Turn on thread page Beta
    • Thread Starter
    Offline

    14
    ReputationRep:
    I have dealt with the formatting. Included questions, marks and answers. Added pictures for some of the questions (I get error for inserting images so I linked the image links.)
    In second post there is an image link to average boundaries.
    Multiple choice (image link)

    Credit to @sat_freak
    Edited on her photos
    question 1 is 180 degrees, I put that in my exam but I was being an idiot and edited the photo wrong
    http://imgur.com/a/TfguZ


    Question 18 - Hydrolysis of halogenoalkanes - 18 marks

    Part (a)
    (i) Why is ethanol used as a solvent in this experiment? (1)
    - Ethanol as solvent because halogenoalkane is insoluble in water

    (ii) Explain why the apparatus was left for 5 minutes before the silver nitrate was added. (1)
    - To make sure all the test tubes are at the same temperature for a fair test

    (iii) Name the precipitate formed in the test tube containing 1-bromobutane and write an ionic equation for the formation of this precipitate. State symbols are not required. (2)
    - silver bromide
    - Ag+ + Br- ==> AgBr

    (iv) Predict the order (fastest first) in which the halogenoalkanes form precipitates. Explain your answer. (2)
    - Iodo>bromo>chloro
    - Bond length decreases in the order of C-I, C-Br, C-Cl and bond enthalpy increases, therefore it becomes harder to break.

    Part (b): Alcohols are usually prepared from halogenoalkanes using aqueous alkali, rather than water, as the reaction is faster.
    (i) Name the mechanism and type of reaction occurring when 1-bromobutane reacts with aqueous alkali. (2)
    - Nucleophilic substitution

    (ii) Explain why the formation of alcohols is faster with aqueous alkali than with water. (1)
    - OH- is a stronger nucleophile than water.

    (iii) Give the mechanism for the reaction of 1-bromobutane with aqueous alkali. Show the lone pair involved in the mechanism and any relevant dipoles and curly arrows. (3)
    See the mechanism here
    - SN2 mechanism
    - Double headed arrow from the bond C-Br to Br
    - Partially negative Br, partially positive C
    - Double headed arrow from the lone pair on OH-

    (iv) One student suggested that the final reaction mixture could be tested for the presence of an alcohol using phosphorus (V) chloride while another suggested using potassium dichromate (VI) with sulphuric acid.
    Describe the result of a positive test for alcohols using each of these reagents.
    Explain which test is better for the final reaction mixture. (3)
    - PCl5: Steamy white fume
    - Dichromate: colour change from orange to green
    - Dichromate is more suitable as PCl5 will give a false positive with the water and sodium hydroxide in the reaction mixture (My teacher's answer
    - PCl5 is more suitable because the excess sodium hydroxide can neutralise the sulphuric acid therefore the test may fail to work (my answer)
    There are more points you could come up with.

    Part (c) 1-bromobutane is classified as a primary halogenoalkane and is one of the four structural isomers with a molecular formula C4H9Br.
    Give the skeletal formula of the three isomers, other than 1-bromobutane, classifying the halogenoalkane in each case. (3)
    See the structures here
    - 2-bromobutane (secondary)
    - 2-methyl-2-bromopropane (tertiary)
    - 2-methyl-1-bromopropane (primary)


    Question 19 - Inorganic analysis - 17 marks

    Part (a)
    (i) The presence of the carbonate ion is usually confirmed using a simple test carried out in two stages at room temperature. Describe this test and its results. (2)
    - Add dilute HCl, pass the gas through lime water
    - Fizzing, lime water turns milky.
    HCl is better than H2SO4, as many sulphates are insoluble in water, the reaction stops immediately due to insoluble layer of sulphate.

    (ii) Using barium carbonate as your example, write the equation for each of the stages of the carbonate test. Include state symbols in both equations (3)
    - BaCO3(s)+2HCl(aq)==>BaCl2(aq)+H2 O(l)+CO2(g)
    - CO2(g)+Ca(OH)2(aq)==>CaCO3(s)+H2 O(l)

    Part (b): When a flame test is carried out on the two powders, A gives no colour and B gives a yellow-red flame.
    (i) Describe how a flame test is carried out. (3)
    - Dip nichrome wire into concentrated HCl, heat at the hottest part of Bunsen flame until it produces no colour, then dissolve the samples in HCl, dip the wire in, heat it again

