1.4 Energetics Revision Questions

8/10/2019 1.4 Energetics Revision Questions

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1.4 a-g Examination Questions

Q1.If the mean CH bond enthalpy is +x, which of the following represents a process withan enthalpy change of +4x?

A C(g) + 4H(g) CH4(g) B CH4(g) C(g) + 4H(g)

C CH4(g) C(s, graphite) + 2H2(g) D C(s, graphite) + 2H2(g) CH4(g)

(Total for question = 1 mark)

Q2.

For which of the following changes is the value of Hnegative?

A K(g) K+(g) + e B K+Cl(s) K+(g) + Cl(g)

C Cl(g) + eCl(g) D Cl2(g) 2Cl(g)


Q3.The standard enthalpy changes of formation of carbon dioxide and of methanoic acid are 394 kJ mol 1and 409 kJ mol1respectively. Calculate the enthalpy change for the reaction

H2(g) + CO2(g) HCOOH(l)

A 803 kJ mol1 B 15 kJ mol1

C +803 kJ mol1 D +15 kJ mol1

1.4. a-g Energetics SpecificationStudents will be assessed on their ability to:

a. demonstrate an understanding of the term enthalpy change,H

b. construct simple enthalpy level diagrams showing the enthalpy change

c. recall the sign ofH for exothermic and endothermic reactions,

eg illustrated by the use of exo- and endothermic reactions in hot and cold packs

d. recall the definition of standard enthalpy changes of reaction, formation, combustion, neutralization and atomizationand use experimental data to calculate energy transferred in a reaction and hence the enthalpy change of thereaction. This will be limited to experiments where substances are mixed in an insulated container, and combustionexperiments

e. recall Hesss Law and apply it to calculating enthalpy changes of reaction from data provided, selected from a tableof data or obtained from experiments and understand why standard data is necessary to carry out calculations of thistype

f. evaluate the results obtained from experiments using the expression:

energy transferred in joules = mass x specific heat capacity x temperature change

and comment on sources of error and assumptions made in the experiments. The following types of experiments

should be performed:i. experiments in which substances are mixed in an insulated container and the temperature rise measured

ii. simple enthalpy of combustion experiments using, eg a series of alcohols in a spirit burner

iii. plan and carry out an experiment where the enthalpy change cannot be measured directly, eg theenthalpy change for the decomposition of calcium carbonate using the enthalpy changes of reaction ofcalcium carbonate and calcium oxide with hydrochloric acid

g. demonstrate an understanding of the terms bond enthalpy and mean bond enthalpy, and use bond enthalpies inHess cycle calculations and recognise their limitations. Understand that bond enthalpy data gives some indicationabout which bond will break first in a reaction, how easy or difficult it is and therefore how rapidly a reaction will takeplace at room temperature.


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Q4.In an experiment performed to measure the enthalpy change for the reaction

3.0 g of zinc powder (an excess) was added to 30.0 cm3of copper(II) sulfate solution

of concentration 1.00 mol dm3

. The temperature rise of the mixture was 47.6 K.Assuming that the heat capacity of the solution is 4.2 J K1g1, the enthalpy change forthe reaction is given by

A H= (30 x 4.2 x 47.6) 0.03 B H= (33 x 4.2 x 47.6) 0.03

C H= (30 x 4.2 x 47.6) x 0.03 D H= (33 x 4.2 x 47.6) x 0.03


Q5.In an endothermic reaction in aqueoussolution, which of the following is correct?


Q6.For which of the following reactions is the enthalpy change equal to the bond enthalpyof H-I?

A B

C D

(Total for Question = 1 mark)

Q7.The enthalpy change for the reaction between hydrochloric acid and sodium hydroxideis 56 kJ mol1. Therefore

A the reaction is exothermic and the temperature rises.

B the reaction is exothermic and the temperature falls.

C the reaction is endothermic and the temperature rises.

D the reaction is endothermic and the temperature falls. (Total for Question = 1 mark)

Q8.Some mean bond enthalpy values are given in the tablebelow.

What is the enthalpy change for the reaction shown below inkJ mol1?

A +436 + 151 299 = +288 B 436 151 + 299 = 288


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C +436 +151 (2 299) = 11 D 436 151 + (2 299) = +11


Q9.The standard enthalpy changes of formation ofsome sulfur species are:

The enthalpy of atomization of sulfur is (in kJ mol1)

A 103 8 B 279 8

C 279 D (103 8) + 279


Q10.Which equation represents the reaction for which the enthalpy change, H, is the meanbond enthalpy of the CH bond?

A CH4(g) C(g) + H(g)

B CH4(g) C(s) + 2H2(g)

C CH4(g) C(g) + 4H(g)

D CH4(g) C(g) + 2H2(g)


Q11.When 10 cm3of 2 mol dm3hydrochloric acid is reacted with10 cm3of 2 mol dm3sodium hydroxide solution, the temperature change is T.

When the reaction is repeated with 50 cm3of each solution, the temperature change is

A T B 5 T C T D 10 2 T


Q12.The molar enthalpy change of combustion of some alkanes is given below inkJ mol1.

Another alkane was found to have an enthalpy change of combustion of 6125kJ mol1The alkane is

A C7H16 B C8H18 C C9H20 D C10H22


Q13.Which equation represents the reaction for which the enthalpy change is the standardenthalpy change of formation, Hf ?, of sodium nitrate, NaNO3?

A 2Na(s) + N2(g) + 3O2(g) 2NaNO3(s)

B Na(s) + N2(g) + 1O2(g) NaNO3(s)

C Na(s) + N(g) + 3O(g) NaNO3(s)


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D Na(g) + N2(g) + 1O2(g) NaNO3(g)


Q14.

The enthalpy change of atomization of iodine is the value of Hfor the process

A I2(s) I2(g) B I2(s) 2I(g) C I2(g) 2I(g) D I2(s) I(g)


Q15.The standard enthalpychanges of combustion ofcarbon, hydrogen andmethane areshown in the table below.

