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AQA CHEM4 discussion/unofficial mark scheme Watch

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    (Original post by Anerror)
    Question 9 wasn't it 6 carbons?
    Yep. It was after something with the molecular formula C6H14O2
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    (Original post by logicmaryam)
    Heey guys, I think we can all agree this paper was one of the hardest CHEM4 papers ever.
    We'll discuss the answers and compile an unofficial mark scheme. Post your answers here!


    Q1 20 marks
    a) Ethanoic acid in water
    CH3COOH ⇌ CH3COO- + H+ (1)
    b) Ethanoic acid as base with HNO3
    HNO3 + CH3COOH = CH3CHOOH2+ + NO3- (1)
    c) pH of acid? 2.08 (3)d) pH when alkali added to acid 2.32 (4)
    e) [salt]/[acid] = 3.33 (3) or 1.20?

    For the Kc - 9.09x10^3?
    k= 3.12x10-5 mol-1 dm3 s-1

    Q8
    Condition for step 1 (3)NaOH heat under reflux?
    use distillation or filtration to separate

    test for acyl chloride Q and some chlorohaloalkane P
    acidified AgNO3
    P - NVC
    Q - white ppt of AgCl Some people put H2O which would give fumes of HCl.

    Lidocaine 9 peaks is a or b more likely to be protonated, why?
    B because ethyl group is electron releasing so the lone electron pair on N is more available because benzene ring is withdrawing the pair at A.

    percentage mass of H = 9.4% because 22/233
    why isn’t Ka used in tables for strong acids? (2)
    Strong acids fully dissociates
    Ka = [H+]??
    Relationship between stereoisomers (1 or 2)
    mirror images and non-superimposable?

    Rate equation when propanone or something is increased
    rate = [H+]
    because propanone is in excess, H+ is limiting factor.

    Why is “X” zero order?
    not involved in the RDS and rate equation

    with knowledge of carbonyls, deduce name of mechanism
    haemacetol ****
    Nucleophilic addition

    1-Chloroethanoylchloride

    some other compound was 2-amino-4-methylpentanoic acid

    why is lidocaine hydrochloride used in medicine?
    More soluble as it forms stronger hydrogen bonds?the other one was polar too but lidocaine had N-H+

    Organic synthesis question (4)?
    KCN aqueous and ethanolHydrogen and Ni catalyst high pressure or LiAlH4 in ether

    orders of reaction (2)
    first, 1
    second, 2

    Why are “X” sutures used in medicine instead of polythene one? (2 or 3)
    Because X has polar bonds so is susceptible to attack by nucleophiles such as H2O in body so no need to remove it as it will break down over time. other one was non-polar

    Benzene one was just acylation
    electrophile CH3CO+
    [ClCH2COCl]+* -> [COCl+](37Cl) + [ClCH2*](35 Cl)

    Q9 potassium dichromate turned green so must be primary or secondary HO-CH2-C(CH3)2-O-CH3??

    This one is by bluhbluhbluh
    Equation representing ethanoic acid as a base with nitric acid:

    CH3COOH + HNO3 -> CH3CO+NO3- + H2O

    Identify what reagents are needed to distinguish between the two chlorine molecules, incl. observation. (Acyl chloride vs haloalkane (?))
    Reagent: water/AgNO3 (?)/named carbonate
    Observation: HCl fumes/White ppt./Effervescence -for the acyl chloride.

    Which Nitrogen acts as a stronger base a vs. b (where a is attached to the benzene, and b is a tertiary amine) and why?
    Nitrogen b due to the positive inductive effect of the carbons on the nitrogen, therefore making the lone pairs more available. Consequently nitrogen a is joined to a benzene so the lone pair delocalises into the benzene ring.

    Order of reactions:
    D 1
    E 2

    What is a racemic mixture?
    Mixture containing equal quantities of each enantiomer.

    How to distinguish between the two:
    Expose to plane polarised light, rotates in opposite directions

    Chiral Carbon relationship:
    They are both mirror images of each other/enantiomers.

    4.
    Electrophile structure: CH3C(+)O

    Reaction mechanism:
    Arrow from benzene ring to C+ in CH3C(+)O, then arrow from hydrogen attached to the same carbon as the CH3CO back into the incomplete benzene ring (from carbon 1-4 i believe).