    (ii) Give the formulae of metal ions in A and B. (2)
    A: Mg2+
    B: Ca2+

    (iii) Explain the origin of the flame colour. (3)
    - electrons are promoted to higher energy levels when heated. when they drop to ground state they release photons of characteristic frequencies, some of which are in visible spectrum and appear as flame colours

    (iv) Suggest why compound A produces no flame colour. (1)
    - The photons emitted are outside of visible spectrum

    Part (c): When group 2 metal carbonates are heated strongly, they decompose forming the metal oxide and carbon dioxide.
    Explain why the thermal stability of the metal carbonates increase as the group is descended. (3)
    - Down the group cationic radius increases whilst the charge stays 2+. Charge density decreases so the polarising power decreases. Anion is distorted less and the C-O bonds are more stable, requiring more energy to break. Results in high thermal stability.


    Question 20 - Green chemistry - 4 marks

    (a) Suggest what is meant by 'natural gas is a much cleaner fuel than coal'. (1)
    - Natural gas releases less pollutants than coal when burnt.

    (b) Explain how greenhouse gases cause global warming. (2)
    - Greenhouse gases have polar bonds which change their dipoles and absorb IR reflected off earth, and re-emit them. Have trapping effect of heat within earth’s atmosphere.

    (c) Suggest why methane is a far more potent greenhouse gas than carbon dioxide. (1)
    - A methane molecule has four polar bonds whereas a carbon dioxide molecule only has two. A molecule of methane absorbs more IR.


    Section C - 21 marks

    Part (a): The equation for the reaction of chlorine with water is:
    Cl2(aq) + H2O(l) = HOCl(aq) + HCl(aq)
    By referring to the relevant oxidation numbers, explain why this is a disproportionation reaction. (3)
    - ON of Cl goes from 0 in CL2 to +1 in HOCl and to -1 in HCl simultaneously

    Part (b)
    (i) Complete the dot and cross diagram for chloric(I) acid (HOCl). Use a dot to represent the hydrogen electron, circles to represent the oxygen electrons and crosses to represent the chlorine electrons.
    Show the outer electrons only, but include non-bonding electrons. (2)
    See the diagram here
    - H:O:Cl with two lone pairs on O, three lone pairs on Cl.

    (ii) Predict the bond angle in chloric(I) acid. Explain your answer fully. (5)
    - Bond angle 104.5 degrees.
    - Around the central oxygen atom, there are two lone pairs and two bonding pairs
    - which arrange themselves in the position of minimum repulsion.
    - lone pairs repel more strongly
    - Based on tetrahedral/V-shaped/from 109.5 degrees 2.5 degrees of bond angle depression per lone pair

    Part (c): Concentration of Cl2 in swimming pools is determined by titration. Recommended concentration is within the range of 1-2mgdm^-3
    1.00dm^3 of sample
    9.65cm^3 of 0.00550 moldm^-3 sodium thiosulphate
    The equations for the reactions are:
    Cl2 + 2I- => I2 + 2Cl-
    I2 + 2S2O3 2- => 2I- + S4O6 2-

    (i) Show that the concentration of chlorine, Cl2, in this sample meets the UK recommended requirements.
    All steps in your calculation must be shown. (3)
    See the working out here
    - 1.88mgdm^-3, so within the range

    (ii) Analysing a single sample of swimming pool water in this way is likely to give unreliable results because it is not possible to repeat the titration. Suggest another way in which this method is unreliable. (1)
    - The concentration may vary depending on the position from which the sample is taken

    Part (d): One advantage of the bromine disinfection system is that, while chloric(I) acid decomposes in sunlight forming hydrogen chloride and oxygen, bromic(I) acid (HOBr) is stable. Another advantage is that bromine is less volatile than chlorine.

    (i) Write an equation for the decomposition of chloric(I) acid in sunlight. State symbols are not required. (1)
    - HOCl=>HCl+1/2O2 (or multiples)

    (ii) Explain why bromine is less volatile than chlorine. (2)
    - A bromine molecule has more electrons than a chlorine molecule so stronger dispersion force between the molecules, requiring more energy to break the attractions between them.

