OCR Chemistry A 2016 unofficial mark scheme 27/05/16

Bosssman

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Here's my attempt at an unofficial mark scheme, it is for the OCR Chemistry A, new spec, breadth in chemistry paper, really can only put the maths questions down, but if there are written ones I can remember I will put them down. This is only really for comparison of numbers, and general ideas

Multiple choice:
BCDACCBDCBCBBCBDDBBD [20] (if anyone happened to store their multiple choice answers on their calculator, please post them below to compare)

Similarities and differences of isotopes: Same number of protons and electrons, different number of neutrons [2]
Relative atomic mass: 63.62 [2]
No of atoms in 5.00g coin: 3.98 x 10^22 (3sf) [2]
Equation with nickel (II) oxide and nitric acid: NiO + 2HNO3 --> Ni(NO3)2 + H2O [1]
Dot and cross: [2]
Empirical formula with water of crystallisation: CrCl3 • 6H2O [3]
Percentage error of water removed mass: 1.72% [2]
Ways to reduce percentage uncertainty: use a larger mass of crystals to get larger mass of water removed, reducing % error [1]
To make sure mass of water removed is correct: reweigh and reheat, repeat until constant mass is achieved [1]
Volume of H2SO4 required to neutralise NaOH: 24.8cm^3 [3]
Oxidation: Al oxidised from 0 to +3 and H reduced from +1 to 0 [2]
Electron configuration of bromide ion: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 [1]
Displacement question: Cl2(g) + 2Br-(aq) = 2Cl- (aq) + Br2 (aq) [1] chlorine is more reactive than bromine [1]
Adding chlorine to water: benefit, kills bacteria, risk, chlorine is toxic [1]
Reagent for halide precipitate: AgNO3 (aq) [1]
Halogen precipitate colours: chloride, white; bromide, cream; iodide, yellow [1]
Enthalpy change of reaction: -58.5 kJmol^-1 [4]
Ionic equation: Pb2+(aq) + 2I-(aq) = PbI2 (s) [1]
Concentration of KI required: 3.3 moldm^-3 [2]
Equilibrium question: The equilibrium position will shift to the right, as the forward reaction is exothermic, in an attempt to reduce the effect of the temperature decrease. The equilibrium position will also shift to the right as there are fewer gas molecules in the products of the forward reaction, in an attempt to reduce the effect of the pressure increase. This may vary to the actual conditions as low temperature reduces rate of reaction, and it's expensive to have a high pressure. [5]
Concentration in equilibrium question: 0.876 moldm^-3 [4]
Homologous series: alkenes, general formula: CnH2n [1]
Alkene to alkane: H2, nickel catalyst, 150 degrees C [1]
Alcohol --> H2SO4 + heat --> alkene (this question was to draw the alcohol I think) [1]
Empirical formula: C2H5O [1]
Last question: step 1 reagent, Cl2/Br2/I2, step 2 reagent, NaOH (KOH and LiOH probably accepted) [3]

Any incorrect (although I don't think there are) please tell and I'll correct

Also, post about how you found the exam, as this will give a good indication of grade boundaries

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X_IDE_sidf

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Concentration in equilibrium question: 0.876 moldm^-3

I never got that I don't think

$K_c = \frac{|SO_3|^2}{|2|^2|1.6|}$ cannot remember

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laney1999

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Can you upload pics of he paper with these answers filled in?

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X_IDE_sidf

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Percentage error of water removed mass: 1.72% again I got 3.57%.
$\pm 0.01 \because$ two measurements
$\frac{0.57-0.55}{0.56}\times 100$

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Bosssman

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(Original post by X_IDE_sidf)
Concentration in equilibrium question: 0.876 moldm^-3

I never got that I don't think

$K_c = \frac{|SO_3|^2}{|2|^2|1.6|}$ cannot remember

Equilibrium constant was 0.160, concentrations given were 2.00 and 1.20 moldm^-3

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Bosssman

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(Original post by laney1999)
Can you upload pics of he paper with these answers filled in?

