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Answer all the questions.
1(a). The reversible reaction between carbon dioxide and hydrogen makes methane and water.
In a sealed container this reversible reaction forms a dynamic equilibrium.
What is meant by the term dynamic equilibrium?
Refer to both concentration and rate of reaction in your answer.
(b). * Kayvan investigates the effect of changing the pressure and changing the temperature on this reaction.
The table shows the percentage yield of methane in the equilibrium mixture under different conditions.
Pressure in atmospheres
100 200 300 400
Temperature in °C 300 35% 52% 65% 80%
600 30% 46% 58% 74%
900 23% 37% 47% 62%
1200 14% 25% 36% 48%
Describe what happens to the percentage yield as the pressure and temperature change and explain the effectof increasing the pressure on the rate of reaction.
9. Ammonia is made from nitrogen and hydrogen in an equilibrium reaction.
N2(g) + 3H2(g) ⇌ 2NH3(g)
The forward reaction is exothermic.
Look at Table 1.
It shows the percentage of ammonia in the equilibrium mixture at 450 °C and different pressures.
Pressure in atmospheres Percentage (%) of ammonia at 450 °C
1 0.2
50 9.5
100 16.2
200 25.3
Table 1
Look at Table 2.
It shows the percentage of ammonia in the equilibrium mixture at 300 atmospheres and different temperatures.
Temperature in °C Percentage (%) of ammonia at 300 atmospheres
400 50
450 35
500 25
550 17
Table 2
Describe and explain how changing the pressure and changing the temperature affect the position of equilibriumin the reaction between nitrogen and hydrogen.
The quality of written communication will be assessed in your answer to this question.
b * Please refer to the marking instructionpoint 10 for guidance on how to mark thisquestion.
Level 3 (5–6 marks)Describes the effect of changing thetemperature and pressure on thepercentage yield from the table andincludes clear explanations on theeffect of increasing the pressure on therate of reaction.There is a well-developed line of reasoningwhich is clear and logically structured. Theinformation presented is relevant andsubstantiated.
Level 2 (3–4 marks)Describes the effect of changing thetemperature and pressure on thepercentage yield from the table andeither describes the effect of increasingthe pressure on the rate of reaction orexplains the effect increasing thepressure on the rate of reaction.There is a line of reasoning presented withsome structure. The information presentedis relevant and supported by someevidence.
Level 1 (1–2 marks)Describes the effect of changing thetemperature and pressure on thepercentage yield from the table ordescribes the effect of increasing thepressure on the rate of reaction.The information is basic andcommunicated in an unstructured way. Theinformation is supported by limitedevidence and the relationship to theevidence may not be clear.
0 marksNo response or no response worthy ofcredit.
6 AO1.1: Knowledge of pressure on rateof reaction
Increasing the pressure increases therate of reaction.Increasing the pressure meansparticles are closer together.Increasing the pressure means morecrowded particles / more particles inthe same space.Increasing the pressure means morecollisions between particles.More collisions the quicker thereaction.More collisions more percentage yield.
AO3.1a: Analyse information in thetable to interpret percentage yield
As temperature increases thepercentage yield decreases.As pressure increases the percentageyield increases.The highest yield is when thetemperature is low and the pressure ishigh.
b *Please refer to the marking instructions onpage 4 of this mark scheme for guidanceon how to mark this question.
Level 3 (5–6 marks)Describes and explains the effect ofchanging the temperature and pressureon the position of equilibrium in boththeoretical terms and from the table andexplains that one prediction issupported and the other prediction isnotThere is a well-developed line of reasoningwhich is clear and logically structured. Theinformation presented is relevant andsubstantiated.
Level 2 (3–4 marks)Describes and explains the effect ofchanging the temperature and pressureon the position of equilibrium in boththeoretical terms and from the tableThere is a line of reasoning presented withsome structure. The information presentedis relevant and supported by someevidence.
Level 1 (1–2 marks)Describes the effect of changing thetemperature and pressure on theposition of equilibrium in theoreticalterms or describes the effect ofchanging thetemperature and pressure on theposition of equilibrium from the tableThe information is basic andcommunicated in an unstructured way. Theinformation is supported by limitedevidence and the relationship to theevidence may not be clear.
0 marksNo response or no response worthy ofcredit.
6 AO1.1: Knowledge of temperature andpressure on percentage yield
As temperature increases the positionof equilibrium shifts to the left in anexothermic reaction.As pressure increases the position ofequilibrium shifts to the side with theleast number of moles of gas.Decreasing the temperature of asystem in dynamic equilibrium favoursthe exothermic reaction.
AO3.1a: Analyse information in thetable to interpret equilibrium position
As temperature increases thepercentage yield decreases.As temperature increases position ofequilibrium moves to the left.As the pressure increases thepercentage yield increases.As the pressure increases position ofequilibrium moves to the right.
AO3.2a: Analyse information in thetable / equation to make judgements /predictions
The prediction is not supported sincereaction must be exothermic ratherthan endothermic because position ofequilibrium moves to the left astemperature increases. The prediction is supported in terms ofthe moles of gas as pressure increasesthe position of equilibrium moves to theright. The prediction is supported becauseas the pressure increases thepercentage yield increases.
if a named catalyst is given it must becorrect including oxidation number except allow vanadium oxide catalyst
if formula and name given both must becorrect
Examiner's Comments
This question was about the ContactProcess.
