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Unofficial Mark Scheme CHEM2 June 2013 COMPLETED watch

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    1)
    a)
    i) State and explain the effect of a decrease in temperature on the equilibrium yield of SO3.

    Increase the yield [1]
    The forward reaction is exothermic / The backward reaction is endothermic [1]
    The equilibrium will shift to the right to oppose the decrease in temperature [1]

    ii) Give two features of a reaction at equilibrium

    The rates of the forward and backward reactions are the same. [1]
    The concentrations of the reactants and products are constant. [1]

    b) Write an equation for the reaction of concentrated sulfuric acid and potassium bromide to form potassium hydrogensulfate and hydrogen bromine.

    KBr+H2SO4 -----> KHSO4 + HBr [1] (Multiples accepted)

    c) Bromine is one of the products formed when concentrated sulfuric acid reacts with hydrogen bromide. Write an equation for this reaction. State the role of sulfuric acid in this reaction.

    Correct Equation [1] (Can't remember what I put)
    Balanced Equation [1]
    H2SO4 is an oxidising agent [1]


    d) Name and outline a mechanism for Stage 1 of this conversion.
    (CH3)2CCH2 + H2SO4 ----> (CH3)2C(OSO2OH)CH3

    Electrophilic Addition [1]
    Arrow from double bond to H [1]
    Breaking of the O-H bond with an arrow pointing from the bond to OSO2OH [1]
    Tertiary Carbocation structure [1]
    An arrow from the lone pair on OSO2OH to the carbocation [1]

    e) Deduce the type of reaction in Stage 2 of this conversion.
    (CH3)2C(OSO2OH)CH2 + H2O ----> (CH3)2C(OH)CH3 + H2SO4

    Hydration / Hydrolysis [1] Debated

    f) State the overall role of sulfuric acid in this conversion.

    Catalyst [1]

    2)
    a) AgBr(s) and AgI(s)

    Reagent: Concentrated NH3 [1] (May accept Excess NH3)
    Observation with AgBr(s): Ppt will dissapear [1]
    Observation with AgI(s): Ppt will remain [1]

    b) HCl(aq) and HNO3(aq)

    Reagent: AgNO3 [1]
    Observation with HCl(aq): White precipitate / white solid [1]
    Observation with HNO3(aq): No change [1]

    c) Cyclohexane and cyclohexene

    Reagent: Bromine Water / Br2 / Bromine [1]
    Observation with cyclohexane: No change [1]
    Observation with cyclohexene: Turns colourless [1]

    d) Butanal and butanone

    Reagent: Fehlings or Tollens [1]
    Observation with butanal: If Fehlings, turns brick red. If Tollens, silver mirror occurs [1]
    Observation with butanone: No change [1]

    3)
    a) On the appropriate axis of this diagram, mark the value of Emp for this distribution. On this diagram, sketch a new distribution for the same sample of gas at a lower temperature.

    The Emp marked on the X-Axis level with the highest point of the curve [1]
    New curve is higher than original [1]
    New curve peaks earlier than original [1]

    b) With reference to the Maxwell-Boltzmann distribution, explain why a decrease in temperature decreases the rate of decomposition of this gas.

    Less molecules would have the required energy to react[1]
    Less molecules react when they collide [1]

    4)
    a)
    i) Balance the equation for this reaction.

    3CuS(s) + 8HNO3(aq) ----> 3CuSO4(aq) + 8NO(g) + 4H2O(l) [1]

    ii) Give the oxidation state of nitrogen in each of the following.

    HNO3: +5 [1]
    NO: +2 [1]

    iii)Deduce the redox half-equation for the reduction of the nintrate ion in acidified solution to form nitrogen monoxide and water.

    NO3- + 4H+ + 3e- ----> NO + 2H2O [1]

    iv) Deduce the redox half-equation for the oxidation of the sulfide ion in aqueous solution to form the sulfate ion and H+(aq) ions.

    S(2-) + 4H2O ----> SO4(2-) + 8H+ + 8e- [1]

    b) Use your knowledge of metal reactivity to state and explain a low-cost method for the extraction of copper from a dilute aqueous solution of copper(II) sulfate. Write the simplest ionic equation for the reaction that occurs during this extraction process.