Which one of the followingexpressions gives the correct value for the standardenthalpy change of formation of methane in kJ mol1?

A 394 + (2 286)891 B 394 (2 286) + 891

C 394 + 286891 D 394 286 + 891


Q16.In an experiment to measure the enthalpy change of a reaction involving gases,which of the following conditions must always be kept constant?

A Pressure B Temperature

C Volume D Temperature and pressure


Q17.

This question is about some standard enthalpy changes, H

A enthalpy of reaction

B enthalpy of combustion

C mean bond enthalpy

D bond enthalpy

(a) Which enthalpy change is represented by p?

(1)

A B C D

(b) Which enthalpy change is represented by q?


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(1)

A B C D

(c) Which enthalpy change is represented by r?

(1)

A B C D

(Total for question = 3 marks)

Q18.When a solution of barium chloride is added to sulfuric acid, a white precipitate isformed. The ionic equation (including state symbols) for this reaction is

A

B

C

D


Q19.The concentration of blood glucose is usually given in millimoles per dm 3ormmol dm3. A reading of 5.0 mmol dm3is within the normal range. Glucose has amolar mass of 180 g mol1. What mass of glucose dissolved in 1 dm3of blood wouldgive this normal reading?

A 0.090 g B 0.18 g C 0.90 g D 9.0 g


Q20.The enthalpy change for the reaction

is +1648 kJ mol1. Hence the mean bond enthalpy for the CH bond is

A +329.6 kJ mol

B +412.0 kJ mol

1

C +1648 kJ mol1 D +6592 kJ mol1



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Q21.Given the following data:

select the expression which gives the enthalpy change, in kJ mol1, for the reaction:

A (820 ) + 270 = 140 B (+820 ) 270 = +140

C 820 + (270 2) = 280 D +820 (270 2) = +280


Q22.


Q23.In an experiment to determine the enthalpy change of combustion of an alcohol, a spirit burner containingthe alcohol was weighed, lit and placed under a copper can containing a known volume of water. Thetemperature rise of the water was measured and the burner re-weighed. The enthalpy change calculatedfrom the results was much less exothermic than the value reported in the literature.

Which of the following factors is mostlikely to be the cause of this error?

A Heat loss around the side of the copper can.


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B The use of a thermometer with a range of 0 - 110 C rather than 0 - 50 C.

C The use of a measuring cylinder for measuring the water rather than a pipette.

D Evaporation of the alcohol during the weighing.


Q24.The enthalpy change for the reaction to form hydrated sodium thiosulfate crystalscannot be measured directly.

The following Hess cycle can be used.

The enthalpy change for the reaction, Hr, is equal to

A H1 + H2 B H1 H2

C H1 H2 D H1 + H2


Q25.The enthalpy change for the reaction

cannot be measured directly since some carbon dioxide is always formed in the reaction.It can be calculated using Hess's Law and the enthalpy changes of combustion ofgraphite and of carbon monoxide.

The enthalpy change for the reaction of graphite with oxygen to give carbon monoxideis

A 677 kJ mol1 B +111 kJ mol

C 111 kJ mol1 D +677 kJ mol1


Q26.(a) State the general formula of the alkanes, using the letter nto denote the number of

carbon atoms in each molecule.

(1)


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(b) Alkanes are used as fuels. In the petrochemical industry, useful hydrocarbons areoften produced from longer chain molecules.

Name the type of reaction shown below.

(i)

(1)

Type of reaction ....................................

(ii)

(1)

Type of reaction ....................................

(c) By what typeof formula are the organicmolecules in (b) represented?

(1)

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(d) By what typeof formula are theorganicmolecules in (b) represented?

(i) Give the molecular formula ofcompound 2.

(1)

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(ii) Give the name of compound 2.

(1)

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(e) An equation for the reaction between methane and chlorine is:

The reaction occurs in the presence of ultraviolet (UV) light via a free-radical chainmechanism.

The initiation step is Cl2 2Cl

The next step could be


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(i) Use the following data to calculate a value for the enthalpy change for each ofthe Steps, Aand B.

(3)

Answer ....................................... kJ mol1

Answer ....................................... kJ mol1

(ii) Use your answer to (i) to justify which of the Steps, Aor B, is the more likely.

(1)

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(f) Another halogenoalkane, bromomethane, CH3Br, is a toxic gas used to protect plantsagainst insects.

Health and Safety advice states that concentrations above 5 parts per million (ppm)by volume of this gas are harmful.

A research laboratory contains 2.5 105dm3of air. Calculate the maximum volumeof bromomethane, in dm3, allowed in the laboratory to comply with the advice given.

(1)


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Q27.This question is about the gas ethane, C2H6, and its reactions.

(a) Write the equation, including state symbols, which represents the reaction takingplace when the standard enthalpy change of combustion of ethane is measured.

(2)

(b) Ethane can react with chlorine to form chloroethane and hydrogen chloride.

Rewrite this equation using displayed formulae.

Use the equation you have written, together with the bond enthalpy data, tocalculate the enthalpy change for the reaction.

(4)


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(c) This reaction takes place in a number of steps, some of which are shown below.

(i) State the type of reaction occurring in step 1 and the conditions needed for

this step.

(2)

Type.................................................................................................................................................................................................................

Conditions.........................................................................................................................................................................................................

(ii) Complete the equation below for the third step of the reaction, and show themovement of electrons using the appropriate arrows.

(3)

(iii) Write equations for twotermination steps in this reaction.

(2)

(d) Ethane can be cracked in industry. Write an equation for the cracking of ethane.

(1)

(e) Suggest tworeasons why cracking of larger alkane molecules is important inindustry.

(2)

Reason 1:......................................................................................................................................................................................

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Reason 2:......................................................................................................................................................................................