    5. Can't remember exact questions, let me know i'll update ! =)
    Notes:
    Zwitterions have R-N(+)H3 and a COO-
    Hydrogen bond between two amino acids: presumably show any partial bond from one hydrogen on one amino acid to either the nitrogen or oxygen on the other.
    Dipeptides- essentially amides formed from 2 amino acids.

    Advantage of using the polyester suture rather than the polyalkene?
    the ester bond in polyester can hydrolyse whereas polyalkene C-C, C-H bonds are too strong thus cannot. Therefore the former can dissolve naturally whereas the former would require manual removal.

    7. Conc. Of propanone increases by 100, What is the new rate equation and explain pls:
    rate = k[H+]Due to the large excess of propanone, it is no longer affecting the rate as only the H+ concentration limits the speed at which the reaction proceeds.

    Why is the initial rate of the reaction used to determine the order of the reactants rather than at any other point during the reaction? Or something
    Initial rate takes into account the initial concentration of the reactants which is more accurate since, as the reaction proceeds the concentrations of the different species change. (?)

    How do you calculate the initial rate of the reaction from a graph of concentration against time?
    Calculate the gradient of the tangent to the slope. (?)

    8. Organic synthesis to produce the branched primary amine:
    Structure: Same as previous however Br is substituted with CN
    Step 3: KCN w/ ethanol (?)
    Step 4: H2/Ni, LiAlH4

    Question 9 Notes
    Potassium dichromate turns green, either a primary or secondary alcohol, or aldehyde
    O-H absorption in the Mass spec
    O-H in NMR spectrum, singlet obvs
    CH3-O present in NMR spectrum, singlet
    RCH2 in NMR spectrum, triplet ?
    R(CH3)2 in NMR spectrum, doublet ?
    Wasn't it 2-chloroethanoyl chloride? Carbon 1 is on the R-COCl group.
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    (Original post by shiney101)
    I'm pretty sure apart from heat was mentioned in part b not a though
    I put dilute but I didn't put aqueous aswell,do you think I'll still get the mark?
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    (Original post by arsan)
    C6H1402

    61% Carbon

    11.9% Hydrogen

    27.1% Oxygen (100-11.9-61)

    what was that question, that's as much as I can remember!!
    Oh was that the one that wanted you to prove the molecular formula for the compound was C6H14O2?
    It was 3 marks.
    It gave you the percentages of Carbon and Hydrogen (so you could work out Oxygen) and then gave you an m/z ratio.
    Totally messed that one up. Couldn't do it. Did anyone manage to do it?
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    (Original post by Saldudsjen123)
    I put dilute but I didn't put aqueous aswell,do you think I'll still get the mark?
    Need ethanolic and aqueous (or just ethanolic, depending on how strict the MS is).

    Aqueous is different from dilute - something can still be aqueous and very concentrated.
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    (Original post by mgill17)
    Oh was that the one that wanted you to prove the molecular formula for the compound was C6H14O2?
    It was 3 marks.
    It gave you the percentages of Carbon and Hydrogen (so you could work out Oxygen) and then gave you an m/z ratio.
    Totally messed that one up. Couldn't do it. Did anyone manage to do it?
    Think so - you get the empirical formula by getting ratio of C : H : O.

    Then, you multiply the empirical formula by ratio (Mr of molecular ion fragment/Mr of empirical formula), which multiplies the empirical formula by 2 to get the molecular formula.
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    Can someone make a Facebook page?
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    (Original post by Cadherin)
    Think so - you get the empirical formula by getting ratio of C : H : O.

    Then, you multiply the empirical formula by ratio (Mr of molecular ion fragment/Mr of empirical formula), which multiplies the empirical formula by 2 to get the molecular formula.
    As soon as I read it I thought empirical formula.
    I did that first but the numbers I got were really odd so I must've done my maths wrong somewhere. Maybe I divided the wrong way around that's something I'd do.

    Hopefully I can get two for working out the % of O then the empirical formula..
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    (Original post by Cadherin)
    Wasn't it 2-chloroethanoyl chloride? Carbon 1 is on the R-COCl group.
    There was only 1 place the chlorine could be so the number isnt needed, but I think 1- is wrong whereas 2- would be in brackets in the MS
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    (Original post by Anon_98)
    I'm sorry but I'm actually still traumatised.- No comment.
    same only just stopped feeling sick over it 🤕🤕
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    (Original post by logicmaryam)
    Heey guys, I think we can all agree this paper was one of the hardest CHEM4 papers ever.
    We'll discuss the answers and compile an unofficial mark scheme. Post your answers here!