    Cu(2+) + Fe ----> Cu + Fe(2+) [1] (State symbols not required)
    Scrap iron [1]
    Scrap iron is cheap [1]
    Iron is more reactive than copper [1]


    5)
    a)
    i) Name and outline a mechanism for the conversion of 2-bromo-3-methylbutane into 2-methylbut-2-ene according to the equation.
    (CH3)2CHCHBrCH3 + KOH ----> (CH3)2CCHCH3 + KBr + H2O

    Elimination [1]
    Arrow showing OH- attacking H to the left of Br [1]
    Arrow from C-H bond (the bond which is attached to the H that OH- attacked) to C-C bond where the double bond will form as shown be the equation [1]
    Arrow from C-Br bond to the Br atom to eliminate the Br [1]

    ii) Draw the displayed formula for the other isomer that is formed.

    Drawn structure of the product that occurs when OH attacks the H on the right of the Br [1]
    Showing all the bonds should be accepted.

    iii) State the type of structural isomerism shown by these two alkenes.

    Position(al) [1]

    b)
    i) A small amount of another organic compound, X, can be detected in the reaction mixture formed when hot concentrated enthanolic potassium hydroxide reacts with 2-bromo-3-methylbutane. Compound X has the molecular formula C6H12O and is a secondary alcohol.
    Draw the displayed formula for X

    Structure of 3-methylbutan-2-ol [1] (Again, all bonds shown should be fine.)

    ii) Suggest one change to the reaction conditions that would increase the yield of X.

    React under reflux [1]

    iii) State the type of mechanism for the conversaion of 2-bromo-3-methylbutane into X

    Nucleophilic Substitution [1]

    iv) Identify one feature of this infared spectrum of a pure sample of X that may be used to confirm that X is an alcohol.

    Absorbance at the correct range (range stated) which suggests an O-H bond. [1]

    6)
    a) Chlorine displaces iodine from aqueous potassium iodide.
    i) Write the simplest ionic equation for this reaction.

    Cl2 + 2I- ----> 2Cl- + I2 [1]

    ii) Give one observation that you would make when this reaction occurs.

    Brown solution [1]

    b) In bright sunlight, chlorine reacts with water to form oxygen as one of the products. Write an equation for this reaction.

    Cl2 + H2O ----> 1/2O2 + 2HCl [1]

    c) Explain why chlorine has a lower boiling point than bromine.

    Chlorine is smaller / Bromine is larger [1]
    Chlorine has weaker vdW / Bromine has stronger vdW [1]

    7) Chlorine can be used to make chlorinated alkanes such as dichloromethane.
    a) Write an equation for each of the following steps in the mechanism for the reaction of chloromethane (CH3Cl) with chlorine to form dichloromethane (CH3Cl2)

    Cl2 ----> 2Cl. [1]
    CH3Cl + Cl. ----> .CH2Cl + HCl [1]
    .CH2Cl + Cl2 ----> CH2Cl2 + Cl. [1]
    2.CH2Cl ----> C2H2Cl2 [1]

    b) When chlorinated alkanes enter the upper atmosphere, carbon-chlorine bonds are broken. This process produces a reactive intermediate that catalyses the decomposition of ozone. The overall equation for this decomposition is 2O3 <----> 3O2

    i) Name the type of reactive intermediate the acts as a catalyst in this reaction.

    (Chlorine) Free Radical [1]

    ii) Write two equations to show how this intermediate is involved as a catalyst in the decomposition of ozone.

    O3 + Cl. ----> .ClO + O2 [1]
    O3 + .ClO ----> .Cl + 2O2 [1]

    8)
    a) Draw the structure of the alkene that would form 1,2-dibromo-3-methylbutane when reacted with bromine.

    Drawn structure of 3-methylbut-1-ene [1]

    b) Draw the structure of the alcohol with molecular formula C4H10O that is resistant to oxidation by acidified potassium dichromate(VI)

    Drawn structure of 2-methylpropan-2-ol [1]

    c) Drawn the structure of the alkene that has a peak, due to its molecular ion, at m/z = 42 in its mass spectrum.