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Q28.(a) On strong heating, calcium carbonate decomposes to calcium oxide and carbon

dioxide:


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Owing to the conditions under which the reaction occurs, it is not possible tomeasure the enthalpy change directly.

An indirect method employs the enthalpy changes when calcium carbonate andcalcium oxide are neutralized with hydrochloric acid.

(i) Write the equation for the reaction of calcium carbonate with hydrochloric acid.State symbols are not required.[H1is the enthalpy change for this reaction]

(1)

........................................................................................................................................................................

...................... H1

(ii) The reaction of calcium oxide with hydrochloric acid is

Use the equations in parts (i) and (ii) to complete the Hess's Law cycle belowto show how you could calculate the enthalpy change for the decomposition ofcalcium carbonate, Hreaction. Label the arrows in your cycle.

(3)

(iii) Complete the expression for Hreactionin terms of H1and H2.

(1)


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Hreaction=

(b) Suggest tworeasons why the value obtained by carrying out these two experimentsand using the equation gives a value different to the data booklet value for thedecomposition reaction of calcium carbonate.

(2)

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Q29.The enthalpy change of combustion of ethanolwas determined using the apparatus

shown in the diagram below. In the

experiment, the temperature increase of thewater in

the beaker is measured when a knownmass of the ethanol is burned.

(a) The results of the experiment aresummarised in the table below.

(i) Calculate the heat energy produced by the combustion of the alcohol using the equation.

heat energy produced (J) = mass of water x 4.18 x temperature change

(1)


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(ii) Calculate the number of moles of ethanol burned in this experiment (the formulaof ethanol is C2H5OH).

(3)

(iii) Use the equation below to calculate the enthalpy change of combustion ofethanol in kJ mol1. Give the value an appropriate sign.

H= heat energy produced number of moles(2)

(b) The data book value for the enthalpy change of combustion of ethanolis 1370 kJ mol1.

(i) Calculate the percentage error in the value calculated in (a)(iii) in comparisonwith the data book value.

(1)


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(ii) List threeways in which the design of the experiment causes the results to beso different from the data book value. (You should be specific but detailedexplanations are not required.)

(3)

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(iii) Use the data

book values forenthalpy changes ofcombustion given inthe table tocalculate theenthalpy change offormation ofethanol.

(3)


Q30.Copper(II) sulfate exists asblue hydrated crystals andwhite anhydrous crystals.The enthalpy changes of

solution for these twosubstances may berepresented by the followingsimplified equations:


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(a) (i) Fill in the box and add labelled arrows to complete the Hess cycle to enable yoto

calculate(3)

(ii) Calculate a value for the enthalpy change(2)

(b) Suggest why it is not possible to directly measure the enthalpy change for theconversion of the blue hydrated copper(II) sulfate crystals into the white anhydrous

crystals.(1)

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(c) (i)Describe briefly the experimental procedure that you would use to obtain thedata necessary to calculate H1, given a known mass of hydrated copper(II)sulfate crystals, CuSO4.5H2O(s).

You should state the apparatus that you would use and any measurements thatyou would make.

You are notrequired to calculate the amounts of substances or to explain howyou would use the data obtained.

(4)

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(ii) The value for the enthalpy change from (c)(i) obtained by experiments in a schoollaboratory is likely to be significantly different from a data book value. List threepossible reasons for this which do notrelate to the quality of the apparatus orchemicals used or possible mistakes in carrying out the procedure.

(3)

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(Total for Question = 13 mark)Q31.Propanone, C3H6O, undergoes complete combustion to form carbon dioxide and water.

(a) In an experiment to calculate the enthalpy change of combustion for propanone,2.90 g of propanone was burned completely in oxygen.

The heat energy from this combustion raised the temperature of 200 g of water from20.2 C to 78.4 C.

The specific heat capacity of water is 4.18 J g

1

C

1

.(i) Calculate the number of moles of propanone present in 2.90 g.[The molar mass of propanone is 58 g mol1.]

(1)


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(ii) Use the expression

to calculate the heat energy transferred to raise the temperature of 200 g of waterfrom 20.2 C to 78.4 C.

(2)

(iii) Use your answers to (a)(i) and (ii) to calculate a value for the enthalpy changeof combustion of propanone. Give your answer to threesignificant figures andinclude a sign and units.

(3)

(b) In another experiment, the enthalpy change of combustion for butanone, C4H8O, wasfound to be 1300 kJ mol1. A Data Book value for the standard enthalpy change of

combustion for butanone is2440 kJ mol1.(i) Suggest a reason why the value obtained in the experiment is so different from

the Data Book value. (1)

..............................................................................................................................................(ii) This Data Book value (2440 kJ mol1) refers to the following equation.

How would the value be different if it referred to the formation of water in thegaseousstate? Justify your answer.

(2)

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(c) Standard enthalpy changes of combustion can be used to calculate the standardenthalpy change of formation of a compound.

(i) Define the term standard enthalpy change of formation, making clear themeaning of standardin this context.(3)

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..............................................................................................................................................(ii) Use the standard enthalpy

changes of combustion, Hc , givenin the table to find the standardenthalpy change of formation forethanoic acid, CH3COOH, in kJ mol

1.

(3)

(Total for question = 15 marks)Q32.This question is about methanol and the energychanges that accompany some of itsreactions.

(a) Complete the diagram (using dots andcrosses) to show the bonding in methanol,

CH3OH. You should show outer electronsonly.

(2)

(b) The Hess cycle below can be used to calculate the standard enthalpy change ofcombustion of methanol, using standard enthalpy changes of formation.

(i) Complete the cycle by filling in the empty box.(2)


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*(ii) Define the term standard enthalpy change of formationof a compound,making clearthe meaning of standardin this context.

(3)

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..............................................................................................................................................(iii) Use your cycle and the data below to

calculate the standard enthalpy change of

combustion of methanol, Hc .