    Q1 20 marks
    a) Ethanoic acid in water
    CH3COOH ⇌ CH3COO- + H+ (1)
    b) Ethanoic acid as base with HNO3
    HNO3 + CH3COOH = CH3CHOOH2+ + NO3- (1)
    c) pH of acid? 2.08 (3)d) pH when alkali added to acid 2.32 (4)
    e) [salt]/[acid] = 3.33 (3) or 1.20?

    For the Kc - 9.09x10^3?
    k= 3.12x10-5 mol-1 dm3 s-1

    Q8
    Condition for step 1 (3)NaOH heat under reflux?
    use distillation or filtration to separate

    test for acyl chloride Q and some chlorohaloalkane P
    acidified AgNO3
    P - NVC
    Q - white ppt of AgCl Some people put H2O which would give fumes of HCl.

    Lidocaine 9 peaks is a or b more likely to be protonated, why?
    B because ethyl group is electron releasing so the lone electron pair on N is more available because benzene ring is withdrawing the pair at A.

    percentage mass of H = 9.4% because 22/233
    why isn’t Ka used in tables for strong acids? (2)
    Strong acids fully dissociates
    Ka = [H+]??
    Relationship between stereoisomers (1 or 2)
    mirror images and non-superimposable?

    Rate equation when propanone or something is increased
    rate = [H+]
    because propanone is in excess, H+ is limiting factor.

    Why is “X” zero order?
    not involved in the RDS and rate equation

    with knowledge of carbonyls, deduce name of mechanism
    haemacetol ****
    Nucleophilic addition

    1-Chloroethanoylchloride

    some other compound was 2-amino-4-methylpentanoic acid

    why is lidocaine hydrochloride used in medicine?
    More soluble as it forms stronger hydrogen bonds?the other one was polar too but lidocaine had N-H+

    Organic synthesis question (4)?
    KCN aqueous and ethanolHydrogen and Ni catalyst high pressure or LiAlH4 in ether

    orders of reaction (2)
    first, 1
    second, 2

    Why are “X” sutures used in medicine instead of polythene one? (2 or 3)
    Because X has polar bonds so is susceptible to attack by nucleophiles such as H2O in body so no need to remove it as it will break down over time. other one was non-polar

    Benzene one was just acylation
    electrophile CH3CO+
    [ClCH2COCl]+* -> [COCl+](37Cl) + [ClCH2*](35 Cl)

    Q9 potassium dichromate turned green so must be primary or secondary HO-CH2-C(CH3)2-O-CH3??

    This one is by bluhbluhbluh
    Equation representing ethanoic acid as a base with nitric acid:

    CH3COOH + HNO3 -> CH3CO+NO3- + H2O

    Identify what reagents are needed to distinguish between the two chlorine molecules, incl. observation. (Acyl chloride vs haloalkane (?))
    Reagent: water/AgNO3 (?)/named carbonate
    Observation: HCl fumes/White ppt./Effervescence -for the acyl chloride.

    Which Nitrogen acts as a stronger base a vs. b (where a is attached to the benzene, and b is a tertiary amine) and why?
    Nitrogen b due to the positive inductive effect of the carbons on the nitrogen, therefore making the lone pairs more available. Consequently nitrogen a is joined to a benzene so the lone pair delocalises into the benzene ring.

    Order of reactions:
    D 1
    E 2

    What is a racemic mixture?
    Mixture containing equal quantities of each enantiomer.

    How to distinguish between the two:
    Expose to plane polarised light, rotates in opposite directions

    Chiral Carbon relationship:
    They are both mirror images of each other/enantiomers.

    4.
    Electrophile structure: CH3C(+)O

    Reaction mechanism:
    Arrow from benzene ring to C+ in CH3C(+)O, then arrow from hydrogen attached to the same carbon as the CH3CO back into the incomplete benzene ring (from carbon 1-4 i believe).

    5. Can't remember exact questions, let me know i'll update ! =)
    Notes:
    Zwitterions have R-N(+)H3 and a COO-
    Hydrogen bond between two amino acids: presumably show any partial bond from one hydrogen on one amino acid to either the nitrogen or oxygen on the other.
    Dipeptides- essentially amides formed from 2 amino acids.