    Drawn structure of propene [1]

    d) Draw the structure of the organic product with Mr = 73, made from the reaction between 2-bromobutane and ammonia.

    Drawn structure for 2-aminobutane. [1]

    9) The alkene (E)-but-2-enenitrile is used to make acrylic plastics.

    a)
    i) Draw the structure of Z-but-2-enenitrile.

    Same structure as the example except CH3 and CN on the same side [1]

    ii) Identify the feature of the double bond in the E and Z isomers that causes them to be stereoisomers.

    The double bond cannot freely rotate [1]

    b) Draw the repeating unit of the polyalkene formed by addition polymerisation of (E)-but-2-enenitrile.

    Same as the shown example except double bond is single, move the angled bonds straight but keep them in the same position. Put square brackets around and add trailing bonds coming left and right from the carbons. [1] (Mark should be given without brackets also)

    c) Identify two features of the infrared spectrum that support the fact that this is the infrared spectrum for but-2-enenitrile.

    Absorbance at ..... which suggests a C(Triple bond)N. [1]
    Aborbance at .... which suggests a C(Double bond)C. [1]


    10) Ethanol is an important industrial compound.
    a) Ethanol can be produced by the hydration of ethene. The equation for the equilibrium that is established is.
    CH2CH2(g) + H2O(g) <----> CH3CH2OH(g) Enthalpy Change = -42 KJ Mol-1

    The operating conditions for the process are a temperature of 300 degrees and a preassure of 7Mpa. Under these conditions, the conversion of ethene into ethanol is 5%.

    i) Identify the catalyst used in this process. Deduce how an overall yield of 95% is achieved in this process without changing the operating conditions.

    Concentrated H3PO4 / Concentrated H2SO4 [1]
    Add H2O / Remove ethanol as it's made to shift the equilibrium more to the right [1]

    ii) Use your knowledge of equilibrium reactions to explain why a manufacturer might consider using an excess of steam in this process, under the same operating conditions.

    There are less gaseous moles on the right / more on the left [1]
    Adding H2O(g) will shift the equilibrium to the right [1]
    Which increases the yielf of ethanol [1]

    iii) At pressures higher than 7MPa, some of the ethene reacts to form a solid with a relative molecular mass greater than 5000. Deduce the identity of this solid. Give one other reason for not operating this process at pressures higher than 7MPa. Do not include safety reasons.

    Polyethene / Polythene [1]
    Expensive to maintain [1] (Other ideas along the line of costly to setup)

    b) Write an equation for the reaction that has an enthalpy change that is the standard enthalpy of formation of ethanol.

    2C + 3H2 + 1/2O ----> CH3CH2OH
    Correct equation [1]
    Correctly balanced [1]

    c) When ethanol is used as a fuel, it undergoes combustion.

    i) Define the term standard enthalpy of combustion.

    The enthalpy change when 1 mole of a compound [1]
    Is completed combusted in oxygen [1]
    With all reactants and products in their standard states and under standard conditions [1]


    ii) Use these data and the equation to calculate a value for the enthalpy of combustion of gaseous ethanol.

    Enthalpy Change = BE(Reactants) - BE(Products) [1]
    Correct values substituted into equation [1]
    Final Answer = -1279 [1] (Correct answer scores all 3 if not otherwise gained)

    d) Gaseous ethanol can be used to convert hot copper(II) oxide into copper.

    i) Deduce the role of ethanol in this reaction.

    Reducing Agent [1]

    ii) Draw the structure of the organic compound with Mr = 60 that is produced in this reaction.

    Drawn structure of ethanoic acid [1]


    11) Group 2 metals and their compounds are used commercially in a variety of processes.

    a) Strontium is extracted from strontium oxide (SrO) by heating a mixture of powdered strontium oxide and powdered aluminium. Consider these standard enthalpies of formation.
    3Sr + 2Al ----> 3Sr + Al2O3

    Use these data and the equation to calculate the enthalpy change for this extraction of strontium.
    The use of powdered strontium oxide and powdered aluminium increases the surface area of the reactants. Suggest one reason why this increases the reaction rate.
    Suggest one major reason why this method of extracting strontium is expensive.