(2)

(c) An experiment was carried out, usingthe apparatus below, to estimate thestandard

enthalpy change of combustion ofmethanol.

After burning the methanol for a fewminutes, the temperature of water inthe beakerhad risen by 20.7 C and the mass of

methanol burnt was 0.848 g.

(i) Calculate the amount of energytransferred to the water.

Energy transferred (J) = mass of water 4.18 temperature change(1)

(ii) Calculate the number of moles of methanol, CH3OH, burnt during the experiment.

(1)


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(iii) Use your answers to (c)(i) and (ii) to calculate the experimental value for thestandard enthalpy change of combustion. Include a sign and units in youranswer, which should be given to threesignificant figures.

(1)

(iv) Compare your answers to (b)(iii) and (c)(iii) and give TWO reasons to explainany differences.

(2)

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..............................................................................................................................................(Total for question = 14 marks)

Q33.(a) State Hess's Law.

(1)

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(b) Methane burns in a limited supply of oxygen to give carbon monoxide and water.

CH4(g) + 1O2(g) CO(g) + 2H2O(l)

The enthalpy change for this reaction cannot be determined directly, but can befound using thestandard enthalpy changes of combustion of methane and carbon monoxide, together with

Hess's Law.The standard enthalpy changes of combustion needed are for CH4, -890 kJ mol

-1, and for CO,-283 kJ mol-1.(i) Draw a Hess's Law diagram which would enable you to calculate the enthalpy change forthe combustion of methane to carbon monoxide.

CH4(g) + 1O2(g) CO(g) + 2H2O(l)

(2)

(ii) Calculate the enthalpy change for this reaction, in kJ mol-1.(2)


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(iii) Explain why the enthalpy change for this reaction cannot be determined directly.(1)

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..............................................................................................................................................(c) Explain why the calculation in part (b)(ii) would give an incorrect result for the enthalpychange for the reaction below.

CH4(g) + 1O2(g) CO(g) + 2H2O(l) (2)

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(Total for question = 8 marks)Q34.

The standard enthalpy change, H1 , for the decomposition of potassiumhydrogencarbonate, KHCO3, is impossible to determine directly.

The value of H1 can be calculated from the standard enthalpy changes whichaccompany the reactions below:

Procedure:

The solids were added to separate 30 cm3portions of dilute hydrochloric acid. The

acid was in excess for both solids.The maximum temperature change for each experiment was noted.

Results: The following results were obtained with KHCO3(s).

Mass of KHCO3used = 2.00 g

Temperature change = 4.9 C

The experiment with K2CO3(s) gave a H3 value of 34 kJ mol1.

Assumption: The dilute hydrochloric acid solution has a density of 1 g cm3.


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(a) (i) Calculate the heat energy absorbed, in joules, by the reaction of the KHCO3(s)with the solution of dilute hydrochloric acid. Use the expression:

energy absorbed (J) = mass of solution 4.18 temperature change(1)

(ii) Calculate the number of moles of KHCO3(s) used. Assume that the molar massof KHCO3(s) is 100 g mol

1.(1)

(iii) Use your answers to (a)(i) and (ii) to calculate, in kJ mol1, the enthalpy change

when one mole of KHCO3(s) reacts completely with the acid (i.e. H2 ).Include a sign in your answer.

(2)


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(b) A Hess Cycle based on these reactions is shown below.

(i) Apply Hess's Law to obtain an expression for H1 in terms of H2 and H3 . (1)

H1 =

(ii) Use your answers to (a)(iii) and (b)(i), and the H3 value of 34 kJ mol1, to

calculate a value for H1 in kJ mol1. Include a sign in your answer.

. (2)

(c) The maximum errors for the apparatus used in the experiment with the KHCO3(s)were as follows:

Balance 0.01gMeasuring cylinder 0.5 cm3

(i) Calculate the maximum percentage error in using each of the following piecesof apparatus in the KHCO3(s) experiment:


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(2)Balance Measuring cylinder

(ii) Suggest a piece of apparatus that could have been used to measure the volumeof dilute hydrochloric acid more accurately in this experiment.

(1)

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..............................................................................................................................................(Total for question = 10 marks)

Q35.This question is about hexane, C6H14, and hex-1-ene, C6H12.

(a) What test would you use to distinguish between hexane and hex-1-ene? Give theresults of the test for each substance.

(2)

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(b) Hex-1-ene has a number of isomers, including two stereoisomers of hex-2-ene.

(i) Complete the formula to show the structure of E-hex-2-ene.

(1)

*(ii) Explain why stereoisomerism can occur in alkenes, and why hex-2-ene hasstereoisomers but hex-1-ene does not.

(2)

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(c) The enthalpy change ofcombustion of hexane wasmeasured using a spirit burner toheat a known mass of water in acalorimeter. The temperature riseof the water was measured. Theresults of the experiment areshown below.

The specific heat capacity of water is 4.18 J g1C1.

(i) Calculate the energy in joules produced by burning the hexane. Use the expression

energy transferred = mass x specific heat capacity x temperature change.

(1)

(ii) Calculate the enthalpy change of combustion of hexane. The mass of 1 mole ofhexane is 86 g.

Give your answer to TWO significant figures. Include a sign and units in youranswer.

(3)

(iii) The value for the enthalpy change of combustion in this experiment is differentfrom the value given in data books. Suggest TWO reasons for this difference.

(2)

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(iv) A student suggested that the results would be more accurate if a thermometerwhich read to 0.1C was used. Explain why this would notimprove theaccuracy of the result. A calculation is notrequired.

(1)..............................................................................................................................................

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(d) Hex-1-ene can be converted to hexane in the following reaction.

(i) What catalyst is used in this reaction?

(1)

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(ii) The enthalpy change of thisreaction Hreactioncan be calculatedfrom the following enthalpychanges of combustion.