    Advantage of using the polyester suture rather than the polyalkene?
    the ester bond in polyester can hydrolyse whereas polyalkene C-C, C-H bonds are too strong thus cannot. Therefore the former can dissolve naturally whereas the former would require manual removal.

    7. Conc. Of propanone increases by 100, What is the new rate equation and explain pls:
    rate = k[H+]Due to the large excess of propanone, it is no longer affecting the rate as only the H+ concentration limits the speed at which the reaction proceeds.

    Why is the initial rate of the reaction used to determine the order of the reactants rather than at any other point during the reaction? Or something
    Initial rate takes into account the initial concentration of the reactants which is more accurate since, as the reaction proceeds the concentrations of the different species change. (?)

    How do you calculate the initial rate of the reaction from a graph of concentration against time?
    Calculate the gradient of the tangent to the slope. (?)

    8. Organic synthesis to produce the branched primary amine:
    Structure: Same as previous however Br is substituted with CN
    Step 3: KCN w/ ethanol (?)
    Step 4: H2/Ni, LiAlH4

    Question 9 Notes
    Potassium dichromate turns green, either a primary or secondary alcohol, or aldehyde
    O-H absorption in the Mass spec
    O-H in NMR spectrum, singlet obvs
    CH3-O present in NMR spectrum, singlet
    RCH2 in NMR spectrum, triplet ?
    R(CH3)2 in NMR spectrum, doublet ?
    For qs 4 the electrophile structure was different im sure?
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    (Original post by mgill17)
    Oh was that the one that wanted you to prove the molecular formula for the compound was C6H14O2?
    It was 3 marks.
    It gave you the percentages of Carbon and Hydrogen (so you could work out Oxygen) and then gave you an m/z ratio.
    Totally messed that one up. Couldn't do it. Did anyone manage to do it?
    yeah work out the empirical formula using the data given, then work out the Mr of the molecules empirical formula, then divide the Mr given by this Mr and you'll find you get 2, so you multiply the empirical formula by 2. Looks like they couldn't think of anything to put on the paper so they put a chem 1 question, disgusting.
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    (Original post by Cadherin)
    Need ethanolic and aqueous (or just ethanolic, depending on how strict the MS is).

    Aqueous is different from dilute - something can still be aqueous and very concentrated.
    I'm not talking about the KCN question I'm talking about the NaOH base hydrolysis of the ester
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    (Original post by mgill17)
    Oh was that the one that wanted you to prove the molecular formula for the compound was C6H14O2?
    It was 3 marks.
    It gave you the percentages of Carbon and Hydrogen (so you could work out Oxygen) and then gave you an m/z ratio.
    Totally messed that one up. Couldn't do it. Did anyone manage to do it?
    I did but pretty much last minute tbh :/
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    (Original post by Saldudsjen123)
    I'm not talking about the KCN question I'm talking about the NaOH base hydrolysis of the ester
    Sorry!

    So, reagent is NaOH and for a base hydrolysis like that, you need to reflux it (so high temperature).
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    (Original post by Lolaturface)
    yeah work out the empirical formula using the data given, then work out the Mr of the molecules empirical formula, then divide the Mr given by this Mr and you'll find you get 2, so you multiply the empirical formula by 2. Looks like they couldn't think of anything to put on the paper so they put a chem 1 question, disgusting.
    I'm guessing I've got 2 of the marks then. I don't know whether it's a bad thing or a good thing I didn't have to resit unit one because I would've actually have revised empirical formula then, I tried to work out an empirical formula with minimal confidence in what I was doing, then I think I did the last step the wrong way around...thanks aqa
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    (Original post by Anam)
    You know the one about A or B being protonated i put B and said at A the lone pair on the N is delocalised into the ring/interacts with the delocalised electrons so N is less available for bonding (negative inductive effect) is that right
    I said that.
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    how did you work out salt/acid question
    • Thread Starter
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    (Original post by Cadherin)
    Wasn't it 2-chloroethanoyl chloride? Carbon 1 is on the R-COCl group.
    oops typo! will change it now
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    (Original post by haj101)
    I did but pretty much last minute tbh :/
    It was synoptic and incredibly sneaky of aqa. They're the worst
 
 
 
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