    Part (e): The pH of the swimming pool affects the performance of the disinfectant. The data below show how the concentrations of bromine species vary with pH.
    The equation for the reaction of bromic(I) acid in water is
    HOBr(aq) = H+(aq) + OBr-(aq)

    (i) Assume the pH of the swimming pool is neutral. Explain how any changes in the pH of the swimming pool affect the concentration of the bromine species. (3)
    - As pH is increased the concentration of H+ is decreased. Therefore the forward reaction is favoured to conteract the change in concentration of H+ so equilibrium shifts to the right. Resulting in increased concentration of OBr- but decrease in concentration of HOBr (or other way around)

    (ii) The higher the concentration of bromate(I) ions, the more effective the disinfectant.
    Suggest a disadvantage of too high a pH. (1)
    - Corrosive, harmful to the swimmers.
    • Thread Starter
    Offline

    14
    ReputationRep:
    Average boundaries since January 2010, including full UMS raw marks
    http://i.imgur.com/QzaxRDp.jpg

    I cannot insert images in OP for some reason
    Offline

    15
    ReputationRep:
    Lel, would rep for this but prsom For the last bit of section C, I think I said it could be an irritant to swimmers skins? Reckon thats sufficient to get the mark ?
    Offline

    3
    ReputationRep:
    (Original post by C0balt)
    Cant remember many, feel free to remind me of the questions

    About hydrolysis of halogenoalkanes
    - Ethanol as solvent because halogenoalkane is insoluble in water
    - 5 mins to make sure all the test tubes are at the same temperature for a fair test
    - silver bromide, Ag+ + Br- ==> AgBr
    - 2-bromobutane (secondary), 2-methyl-2-bromopropane (tertiary), 2-methyl-1-bromopropane (primary)
    - Iodo>bromo>chloro because C-I bond is the longest and the weakest
    - sodium hydroxide is faster because OH- is a stronger nucleophile
    - PCl5: steamy white fume, dichromate: from orange to green, PCl5 is more suitable because the sodium hydroxide in the reaction mixture can neutralise the dilute sulphuric acid, so the test may fail to work with dichromate (that is my answer, and I still believe in it)

    About carbonate
    - add dilute HCl, pass the gas through lime water
    - BaCO3(s)+2HCl(aq)==>BaCl2(aq)+H2 O(l)+CO2(g)
    - CO2(g)+Ca(OH)2(aq)==>CaCO3(s)+H2 O(l)
    - Dip nichrome wire into conc HCl, heat at the hottest part of flame until it produces no colour, then dissolve the samples in HCl, dip the wire in, heat it again
    - electrons are promoted to higher energy levels when heated. when they drop to ground state they release photons of characteristic frequencies, some of which are in visible spectrum and appear as flame colours
    - Mg2+ Ca2+
    - Mg no colour because the photons are outside of visible spectrum
    -Down the group cationic radius increases whilst the charge stays 2+. Charge density decreases so the polarising power decreases. Anion is distorted less and the C-O bonds are more stable, requiring more energy to break. Results in high thermal stability

    Green chemistry
    - clean fuel releases less pollutants when burnt
    - greenhouse gases have polar bonds which change their dipoles and absorb IR reflected off earth, and re-emit them. Have trapping effect of heat within earth’s atmosphere
    - Methane is more potent because it has four polar bonds whereas CO2 only has two

    Section C
    - 1.88mgdm^-3, so within the range
    - not reliable because concentration may vary depending on the position from which the sample was taken
    -Br2 less volatile because it’s got more electrons so stronger dispersion force, requiring more energy to break the attractions between the molecules
    - As pH is increased the concentration of H+ is decreased. Therefore the forward reaction is favoured to conteract the change in concentration of H+ so equilibrium shifts to the right. Resulting in increased concentration of OBr- but decrease in concentration of HOBr
    - Too high pH, corrosive, harmful to swimmers
    If this is all right then the test was great! How many marks was the carbonate question?
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by jshep000)
    Lel, would rep for this but prsom For the last bit of section C, I think I said it could be an irritant to swimmers skins? Reckon thats sufficient to get the mark ?
    yeah should be
    Offline

    1
    ReputationRep:
    (Original post by C0balt)
    Cant remember many, feel free to remind me of the questions

    About hydrolysis of halogenoalkanes
    - Ethanol as solvent because halogenoalkane is insoluble in water
    - 5 mins to make sure all the test tubes are at the same temperature for a fair test
    - silver bromide, Ag+ + Br- ==> AgBr
    - 2-bromobutane (secondary), 2-methyl-2-bromopropane (tertiary), 2-methyl-1-bromopropane (primary)
    - Iodo>bromo>chloro because C-I bond is the longest and the weakest
    - sodium hydroxide is faster because OH- is a stronger nucleophile
    - PCl5: steamy white fume, dichromate: from orange to green, PCl5 is more suitable because the sodium hydroxide in the reaction mixture can neutralise the dilute sulphuric acid, so the test may fail to work with dichromate (that is my answer, and I still believe in it)