Complete the Hess cycle by addinglabelled arrows. Use your cycle tocalculate the enthalpy changeHreaction.

(3)


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(iii) The enthalpy change for thereaction of some other alkenes withhydrogen is shown below.

Explain why the values are sosimilar.

(1)

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.............................................................................................. (Total for question = 17 marks)


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Examiners ReportQ1.-Q25No Examiner's Report available for this question

Q26.In (a), a sizeable majority of candidates were able to state the general formula of analkane. In (b) (i), cracking was frequently stated as the correct response, but in (b)(ii) the process of reforming was less well known. Answers to (c) and (d) (i) showed asound knowledge of skeletal formulae, but the branched isomer (compound 2) proveddifficult for many to name correctly. In (e) (i), the signs for the two enthalpychanges were often reversed as candidates had used a "products - reactants" formulainstead of adding the mean bond enthalpies for the bonds broken (positive sign) tothe mean bond enthalpies for the bonds made (negative sign). Consequentialmarking then allowed credit to be earned for a logically correct answer to (e) (ii).Part (f) was found to be straightforward by those with a clear understanding of the

concept of parts per million (ppm).

Q27.(a)Only a few candidates gave the incorrect formulae, but the states gave more problems. Inspite of the standard enthalpy of combustion being mentioned water was often given as agas. Much worse were the candidates who gave ethane as a liquid or solid, suggesting theydid not know that the first four alkanes are all gases under standard conditions.

Some could not balance the equation, oxygen being the major problem.

Results Plus: Examiner CommentsThis equation has the correct entities with their states.It is balanced for hydrogen and carbon, but not for oxygen.

Results Plus: Examiner TipRead the question carefully and remember to give Advanced level answers.

Results Plus: Examiner Comments

The state of ethane is wrong as is probably the state of water and the equation is notbalanced.

Results Plus: Examiner TipBe familiar with the states and uses of the first four alkanes.Make state symbols very clear - if a mistake is made cross it out and write the correctstate next to it.Remember equations will always be expected to be balanced.

(b)

The majority of candidates gained full marks.Some candidates gave structural and not displayed formulae in the equation losing the firstmark. Remember displayed formulae show all the atoms and all the bonds.


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Good candidates made it clear which bonds were breaking and which were making andrealised that bond enthalpies are for dissociating or breaking bonds so the sign needs to bereversed when bonds are made.

Results Plus: Examiner CommentsThe candidate has done much unnecessary work including bonds which do not break or

make and then failed to appreciate that bond enthalpies are for bond breaking.

Results Plus: Examiner TipIn calculations it is important to think about what you are doing.

(c) (i)Many candidates failed to get the first mark as they misread the question and gave theoverall reaction type rather than the type of step.

The condition of UV radiation was well remembered.

Results Plus: Examiner CommentsA typical one mark answer. The candidate has given the overall reaction type, ratherthan the name of the step. The condition is correct.

Results Plus: Examiner TipBe careful in reading the question - it is a good idea to highlight or underline key wordslike 'step'.


The candidate has been fortunate as both parts contain additional information.In the first part it could be construed that the candidate meant an initiation step in a freeradical substitution.In the second part the candidate is fortunate that the additional information happens towork.

Results Plus: Examiner TipOnly give additional information if you are absolutely sure it is correct.

(c) (ii)

Many candidates managed to get the correct products, chloroethane and a chorine freeradical, but the use of half arrows was rarely given correctly. Full arrows were sometimesused. Often only one half of the Cl-Cl bond was broken, or there was no pairing of electronsto form the C-Cl bond.

(c) (iii)Most candidates were familiar with termination steps, some offering more than two whichproviding they were all correct were acceptable. Some misread the question and gavepropagation steps. Some omitted the dots representing the unpaired electrons which losta mark. Some put dots in the wrong places, some in the products. Some gave the electronmovements correctly as well, though this was not required.

Results Plus: Examiner CommentsThis was an instructive error. The first equation has the correct reactants but theproduct has a five bond carbon and a three bond carbon.


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Results Plus: Examiner TipAlways check the number of bonds formed by each carbon in organic formulae.

(d)Ethane is produced in a number of oil refinery processes and is then cracked producing

high percentages of ethene and hydrogen. There were many correct answers. Eventhough candidates may have been unfamiliar with this starting material for cracking, manycandidates applied their knowledge that ethane is always one of the products of cracking.Methane is also a product of this cracking so the equation for its formation with ethene wasacceptable.

Weaker candidates lost atoms in the equation forming methane and hydrogen, or failed toproduce stable compounds giving two methyl groups or even methane and CH2.

Results Plus: Examiner CommentsThe addition of the word heat above the arrow in the correct equation is fine, as it is

true.

Results Plus: Examiner TipIt is a good idea to give extra relevant information providing it is correct.

Results Plus: Examiner CommentsA typical incorrect response.

Results Plus: Examiner TipAlways check that carbon has formed four bonds.

(e)Clear appreciation of why cracking of larger alkane molecules is carried out was neededhere. Candidates were prone to rather vague reasons like 'more uses for the products'(which fails to say which products) or 'to form alkenes' (without reference to why alkenesare important). With this type of question it is important that candidates make a responseof A level quality, expressing as much as they should know, for example 'Short chainalkenes are produced from which many other useful chemicals like poyalkenes are made'.It is also essential that candidates make their writing clear. Benefit of the doubt cannot begiven to candidates where alkAnes could not be distinguished from alkEnes.

Results Plus: Examiner CommentsThe first reason is just acceptable for one mark, though it would have been better if ithad ended '...are used as fuels like petrol'.The second is insufficient.

Results Plus: Examiner TipAlways relate as much as you know which is relevant to the question.