    Correct enthalpy change equation [1]
    Correct answer = +101 [1] (Correct answer gains both marks)
    More chance of a successfully collision [1]
    Aluminium has to be extracted by electrolysis [1]
    Electrolysis is expensive [1]

    b) Explain why calcium has a higher melting point than strontium.

    Calcium's delocalised electrons are closer to the cation than in strontium [1]
    It therefore has stronger metalic bonding. [1]

    c) Magnesium is used in fireworks. It reacts rapidly with oxygen, burning with a bright white light. Magnesium reacts slowly with cold water.
    Write an equation for the reaction of magnesium with oxygen.
    Write an equation for the reaction of magnesium with cold water.
    Give a medical use for the magnesium compound formed in the reaction of magnesium with cold water.

    Mg + 1/2O2 ----> MgO [1]
    Mg + 2H2O ----> Mg(OH)2 + H2 [1]
    Antacid [1] (Should get mark if you explain the use, putting gaviscon probably won't
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    Looking good so far.

    You missed the question where it asks how to differentiate between Butanal and Butanone.
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    There was a question for the observations with an Aldehyde and a Ketone So, Fehlings or Tollens
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    I remember a chlorine free radical question, chlorine and water in sunlight, Mg and cold water, Mg and oxygen, use of Mg(OH)2, definition of enthalpy of combustion
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    Missing 2d)

    For an aldehyde and a ketone

    Benedicts/Fehlings Solution - Blue to red, then stays blue
    OR
    Tollen's Reagent - Silver mirror, no silver mirror
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    what was Q4?
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    I know there was a whole page on structures.
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    (Original post by Blobar)
    I know there was a whole page on structures.

    that was question 8
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    Im sure for AgBr and AgI you can use H2SO4 as a reagent.
    obs with br ion: turns brown?
    Obs with I : bad egg smell.

    Can't believe I didn't just put concentrated ammonia just to play it safe.
    Also for the enthalpy change of combustion of ethanol i got like +209.
    How did a lot of you get -1700 or something?
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    I appreciate the effort but I could probably recall all the questions, problem is we need the numbers and marks. Provide as much as you can. If we just throw it all in it's harder to recall what you put.
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    (Original post by Danny-r)
    I appreciate the effort but I could probably recall all the questions, problem is we need the numbers and marks. Provide as much as you can. If we just throw it all in it's harder to recall what you put.
    how about you do the answers and from there it may be easier for us to recall the marks.
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    For the balancing question the ratio was 8 3 8 4
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    (Original post by Blobar)
    how about you do the answers and from there it may be easier for us to recall the marks.
    I can easily recall the answers, but we need the numbers first, does anybody remember what Q4 was?
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    (Original post by me0)
    For the balancing question the ratio was 8 3 8 4
    Can't remember exactly but the first one was 8. I took a pot luck guess to see if it would work and luckily it did and saved me a lot of time!
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    (Original post by Danny-r)
    I can easily recall the answers, but we need the numbers first, does anybody remember what Q4 was?
    Nope, for some reason im just drawing blanks on what questions was what.
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    You had to define enthalpy of combustion (3)
    There was a question asking to work out something using hess's law, and there were two other marks in that question, it was 5 overall.
    4 marks for the one about the extraction of copper and the ionic equation.
    (3) something about the group 2 metals and a trend...?
    (3) for magnesium and two of it's reactions and it's use in medicine
    (4) for the page of structures

    bit vague.. but!
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    on the question about the Emp I drew the line all the way up so it touched the peak, which I now realise isn't what they asked for.. Do you think I could get a mark?
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    (Original post by Jpeccarino)
    on the question about the Emp I drew the line all the way up so it touched the peak, which I now realise isn't what they asked for.. Do you think I could get a mark?
    It's what I also did, it marks it either way so it's fine.
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    (Original post by Jpeccarino)
    on the question about the Emp I drew the line all the way up so it touched the peak, which I now realise isn't what they asked for.. Do you think I could get a mark?
    What did they ask for, thats what I did and labelled the line all the way up Emp?
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    Could you have used acidified potassium dichromate to distinguish between the aldehyde and keytone? Aldehyde goes green, keytone no change??
 
 
 
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