The need for a catalyst was well known butmany candidates gave the name of thewrong catalyst for example iron or nickel. Asmall proportion of candidates referred to ahigh concentration as a condition.
reference to it is a compromisetemperature is not sufficient
allow good product (percentage) yield atlow pressure
not use low pressure to shift equilibrium tothe right
Examiner's Comments
This question was about the ContactProcess.
There were many different marking pointsthat could be accessed by the candidates.In terms of temperature the idea that thereaction would be fast and was low enoughto keep the equilibrium on the right wassufficient – many candidates onlymentioned the reference to rate. In termsof the pressure many candidates thoughtthat the low pressure was to force theposition of equilibrium to the right. Only thebest answers appreciated that even at lowpressure the position of equilibrium was onthe right. Many candidates appreciated thata higher pressure would be moreexpensive. Most candidates appreciatedthat the catalyst increases the rate ofreaction but the candidates did not oftenmention that the position of equilibrium wasunaffected.
4 [Level 3]Extracts the correct conditions from thegraphANDstates at least two costs of makingammonia.Quality of written communication does notimpede communication of the science atthis level.
(5 – 6 marks)
[Level 2]Extracts one correct condition from thegraph and states at one cost of makingammonia.ORExtracts the correct conditions from thegraphORStates at least two costs of makingammonia.
Quality of written communication partlyimpedes communication of the science atthis level.
(3 – 4 marks)
[Level 1]Extracts one correct condition from thegraphORStates one cost of making ammonia.Quality of written communication impedescommunication of the science at this level.
(1 – 2 marks)[Level 0]Insufficient or irrelevant science. Answernot worthy of credit.
(0 marks)
6 This question is targeted at grades upto C.
Costs
cost of starting materialslabour costscost of the plant / apparatus orequipmentcost of catalystrent or rateshealth and safetypollution control
ignore references to advertising /environment / transport / storage /packaging / energy / temperature andpressure
Conditions
any temperature from 350oC or belowany pressure from 400 atmospheres orabove
Use the L1, L2, L3 annotations inScoris; do not use ticks.
Examiner's Comments
This question was about the manufactureof ammonia by the Haber process.
This was a six-mark question that involveddate interpretation. Many candidates couldinterpret the information in the graphs andgave a possible temperature and apossible pressure to get the highest yield.Most candidates chose figures from thegraphs although some gave lowertemperatures and/or higher pressuresinstead. The other part of this questioninvolved stating possible costs of makingammonia by the Haber process. Typicallycandidates were able to give one or twocosts. Some candidates ignored the stemto the question and gave costs that were
related to the energy e.g. maintaining thepressure or temperature, these answerswere not given any credit.
Total 6
5 i 160 atm and 200 oC (1) 1 both required
Examiner's Comments
Most candidates could identify theconditions giving the highest yield. Only aminority could explain why these conditionswere not used by reference to higher costto generate high pressures or higher plantcosts to withstand high pressures.
ii any two from:idea that higher energy cost with higherpressure (1)idea that higher plant costs with higherpressures (1)idea that greater safety risks with higherpressure (1)
2 if in doubt assume answer refers to 70atmallow it is cheaper (1) if no other marksawardedallow 70atm to avoid breaking theequipment (1)allow uses less energy (1)
Examiner's Comments
Most candidates could identify theconditions giving the highest yield. Only aminority could explain why these conditionswere not used by reference to higher costto generate high pressures or higher plantcosts to withstand high pressures.
Most candidates did not choose both of thecorrect answers. The answers to the lasttwo boxes were common errors.
b
(yes) it is exothermic because thepercentage yield goes down as temperature increases (1)
(no) there are less moles on right handside because the percentage yield goes upas pressure increases (1)
2 Answers must refer to yield, or amountof product reference to only position ofequilibrium is not sufficient
ignore references to bond making andbond breaking
allow ora if specified
Examiner's Comments
This question that assessed evaluationskills was extremely demanding.Candidates had to apply theirunderstanding of le Chatelier’s principle todecide if the statements were supported.Many candidates were not able to relatethe pattern shown by a graph to the correctstatement and then decide if the patternsupported the statement.
8 a i increases / gets bigger / AW (1) 1Examiner's Comments
This question was about the manufactureof ethanol by the hydration of ethene.
Most candidates could interpret the data inthe table.
ii decreases / gets less / AW (1) 1Examiner's Comments
Most candidates could interpret the data inthe table.
b idea of catalyst used to speed up thereaction or increase the rate of reaction (1)
70 atm used as is cheaper to generatethan higher pressures (1)
300 °C is used to increase the rate ofreaction but sacrifice percentage yield / it isa compromise or optimum temperature (1)
3 allow catalyst does not affect percentageyield (1)
allow answer relating to the risksassociated with high pressure (1)
Examiner's Comments
Was a challenging question and only asmall proportion of candidates were able toexplain why the temperature chosen was acompromise. Candidates were most likelyto be awarded a mark for stating that thecatalyst increased the rate of reaction orfor the idea that a higher pressure will costmore money.
c idea of reduction of wage bill / idea of reduction of number of workers (1)
1 ignore rule out human errorignore to make the process work fasterignore references to safetyignore it is a continuous processnot no labour costs
Examiner's Comments
Many candidates appreciated that fewerworkers would be employed. Howeversome stated that no workers would beneeded, which was not allowed on themark scheme.