Results Plus: Examiner CommentsThe first reason is just acceptable but would have been much better if the reason for thehigher demand had been given mentioning petrol for internal combustion engines.The second reason would have been sufficient if alkenes had been mentioned, though


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the use of the term plastic is not strictly appropriate. Plastic is a property of a material(that allows its shape to be permanently changed when a stress is applied). The correctterm here is polymer.

Results Plus: Examiner TipBe very careful to use scientific terms correctly.


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Q28.(a) (i)This part was generally well-answered. The most frequent error, however, was to omit the"2" in front of the HCl in the equation.

Results Plus: Examiner CommentsIncorrect, as the "2" is missing from in front of the HCl.

Results Plus: Examiner TipCheck that your equations are balanced!

(a) (ii-ii i)Answers to (a)(ii) showed that many candidates have a good understanding of energycycles. The most common error in (a)(ii) was to omit one of the products in the box on thecycle.


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Results Plus: Examiner CommentsNote "CO2" is missing from the box in the Hess cycle in (a)(ii).

Results Plus: Examiner TipCheck that all relevant species are included in your Hess cycles.

(b )The most frequently seen correct answer related to heat losses from the apparatus. Manyincorrect answers referred to "incomplete combustion", despite the fact that the questionreferred to the thermal decomposition of calcium carbonate.

Results Plus: Examiner CommentsPart (b) scored the mark for "heat loss". References to inaccuracy of measurements orthe equipment did not earn a mark.

Results Plus: Examiner TipRemember that data book values for reaction enthalpy changes are normally givenunder standard conditions.

Q29.(a-b (i))In (a)(i), the majority of candidates demonstrated competence in calculating the heat energy

produced.In (a)(ii), candidates were, in general, successful at calculating the number of moles of ethanolburned. In some cases, however, answers were rounded up to 0.02 (i.e. to only one significantfigure).In (a)(iii), the correct value for the enthalpy change for the combustion of ethanol was calculated,but the negative sign was often overlooked.Part (b)(i) was found to be challenging, with the answer from (a)(iii) divided by 1370 andmultipliedby 100, instead of initially subtracting the value given in (a)(iii) from 1370 and then calculatingthepercentage error.


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Results Plus: Examiner Comments(a)(i) is correct, so one mark is awarded. (a)(ii) scored the first two marks (for the massof ethanol burned, by subtraction, and the molar mass of ethanol). No third mark wasgiven as the moles of ethanol burned were rounded to one significant figure. (a)(iii) is

consequentially correct, so two marks have been awarded. (b)(i) The mark is awardedas this is consequentially correct.


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(b) (ii)Most candidates appreciated that heat loss to the surroundings is a significant factor inexplainingthe discrepancy between laboratory results and those from a data book. Incomplete combustionandevaporation of hot ethanol were mentioned less frequently in candidates' responses.

Results Plus: Examiner CommentsFirst mark was awarded for heat loss from the beaker. The second mark was awardedas not all the heat from the flame reaches the water in the beaker. Third mark wasawarded for mentioning the evaporation of the alcohol.

(b) (iii)A large majority of candidates gained some credit for their answer to this calculation question.Full

marks were frequently awarded, showing that many candidates have a good understanding ofHesscycles.


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Results Plus: Examiner Comments690 scores two marks. [Note: the + sign is in brackets in the mark scheme.] The total ofthe enthalpy changes of combustion of the reactants minus those of the products hasbeen calculated, but without doubling the value for carbon and tripling the value forhydrogen (this counts as one error).


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Results Plus: Examiner CommentsThree marks were awarded. An excellent response.

Results Plus: Examiner TipLearn all thermochemical definitions and practise more on Hess cycle questions.


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Q30.(a)There were many excellent answers to this straightforward application of Hess's Law butthere were a significant number of candidates who were unable to use the data providedto construct the appropriate cycle. The most common error was to insert incorrect species,usually the elements, in the box but both the direction and the labelling of the arrowsalso caused immense difficulty. Many candidates were unable to use the cycle they had

constructed to calculate the required enthalpy of reaction.


It's easy when you know how! The correct answer given with clear simplicity.


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Results Plus: Examiner CommentsNote the reversed right hand arrow with the reversed sign on the H value which thecandidate uses to complete the calculation below. Again an excellent answer.


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Results Plus: Examiner CommentsThis example shows some typical errors. The constituent elements have been put in thebox rather than CuSO4(aq) and the direction of the right-hand arrow has been reversedwithout changing the sign of the H value. However, ignoring the contents of the box,the data in the cycle has been used correctly in (a)(ii) so consequential marking has

been applied to the calculation.

(b-c) (i)Many of the attempts to 18(b) were stock answers to a different question about sources oferror in experimental work or else plainly incorrect assertions, the most common of whichwas that it is impossible to measure the temperature of a solid. Relatively few candidatesappeared prepared to think about the practical issues raised by the question.There were some excellent answers to 18(c)(i) but, sadly, these were few and far between.Very many candidates did not appear to have read the question with any care at all andthere were many answers which described in great detail the preparation of blue copper(II)sulfate crystals and many more which referred to the dehydration of blue copper(II) sulfate;a sigificant number of responses described enthalpy of combustion reactions. Of those thatdid correctly identify the reaction and had some idea of the appropriate procedure, manyomitted crucial experimental details while giving extensive descriptions of the calculationsrequired despite being specifically told that this was not required. A number of candidatesreferred to the use of a bomb calorimeter although it was far from clear that theyunderstood its purpose.


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Results Plus: Examiner CommentsThis candidate makes a reasonable attempt at these two questions.In 18(b) the candidate has appreciated that direct dehydration of the blue copper(II)

sulfate crystals will require an input of heat energy and it will not be possible todiscriminate between the heat supplied by an external source and the heat absorbeddue to the endothermic nature of the reaction the reaction.In 18(c)(i) a correct experimental procedure has been selected but the vital importance

of stirring to ensure thermal equilibrium has not been appreciated.