    About carbonate
    - add dilute HCl, pass the gas through lime water
    - BaCO3(s)+2HCl(aq)==>BaCl2(aq)+H2 O(l)+CO2(g)
    - CO2(g)+Ca(OH)2(aq)==>CaCO3(s)+H2 O(l)
    - Dip nichrome wire into conc HCl, heat at the hottest part of flame until it produces no colour, then dissolve the samples in HCl, dip the wire in, heat it again
    - electrons are promoted to higher energy levels when heated. when they drop to ground state they release photons of characteristic frequencies, some of which are in visible spectrum and appear as flame colours
    - Mg2+ Ca2+
    - Mg no colour because the photons are outside of visible spectrum
    -Down the group cationic radius increases whilst the charge stays 2+. Charge density decreases so the polarising power decreases. Anion is distorted less and the C-O bonds are more stable, requiring more energy to break. Results in high thermal stability

    Green chemistry
    - clean fuel releases less pollutants when burnt
    - greenhouse gases have polar bonds which change their dipoles and absorb IR reflected off earth, and re-emit them. Have trapping effect of heat within earth’s atmosphere
    - Methane is more potent because it has four polar bonds whereas CO2 only has two

    Section C
    - 1.88mgdm^-3, so within the range
    - not reliable because concentration may vary depending on the position from which the sample was taken
    -Br2 less volatile because it’s got more electrons so stronger dispersion force, requiring more energy to break the attractions between the molecules
    - As pH is increased the concentration of H+ is decreased. Therefore the forward reaction is favoured to conteract the change in concentration of H+ so equilibrium shifts to the right. Resulting in increased concentration of OBr- but decrease in concentration of HOBr
    - Too high pH, corrosive, harmful to swimmers
    any multiple choice answers?
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by sj97)
    If this is all right then the test was great! How many marks was the carbonate question?
    3 I think
    can't remember lol
    Offline

    15
    ReputationRep:
    (Original post by C0balt)
    yeah should be
    Sweet. Think you pretty much got the answers bang on for section C, got mostly identical to you.
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by Marco1000)
    any multiple choice answers?
    I will do that now
    Offline

    1
    ReputationRep:
    I think you miss the HClO dot & cross bond angle thing.
    Don't know why I put 167.5 for the angle. Banging my head on the wall :banghead:
    • Thread Starter
    Offline

    14
    ReputationRep:
    Forgot most of multiple choice lol
    Also can anyone tell me how to move thread to chemistry exam
    Offline

    10
    ReputationRep:
    Since when was knowing the REASON for Mg having no colour on the syllabus, we've never been taught that, and it isn't in either of the two As chemistry books i have, utter crap.
    Offline

    2
    ReputationRep:
    (Original post by C0balt)
    Cant remember many, feel free to remind me of the questions

    Multiple choice (order is totally messed up)
    - BeCl2 is 180 degrees
    - For ketone and aldehyde one, the wrong statement was the one which said "they have different molecular ion peaks"
    - H2SO3 acting as oxidising agent in the reaction with H2S, forming 3S
    - IR with ketone under reflux is C=O
    - Maxwell-Boltzmann: shifts to the right and lower peak for increased temperature, no change for concentration, the activation energy line goes to the left without change in peak for catalyst
    - Covalency increases NaCl to AlCl3 (charge density increases so more polarisation of anion)
    - Boiling point increased in this order: branched Cl, non branched Cl, branched I, non branched I
    - Buckyball is soluble because it is a simple molecule

    About hydrolysis of halogenoalkanes
    - Ethanol as solvent because halogenoalkane is insoluble in water
    - 5 mins to make sure all the test tubes are at the same temperature for a fair test
    - silver bromide, Ag+ + Br- ==> AgBr
    - 2-bromobutane (secondary), 2-methyl-2-bromopropane (tertiary), 2-methyl-1-bromopropane (primary)
    - SN2 mechanism
    - Iodo>bromo>chloro because C-I bond is the longest and the weakest
    - sodium hydroxide is faster because OH- is a stronger nucleophile
    - PCl5: steamy white fume, dichromate: from orange to green, PCl5 is more suitable because the sodium hydroxide in the reaction mixture can neutralise the dilute sulphuric acid, so the test may fail to work with dichromate (that is my answer, and I still believe in it)