I don't have a copy of the exam, literally just walked out of the exam room 30 mins ago haha

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X_IDE_sidf

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(Original post by Bosssman)
Equilibrium constant was 0.156, concentrations given were 2.00 and 1.20 moldm^-3

$K_c = \frac{|SO_3|^2}{|2|^2|1.6|} = 0.156$
$\implies SO_3 = \sqrt{|2|^2|1.6|\times 0.156}$

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Bosssman

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(Original post by X_IDE_sidf)
Percentage error of water removed mass: 1.72% again I got 3.57%.
$\pm 0.01 \because$ two measurements
$\frac{0.57-0.55}{0.56}\times 100$

The ideal mass was 0.58g, and so 0.01/0.58 = 1.72% (3sf)

Edit: however my method was to use the upper and lower bound of the value, which may not be the way the spec wants

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X_IDE_sidf

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(Original post by Bosssman)
The ideal mass was 0.58g, and so 0.01/0.58 = 1.72% (3sf)

no the ideal mass was .56g and even the %age difference isn't calculated like that.

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laney1999

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(Original post by Bosssman)
I don't have a copy of the exam, literally just walked out of the exam room 30 mins ago haha

I thought you did, either that or you have a good memory to remember all the multiple choice

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Bosssman

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(Original post by X_IDE_sidf)
$K_c = \frac{|SO_3|^2}{|2|^2|1.6|} = 0.156$
$\implies SO_3 = \sqrt{|2|^2|1.6|\times 0.156}$

The concentration was 1.20 not 1.60

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Bosssman

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(Original post by laney1999)
I thought you did, either that or you have a good memory to remember all the multiple choice

I stored my answers on my calculator haha, did it in case anyone else had the same idea and so to compare multiple choice amswers

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X_IDE_sidf

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(Original post by Bosssman)
The concentration was 1.20 not 1.60

sorry, you are right, ignore me.

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Bosssman

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(Original post by X_IDE_sidf)
no the ideal mass was .56g and even the %age difference isn't calculated like that.

Also, the question stated that the percentage error was +/- 0.005g, and so two measurements is +/-0.01g, which is what I used in method

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X_IDE_sidf

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what did y'all get for the very last reactions's step one and two?

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Bosssman

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(Original post by X_IDE_sidf)
what did y'all get for the very last reactions's step one and two?

This may not be right but step 1 reagent I used as Br2, and then step 2 reagent as NaOH, based off the idea that step 1 was electropholic addition and step 2 was nucleophilic substitution

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medicapplicant

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(Original post by Bosssman)
Here's my attempt at an unofficial mark scheme, it is for the OCR Chemistry A, new spec, breadth in chemistry paper, really can only put the maths questions down, but if there are written ones I can remember I will put them down. This is only really for comparison of numbers, and general ideas

Multiple choice:
BCDACBBDCBCBBCBDDBBD

Relative atomic mass: 63.62
No of atoms in 5.00g coin: 3.98 x 10^22 (3sf)
Percentage error of water removed mass: 1.72%
Volume of H2SO4 required to neutralise NaOH: 24.3cm^3
Enthalpy change of reaction: -58.5 kJmol^-1
Concentration of KI required: 3.3 moldm^-3
Concentration in equilibrium question: 0.876 moldm^-3

Homologous series: alkenes
General formula: CnH2n

Equilibrium question: The equilibrium position will shift to the right, as the forward reaction is exothermic, in an attempt to reduce the effect of the temperature decrease. The equilibrium position will also shift to the right as there are fewer gas molecules in the products of the forward reaction, in an attempt to reduce the effect of the pressure increase. This may vary to the actual conditions as low temperature reduces rate of reaction, and it's expensive to have a high pressure.

If anyone remembers the written answers, please post and I'll add them,
Any incorrect (although I don't think there are) please tell and I'll correct

i think this is what i got for the last question
1st reagent Br2 or Cl2
halogenated compound
2nd reagent NaOh

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Bosssman

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(Original post by medicapplicant)
i think this is what i got for the last question
1st reagent Br2 or Cl2
halogenated compound
2nd reagent NaOh

Yes, so did I, just added it the original post

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medicapplicant

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(Original post by Bosssman)
Yes, so did I, just added it the original post

hey, for one on the mcq's about which compound is nonpolar did you put Z1,4 something or Z2,3 or one of the E ones

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Bosssman

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(Original post by medicapplicant)
hey, for one on the mcq's about which compound is nonpolar did you put Z1,4 something or Z2,3 or one of the E ones

Hi, it was the second E one, as this was the only one where the dipoles always cancelled out, E-2,3-dichlorobut-2-ene

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