(c) (ii)As elsewhere in question 18 there were excellent answers interspersed with responses thatdrew on a stock list of 'errors in practical work' without regard either to the specifics ofthis experiment or, indeed, to the clear exclusions given in the stem of the question. Otherincorrect responses were based on arguments used to discuss the discrepancy betweencalculated and experimental values for quantities such as Lattice Energy.


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Results Plus: Examiner CommentsThis candidate gives two sensible practical possibilities to answer the question and avague proposal of the sort that might be used in a quite different context to explain thediscrepancy between a calculated and experimental value.

Results Plus: Examiner TipThis type of question requires the candidate to think about the specific experiment in thequestion and also the way in which the data will be used to calculate the required value.Avoid vague generalisations!



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Despite the clear instruction in the question, this candidate gives an answer based onsimple mistakes in carrying out the experiment.

Results Plus: Examiner TipDo read the question.

Results Plus: Examiner CommentsParallax errors in measuring a scale arise from faulty technique and are easilyavoidable. It is unclear why accurate timing might have been a factor in this experimentindicating that this suggestion also has been selected from a general list of possibleexperimental errors.


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Q31.Part (a) elicited a full range of marks. All six marks were often awarded. Where they were not, itwas often because in (a)(ii) the mass of fuel rather than the mass of water was used to calculatethe energy transferred to the water. In (a)(iii), some candidates did not round up their answer tothree significant figures, whilst others omitted the negative sign or did not give the correct units ofkJ mol-1.


This response shows an example of a well set-out answer scoring full marks.


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The candidate has missed the negative sign for an exothermic reaction in (iii).

In (b)(ii), contradictory answers were seen particularly when the difference was described as"higher" or


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as seenbelow. This response has also neglected to mention formation from elements(Note: the slight

error ofthe standard conditions not being at 100 kPahas been ignored as 1 atmosphere pressure has

also beenincluded):-


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Results Plus: Examiner CommentsThis example is one of many fully correct answers seen for (c)(ii), showing a soundknowledge of how to construct a Hess cycle.

Use of either the expression Hreaction= HC[products] - HC[reactants] orincorrect algebraic expressionslead to an incorrect answer of +490 kJ mol-1, which was awarded one out of the three marks

available asillustrated below:-


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Even in other incorrect or incomplete answers, candidates were aware of the necessity to doublethestandard enthalpies of combustion of both carbon and hydrogen in this calculation.


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Q32.(a)

This item was very well answered and the majority of candidates gained two marks because theyclearly showed all electron pairs present around the carbon and oxygen atoms.

Results Plus: Examiner CommentsThis response only scored one mark as, although all the bondingelectrons are present, the candidate omitted to show the lonepairs present on the oxygen atom.

Results Plus: Examiner Tip

Check 'dot and cross' diagrams to ensure all bondand lone pairs are shown, where appropriate.

(b )

Whilst most candidates could indicate the formulae of the required elements, the questiondiscriminated to an extent as a significant minority either could not balance the equation(especially the oxygen) or omitted one or more state symbols.


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Results Plus: Examiner CommentsThis answer only scored one mark as the candidate has missedout the state symbols.

Results Plus: Examiner TipIf asked to complete a cycle, look at the formatused to help you see the expected style of youranswer. For instance, here all the species alreadypresent have state symbols. Hence anything youinclude should have state symbols too.

(b) (ii)

Responses to this question showed that most candidates had made the effort to memorise thisbasic definition. A view persists that the enthalpy change of formation is always endothermic, soa small number of answers made statements such as 'the energy required...' so missed the firstmark. A few also ignored the pointer in the question and did not elaborate on what was meantby standard conditions.

Results Plus: Examiner CommentsThis example illustrates the two most common errors identifiedin the introduction.

(b) (iii)-(c)

The calculations showed sound understanding of the use of the Hess cycle, with the mostcommon error being the failure to multiply the standard enthalpy of formation of water by two.


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Parts c (i) and (ii) presented few problems, although a small number candidates rounded uptheiranswers at this early stage, leading to inaccuracies later. Part c (iii) presented more challenges.In fairness, candidates had to follow all the clear instructions given to score here, but it wasfrustrating to see a sound answer miss out as the answer was not quoted to three significantfigures as required in the question. Part c (iv) discriminated well as weaker candidates

tended

to really on generic statements about errors rather than considering this specific experiment.


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Results Plus: Examiner CommentsAlthough this answer does not score the heat loss mark, as they

have not identified where the heat may have gone, the candidatehas considered the experiment and identified two potential sourcesof error. Hence, two marks were scored for recognising thatany combustion experiment may have errors due to incompletecombustion and that, as this example involves a liquid fuel, itsvolatility may lead to some of the fuel escaping by evaporation.This answer would have been awarded two marks had the candidateidentified that the evaporation may have occurred after combustion,but before the reweighing of the sample.

Q33.(a)

This question was answered reasonably well. The essential idea required was that the enthalpychange for a reaction is independent of the pathway (or route) followed.



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This is a typical correct response to this question.

Results Plus: Examiner TipMake sure you understand the fundamental principles underpinning Hess's Law.

(b )

In (b)(i), a mark was often lost for the omission of the '2' in the '2H2O' required. Some of thecycles given had shown confusion between enthalpy changes of combustion and those offormation. Part (b)(ii) was well-answered, but (b)(iii) proved difficult for many candidates.Answers to (b)(iii) often focused on the toxicity of carbon monoxide or simply 'heat loss' ratherthan the formation of carbon dioxide.


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Results Plus: Examiner CommentsParts (b)(i) and (b)(ii) are answered correctly. The response to (b)(iii) is incorrect.

Results Plus: Examiner TipPractise plenty of questions involving Hess Cycles. Understand when to use enthalpychanges of combustion and when to use enthalpy changes of formation. Alwaysconsider carefully the direction of every arrow drawn in any Hess Cycle.