    About carbonate
    - add dilute HCl, pass the gas through lime water
    - BaCO3(s)+2HCl(aq)==>BaCl2(aq)+H2 O(l)+CO2(g)
    - CO2(g)+Ca(OH)2(aq)==>CaCO3(s)+H2 O(l)
    - Dip nichrome wire into conc HCl, heat at the hottest part of flame until it produces no colour, then dissolve the samples in HCl, dip the wire in, heat it again
    - electrons are promoted to higher energy levels when heated. when they drop to ground state they release photons of characteristic frequencies, some of which are in visible spectrum and appear as flame colours
    - Mg2+ Ca2+
    - Mg no colour because the photons are outside of visible spectrum
    - Down the group cationic radius increases whilst the charge stays 2+. Charge density decreases so the polarising power decreases. Anion is distorted less and the C-O bonds are more stable, requiring more energy to break. Results in high thermal stability

    Green chemistry
    - clean fuel releases less pollutants when burnt
    - greenhouse gases have polar bonds which change their dipoles and absorb IR reflected off earth, and re-emit them. Have trapping effect of heat within earth’s atmosphere
    - Methane is more potent because it has four polar bonds whereas CO2 only has two

    Section C
    - 1.88mgdm^-3, so within the range
    - not reliable because concentration may vary depending on the position from which the sample was taken
    - Bond angel 104.5. 2 lone pairs, 2 bonding pairs, position of minimum repulsion, lone pairs repel more. Not sure about last point, maybe for saying bent/based on tetrahedral/2.5 depression etc
    - Br2 less volatile because it’s got more electrons so stronger dispersion force, requiring more energy to break the attractions between the molecules
    - As pH is increased the concentration of H+ is decreased. Therefore the forward reaction is favoured to conteract the change in concentration of H+ so equilibrium shifts to the right. Resulting in increased concentration of OBr- but decrease in concentration of HOBr
    - Too high pH, corrosive, harmful to swimmers
    "boiling point increased in this order: branched Cl, non branched Cl, branched I, non branched I"
    surely branched I, non branched I, branched Cl, non branched Cl as there are stronger permanent dipoles in Cl than with I ?
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by thedylanman)
    "boiling point increased in this order: branched Cl, non branched Cl, branched I, non branched I"
    surely branched I, non branched I, branched Cl, non branched Cl as there are stronger permanent dipoles in Cl than with I ?
    The decrease in dipole moment is outweighed by the increase in dispersion force from Cl to I
    Also extremely similar question has come up before and the answer was the same


    Posted from TSR Mobile
    Offline

    3
    ReputationRep:
    (Original post by C0balt)
    Forgot most of multiple choice lol
    Also can anyone tell me how to move thread to chemistry exam
    FOr the multiple choice, the sulfuric acid one was that A? ANd the keton aldehyde one mass spec was D?
    Offline

    0
    ReputationRep:
    Poll on how you found the exam

    http://www.thestudentroom.co.uk/show....php?t=3375341
    • Thread Starter
    Offline

    14
    ReputationRep:
    (Original post by sj97)
    FOr the multiple choice, the sulfuric acid one was that A? ANd the keton aldehyde one mass spec was D?
    Aldehyde mass spec was D
    Can't remember the letter for the acid but it was C or D


    Posted from TSR Mobile
    Offline

    20
    ReputationRep:
    I've already lost 10 marks from this markscheme. But I think the multiple choice was fine so hopefully I can still get an A.
    Offline

    0
    ReputationRep:
    For the testing for a carbonate question i talked about heating the carbonate releasing co2 and using the lime water to test for it. will i get any marks?
    Offline

    0
    ReputationRep:
    (Original post by sj97)
    If this is all right then the test was great! How many marks was the carbonate question?
    It was 3 Marks
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: October 16, 2015

University open days

  • University of Bradford
    All faculties Undergraduate
    Wed, 21 Nov '18
  • Buckinghamshire New University
    All Faculties Postgraduate
    Wed, 21 Nov '18
  • Heriot-Watt University
    All Schools Postgraduate
    Wed, 21 Nov '18
Poll
Black Friday: Yay or Nay?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.