indicated as vertical difference between starting temperature and extrapolated line at 3 minute (1) TE if cooling curve drawn in 3(b)(i) for both marks.<br />
Acceptable Answers Question Number 3(b)(iii) To check water temperature is Mark Reject Water temperature may change Minerals in water may affect result steady / constant OR To deduce temperature at 3 mins / at start by extrapolation of line ALLOW to allow water temperature to equilibrate with surroundings/ to reach temperature of surroundings/ to acclimatise IGNORE to get initial temperature accurate Heat must be supplied (and cannot be measured) ALLOW impossible to tell when/if reaction is complete reaction goes to equilibrium/ is reversible IGNORE reference to gases escaping / products are gases / hazards Acceptable Answers Reject Mark Justbecause it is endothermic Needs high temperature Question Number 3(c)(i)<br />
Acceptable Answers Question Number 3(c)(ii) NH4Cl(s) H1NH4Cl(aq) Hreaction NH3(g) + HCl(g) H2 +H3H4NH3(aq) +HCl (aq) OR 2 separate parallel arrows for H2 +H3 OR H2 H3 next to one arrow without being separated by + ALLOW Arrows reversed if signs of enthalpy changes are reversed. IGNORE Any water molecules added/ aq signs / other reactant species Arrow size Acceptable Answers Question Number 3(c)(iii) Hreaction = H1H2H3H4 ALLOW any order of terms with correct signs Any correct use of brackets No TE on incorrect cycle Reject Mark Reject Mark Total for Question 3 =13 marks<br />

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