(c )

This question proved to be a good discriminator. Whilst many candidates noticed that water hadbeen produced in the gaseous state, fewer were able to reason the effect on the enthalpychange of the reaction given in (c).

Results Plus: Examiner CommentsThis response scores the first mark as it states that the water is formed in the gaseousstate. It also is awarded the second mark as the candidate realises that the energychange for the reaction as given is less exothermic than that in (b)(ii).

Results Plus: Examiner TipWhen comparing negative values, use terms such as 'less exothermic', rather than just'greater' or 'smaller', to avoid ambiguity.


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Q34.In (a) (i), many candidates were unsure as to what to use for the "mass of solution"in the expression given, but they were able to score consequential marks in (a) (iii)and, subsequently, (b) (ii). In (a) (iii), a sizeable number of candidates thought thatthe reaction described should have a negative enthalpy change value as thetemperature change had been given as 4.9 oC in the stem of the question. Some ofthose candidates who realised that the reaction was endothermic then went on to

omit the positive sign, despite being asked to include a sign in their answer. In (b)(i), relatively few candidates realised that H1= 2 x H

2-

H3and it was evidentthat further practice of this type of Hess's Law calculation would have been ofbenefit to many candidates. In (c) (i), those who understood the concept ofpercentage error were able to score one or both of the marks available and manyexcellent answers to (c) (ii) were seen.

Q35.The test in (a) was very well known.

Diagrams of alkenes with four groups round the carbon-carbon double bond are helpful foridentifyingisomers and predicting the formulae of polymers, so it is a good technique to learn to draw them,but thedouble bond was often not on the second carbon atom.

Results Plus: Examiner TipThe question did not require the formula to be displayed to show all atoms, and it wasenough to show the alkyl groups

Results Plus: Examiner CommentsThis got the mark for showing the four groups round the double bond, even though itwas not totally clear that the bond to the propyl group went to carbon rather thanhydrogen.

The answer to (b)(ii) required clear use of language. The point about the carbon-carbon doublebondrestricting rotation was comparatively easy to express, but some candidates missed out theexplanation of

why hex-1-ene does not have stereoisomers. Others thought it was because the double bondwas at the endof a chain, but did not explain that there must be two different groups on each end, as swappingthe position


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of the two hydrogen atoms on the first carbon in hex-1-ene would not result in a differentstructure.

Results Plus: Examiner TipWhen there are two parts to a question it is important to answer both. Stereoisomerismoccurs because there is no free rotation about a double bond, so if there are differentgroups at each end of the bond, different structures would be produced when they swappositions. Hex-1-ene has two hydrogen atoms at one end of the double bond, so if

these change positions there is no change in structure.

Results Plus: Examiner CommentsThis gets the first mark for the idea of a rigid structure with no rotation, but does notexplain why hex-1-ene is unchanged if the sigma bonds rotate.

Results Plus: Examiner CommentsThis scores full marks. It is not enough to say that the double bond is at the end of thechain, but the answer has developed this and added that the first carbon has twohydrogen atoms on it.

The problem posed in (c)(i) was what number to use for mass. The temperature rise refers to thewater,which is being heated up, but a significant number of candidates used the mass of hexane whichwas burnt.In (c)(ii) many forgot the negative sign in the final enthalpy change and missed the instruction togive theanswer to two significant figures.

Results Plus: Examiner TipThe mass used in the given equation refers to the substance which gets hotter ie thewater.


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Results Plus: Examiner CommentsThis answer does not get the mark for (i) as the mass of the hexane is added to themass of the water. However the method in (ii) is correct, including the sign, unit andsignificant figures in the final answer so it scores full marks.

Rounding the answer to -2600 kJ mol-1in (c)(ii) is sensible because there are so many heatlosses in theexperiment, and these are not improved by a thermometer reading to 0.1C. In (c)(iv) manyanswers statedcorrectly that use of this thermometer increased the precision of the reading, but did not gainmarks as theydid not explain why the accuracy of the result was not improved.

Results Plus: Examiner TipA thermometer reading to 0.1C in a reading of 68C produces a smaller error than abalance readingto 0.5g in a mass of 50g but both of these errors are small in relation

to errors due to heat loss.


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Results Plus: Examiner CommentsThis is enough for the mark.

In (d)(ii) the question was about using data on enthalpy changes of combustion. Adding labels toarrowsshould have helped candidates to apply Hess' Law, and many scored full marks. A minoritystated that the

enthalpy change of a reaction equals the difference in enthalpy of formation of products andreactants. Thisis inappropriate for combustion data, and showed a need for more careful reading of thequestion and thetype of data in it.

Results Plus: Examiner TipThe question asked for arrows to be labelled. Combustion of hex-1-ene and hydrogencould be shown together on one arrow, or on two. Symbols on the arrows wereaccepted if their meaning was clear eg through a key.


This scores full marks. Adding oxygen to balance the equations was not required in thequestion.


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Part (d)(iii) was not well answered. Very few answers referred to bonds, and most just said thatall thereactants were in the same homologous series, or that the same reaction occurred each time.

Results Plus: Examiner TipEnthalpy changes are due to the difference in energy required to break bonds, andenergy released making new ones. Each reaction shown involved addition of hydrogento a C=C bond.

Results Plus: Examiner CommentsThis is an alternative way of giving the answer, rather than specifying exactly whichbonds are broken and formed.


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1.4 a-g markschemeQ1. B Q2. C Q3. B Q4. A Q5. C

Q6. C Q7. A Q8. C Q9. C Q10. A

Q11. A Q12.C Q13.B Q14. D Q15. B

Q16. A Q17 (a) D (b) C (c) A

Q18. D Q19. C Q20. B Q21. C Q22. D

Q23. A Q24. B Q25. C

Q26.


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Q27.


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Q30.


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Q31.


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Documents

1.4 Energetics Revision Questions