Skip to content
Edexcel Jun 2016 (IAL) Paper 3 Q
Answers available below
Show answer
Question
![3 The apparatus below was used in a series of experiments by a group of students to determine the enthalpy change of combustion of some alcohols. thermometer copper can draught shield alcohol clamp water burner (a) In the experiment to determine the enthalpy change of combustion of CH3OH, one student obtained the following results. Measurement Mass of copper can / g Mass of copper can + water / g Mass of burner + CH3OH (start) / g Mass of burner + CH3OH (finish) / g Temperature of water (start) / C Temperature of water (finish) / C Value 300.00 700.00 151.65 150.00 21.5 33.5 Data Specific heat capacity of copper = 0.39 J g1 C1 Specific heat capacity of water = 4.2 J g1 C1 10 *P46663A01016*<br />
(i) Calculate the heat energy transferred. Use the expression heat energy transferred / J = [(0.39mass of copper can) + (4.2mass of water)]temperature rise (2) (3) (ii) Use your answer from (a)(i) to calculate the enthalpy change of combustion of CH3OH. Give your answer in kJ mol1 and include the appropriate sign. *P46663A01116* 11 Turn over<br />
(b) The mean values obtained by the students were collected in a table. Alcohol CH3OH C2H5OH C3H7OH C4H9OH C5H11OH () Enthalpy change of combustion / kJ mol1 450 800 No value obtained 1600 2000 (i) Label the axes below and plot a graph of (the magnitude of) the enthalpy change of combustion (on the vertical axis) against the number of carbon atoms in the alcohol (on the horizontal axis). (2) 12 *P46663A01216*<br />
(ii) Use your graph to estimate the enthalpy change of combustion of C3H7OH. (1)(iii) By considering the combustion equations for the alcohols, explain the trend shown by the graph in terms of the bond changes. The equation for the combustion of CH3OH is given below; you are not expected to write any other equations. (2) H C O H + 3 2O O O C O + 2H O H*P46663A01316* 13 Turn over<br />
(c) The students then compared their results to the values in the Data Booklet. They found that the magnitudes were consistently much smaller; for example, the Data Booklet value for C2H5OH is 1367.3 kJ mol1. The students suggested a number of possible explanations for the discrepancy: I uncertainties in the measurement of mass and temperature II the values used for the specific heat capacities of copper (0.39 J g1 K1) and water (4.2 J g1 K1) are rounded (from 0.385 and 4.18 J g1 K1) III heat losses to the surroundings IV incomplete combustion of the alcohols (i) Calculate the percentage error in the students mean value for the enthalpy change for combustion of C2H5OH compared with the Data Booklet value. Give your answer to two significant figures. (2) 14 *P46663A01416*<br />
(ii) By considering your answer to (c)(i), evaluate the validity of each of the four reasons that the students put forward to explain the discrepancies between their values and those in the Data Booklet. (4) Suggestion ISuggestion IISuggestion IIISuggestion IV(Total for Question 3 = 16 marks) TOTAL FOR PAPER = 50 MARKS *P46663A01516* 15<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJune_2016_IAL_QP_-_Unit_3_Edexcel_Chemistry_3-0.jpg)
![Reject Mark (2) Reject Mark (3) use of values rounded to 1 SF Question Number 3(a)(i) Acceptable Answer E = [(0.39 x 300) + (4.2 x 400)] x 12 (1) = 21564 (J) / 21.564 kJ (ans*) TE on incorrect values in expression ALLOW for MP2 20277(J) / 20.277 kJ (1) IGNORE SF except 1 SF IGNORE sign Correct answer with no working scores (2) Question Number 3(a)(ii) Mr (CH3OH) = 32 Acceptable Answer Amount methanol = 1.65/32 (ans**) (1) (= 0.05156) Hc = ans* / ans** = 21564 x 32 / 1.65 = 418211 J mol1 = 418 (kJ mol1) If 20277 used Hc = 393251 J mol1 = 393 (kJ mol-1) Value (1) Correct sign and units (if given) (1) TE on any value obtained in (a)(i) TE on correctly rounded values from (a)(i) IGNORE SF except 1 SF Correct answer with no working scores (3) If units are given for the final answer they must be fully correct<br />
Mark (2) Reject scale that does not use topand / or right- hand 3/10 of the grid non-linear scales scores zero Question Number 3(b)(i) Acceptable Answer See graph below Axes labelled including units and scale as shown or similar (1) All four points correct and best fit line drawn TE on axes reversed (1) ALLOW Hc label with units on y-axis Graph plotted with negative enthalpy changes Formulae of alcohols on the intervals of the x-axis with or without axis label IGNORE Omission of negative sign before enthalpy of combustion Extrapolation (-)Enthalpy (change of combustion) / kJ mol-1<br />
Question Number 3(b)(ii) Acceptable Answer This mark is stand alone ()120050 (kJ mol1) IGNORE omission of negative sign If units are given they must be fully correct No TE on incorrect graph Reject Mark (1) Reject Mark (2) Question Number 3(b)(iii) Marking Point 1 Acceptable Answer For each successive alcohol 1 extra CC bond and 2 extra CH bonds and 3/2 extra O=O bonds have to be broken and 2 extra C=O bonds and 2 extra OH bonds are formed OR The same extra bonds are broken and formed on each increment ALLOW As the chain length increases more bonds need to be broken but more bonds are formed. OR Each successive alcohol has an extra CH2 group OR Each successive alcohol has two extra C-H bonds and one extra C-C bond (1) Marking Point 2 Breaking (CC and CH ) bonds is endothermic / requires energy and forming (C=O and OH) bonds is exothermic /releases energy (1) If intermolecular forces used at any point as an explanation, max (1)<br />
Mark (2) Reject = 41.5 = 42% Question Number 3(c)(i) Acceptable Answer Correct answer with no working scores (2) % Error = 100 x (1367.3800) / 1367.3 (1) = 41.4905 = 41% (1) TE for SF only on use of 800 as denominator (error = 71% (to 2SF)) IGNORE Use of negative signs on enthalpy changes (1367.3 & 800)in front of answer Answer greater than 100% score zero<br />
Question Number 3(c)(ii) Acceptable Answer I. The % uncertainties in the thermometer and balance readings are very small (compared with the difference between the student mean and the Data Book value) OR The thermometer and balance reading uncertainties are random and would give values both high and low rather than consistently low II. The rounding of the specific heat capacities is small (compared with the observed differences) OR Both specific heat capacities have been rounded up so would produce larger magnitude / more negative values for the enthalpies III. Heat losses will be large. despite the draught shield OR from flame or calorimeter or water OR because the copper can is not / should be insulated OR because the copper can does not / should have (loose fitting) lid IV. Incomplete combustion will cause significant error as there will be insufficient oxygen ALLOW 1 mark in place of either III or IV for Heat loss / incomplete combustion will result in lower magnitude / less negative enthalpy change of combustion values ALLOW Reverse arguments Mark (4) Reject Just valid OR Just invalid Just larger Use of polystyrene cup Just lower Total for Question 3 = 16 marks Total for Paper = 50 marks<br />](https://learnah.org/wp-content/uploads/2018/08/June_2016_IAL_MS_-_Unit_3_Edexcel_Chemistry_3-0.jpg)