9 Level 3 (5–6 marks)Manipulates the data to describe andexplain how the position of equilibriumchanges with pressureANDManipulates the data to describe andexplain how the position of equilibriumchanges with temperatureQuality of written communication does notimpede communication of the science atthis level.
Level 2 (3–4 marks)Manipulates the data to describe andexplain how the position of equilibriumchanges with pressureORManipulates the data to describe andexplain how the position of equilibriumchanges with temperatureQuality of written communication partlyimpedes communication of the science atthis level.
Level 1 (1–2 marks)Manipulates the data to describe howthe position of equilibrium changes withpressure AND with temperatureQuality of written communication impedescommunication of the science at this level.
Level 0 (0 marks)Insufficient or irrelevant science. Answernot worthy of credit.
6 This question is targeted at grades upto A*.
Relevant points at levels 2 and 3include:
increasing the pressure moves theequilibrium to the right because thereare fewer molecules (or number ofmoles)on the rhsincreasing the temperature moves theequilibrium to the left because theforward reaction is exothermic or thebackward reaction is endothermic.
Relevant points at level 1 include:
as the pressure increases the positionof equilibrium moves to the right or viceversaas pressure increases percentage ofammonia increasesas the temperature increases theposition of equilibrium moves to the leftor vice versaas temperature increases thepercentage of ammonia decreases
ignore references to rate
Examiner's Comments
This was a very challenging six markquestion that involved data analysis and leChatelier's principle. It was targeted up tograde A*.
Some candidates only described the trendsshown in the two tables and found it verydifficult to explain the trends. Othercandidates contradicted themselves interms of percentage yield and the positionof equilibrium, incorrectly stating that with alow percentage yield the position ofequilibrium is on the right. A significantproportion of candidates referred tochanging the temperature and pressure but
Candidates were more likely to be able toexplain the effect of increasing temperatureon the position of equilibrium rather thanthe effect of increasing the pressure. Thebest answers for increasing temperaturereferring to the reaction being exothermicor the reverse reaction being endothermic.The best answers for increasing pressurereferred to the reaction moving to the sidewith the least number of moles.
A significant proportion of the candidatesanswered a completely different questionto the one set, since they tried to explainthe conditions used in the reaction andreferred to optimum conditions and rate ofreaction.
10 [Level 3] Deduces how changingtemperature and pressure affects thepercentage yieldAND explains the connection betweenpercentage yield and position ofequilibriumQuality of written communication does notimpede communication of the science atthis level
(5 – 6 marks)
[Level 2]Deduces how changing temperatureand changing the pressure affects thepercentage yieldQuality of written communication partlyimpedes communication of the science atthis level
(3 – 4 marks)
[Level 1]Deduces how changing temperatureaffects the percentage yieldORdeduces how changing pressure affectsthe percentage yieldQuality of written communication impedescommunication of the science at this level
(1 – 2 marks)
[Level 0]Insufficient or irrelevant science. Answernot worthy of credit.
(0 marks)
6 This question is targeted at grades upto C.Indicative scientific points at level 3must include:
As percentage yield increases positionof equilibrium shifts to the right / highpercentage yield has position ofequilibrium is on the right / ora
Relevant points at all levels couldinclude explanations
as temperature increases percentageyield decreases / oraas pressure increases percentage yieldincreases / ora
Use the L1, L2, L3 annotations in scoris.Do not use ticks.
Examiner's Comments
Most candidates could state therelationship between the temperature andthe percentage yield and between pressureand percentage yield and gained level 2 (4marks). If only one of these relationshipswas given then level 1 (1 or 2 marks) wasachieved. Few candidates could describehow the percentage yield was linked to theposition of equilibrium which wasnecessary to reach level 3 (5 or 6 marks)
11 a B (1) 1 allow correct answer ticked, circled orunderlined in list if the answer line is blank
Examiner's Comments
Most candidates identified the conditionsthat give the highest percentage of ethanol.
b any two from:the temperature or pressure chosen is acompromise (1)the high temperature gives a high rate ofreaction (1)high pressure increases the percentageyield of ethanol (1)at higher temperatures the percentageyield is lower (1)higher pressures are expensive to maintainor generate (1)
2
allow answer relating to the risksassociated with high pressure (1)
Examiner's Comments
This was a challenging question and only asmall proportion of candidates were able tosuggest why these conditions are used,even though they do not give the highestpercentage yield. Candidates were mostlikely to be awarded a mark for suggestingthat the high temperature gives a high rateof reaction or that higher pressures areexpensive to generate.
Total 3
12 a reversible (1) 1 allow reaction goes both ways / goesbackwards and forwards / reaction formsan equilibrium mixture