2019 OCR Chemistry (A) - Paper 3: Unified Chemistry [Unofficial Mark Scheme]

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Question 1 - Mixed Chemistry Question

Why does CF₄ have no overall dipole? [2 marks]
Each C-F bond is polarised due to F electronegativity, but all bonds have this dipole, so in every direction, the dipoles cancel out, leaving a symmetrical molecule

Water as a 20gmol^-1 mass [2 marks]
Small percentage of water made up of isotopes of Hydrogen (Deuterium) which have mass of 2g rather than 1g for H. Therefore, small percentage of water exists as D₂O, hence 20gmol^-1

Partial Pressure [1 mark]
Just multiply the percentage (as a decimal) by the total pressure for the partial of O₂ - can't remember the answer

Mass of CO₂formed [2 marks]
Write out the combustion equation, work out the moles of propane (using n=v/24), then moles of CO2, and work out the mass by x by Mr of CO2

New rate of reaction [1 mark]
Can either:

•

Work out rate constant of first reaction, then work out the new rate from the concentration of H+ in second mixture

•

Use proportion to work out the proportional difference in rate, as a proportion of the change of H+ (new rate was x10^{-5 /}100th of the value given in question)

Atoms of oxygen [1 mark]/[2 marks] (cant remember)
Work out moles of substance, then * by Avogadro's number, then * by number of oxygens in the formula (can't remember the number)

Question 2 - Organic Practical (Benzoic Acid)

•

Equation for alkaline conditions: CO₃^2- + H₂O --> CO₂ + 2OH^-[1 mark]

•

Need to react with an HCl for the conjugate base (sodium benzoate) to accept a proton and form the acid (alkaline hydrolysis makes the salt). Also the salt is ionic which is soluble in water, whereas benzoic acid isn't (even though it can form hydrogen bonds), hence the crystals will precipitate out) [2 marks]

•

Percentage yield = 33% (I think) [3 marks]

•

Recrystallisation - Dissolve in min amount of hot solvent, filter insoluble impurities from the hot mixture, allow to cool. Crystal will form on cooling, filter under reduced pressure. Then wash in cold solvent and dry for pure product. [2 marks]

Question 3 - Group 4 Chemistry

•

Equation for reaction: Lead oxide + methane --> Lead + CO₂ + H₂O (needs to be balanced - cant remember the numbers - could have worked it out using redox method) [1 mark]

•

Safety precaution - Carry out in a fume cupboard because Pb is a toxic metal [1/2 marks]

•

Methods for improvement - Heat until constant mass is obtained, heat for longer (unsure of these) [2 marks]

•

Empirical formula - Pb₃O₄[2 marks]

•

SiO2 is giant covalent between ATOMS, other molecules was simple molecular, with London forces and permanent dipoles between MOLECULES. Giant covalent need more energy to break because they're stronger, hence a higher MP [4 marks]

Question 4 - Isomers and related things

•

1 chiral centre at the bottom of the hexagon [1 mark]

•

E/Z could happen because of the double bond, the molecule was a Z isomer, and it could never be E because it had a cyclic ring. The ring cannot be formed if the molecule was an E isomer as 2 points would never join to each other. [4 marks]

•

Tests for each functional group - Alkene = Bromine water, would decolourise and form a new compound with Br attached at both sides of double bond. Alcohol = Cr2O72-/H+, was tertiary alcohol, so no colour change, the molecule/product would be the same as the original [4 marks]

•

New molecule with new functional groups - had Phenol on it - had to test for a phenol (Bromine water, decolourised upon adding, forms a white ppt. - unsure about the ppt because there were limited areas for substitution) [2 marks]

•

Detol Ka Calculation - 1.71x10^-10 I think (can't remember the exact number) [5 marks]

Question 5 - Enthalpy and Entropy stuff

•

6 Marker - Use Q=mcdeltaT for the first equation, with the mass found by subtraction of the table on the left (think it was 50.7g), calc moles of copper sulphate and divide Q by the answer to get enthalpy change of solution. Use Hess's law cycle for the enthalpy of reaction - was exothermic, around -60kJmol^-1 (cant remember exact)

•

Standard entropy calculation - +156 [4 marks]

•

Can't remember any of the other questions in this section

Question 6 - Transition Elements & Reactions

•

Flow diagram - A: [Fe(H₂O)₆]³⁺B: Ag₂S [2 marks]

•

Equation to form A: 2[Fe(H₂O)₆]2⁺+ Cl₂ --> 2[Fe(H₂O)₆]³⁺ + 2Cl^-[1 mark]

•

Equation for Manganese Reaction - Standard redox reaction of MnO4- to Mn2+ combined with H2S to S (Overall = 2MnO₄^- + 5H₂S + 6H⁺ --> 2Mn²⁺ + 5S + 8H₂O [2 marks]

6 Marker -

•

Work out the mass of water lost, moles of water lost, and then divide this by the moles of the whole crystallised compound to get the number of waters of crystallisation (9H₂O), overall compound was Fe(NO₃)₃

•

When burnt, the oxide has to be Fe₂O₃as the Iron is definitely Iron(III). Overall gas volume was 270cm³, but one of the gases remained a gas afterwards, which allows you to identify the amounts of each gas in this 270.

•

Can work out the moles of the gas that formed a solid (n=v/24), and then work out this gases Mr (Mr=m/n), which gave 46. This corresponds to NO₂

•

The other information given allowed you to identify O₂. However, this could be deduced from what was burning, as this was the only other possible gas from the burning of the salt. It could also be calculated (cant remember how I did it).

Feel like I've missed a few questions, but I cant remember them
If I've missed anything/got anything wrong, let me know :smile:

Edit:

•

Thiosulphate question - Tetrahedral Shape, Bond angle 109.5^o[2 marks]

•

Dimer: flip the molecule around, and attach sulphur to sulphur with a single bond [1 mark]

•

NMR: 5 peaks as there was 1 plane of symmetry for the benzene environments [1 mark]

•

Temperature change from uncertainty - 1^oC [1 mark]

Total Marks I've suggested is around 70 marks (some will be wrong I think, can't remember all of the marks)

(edited 4 years ago)

Reply 1

georgew9828

OP

https://www.thestudentroom.co.uk/showthread.php?t=6008998&p=84053818#post84053818 ive done one here if you want to link it perhaps? :smile:

Reply 2

radishcoffee

5

Here are my answers - not sure if all correct so sorry about this
1. CF4 is not polar because all the dipoles cancel out in each direction - each bond is polar because of F being more electronegative
water may have 20 MR because of deuterium
partial pressure was 21000Pa
231000 g of CO2 produced from something
rate when pH 3 is used was 2.4 x 10^-5
number of O2 atoms in P2O5 was 9.03x10^22
2. Carbonate half equation just need to make OH- on RHS
acid added to protonate the anion formed in alkaline conditions
percentage yield 33.8%
recrystallisation - standard mark scheme answer? (dissolve in minimal hot solvent, hot filtration etc.)
3. Question about lead - reduction to Pb equation had 8 Pb on RHS (can't remember the rest)
safety - not too sure, something about avoid heat by heat proof mat/gloves
modify experiment - use gas syringe and pass gas over until constant volume/take mass readings until 2 are the same
the empirical formula was Pb3O4
SiO2 is giant covalent lattice, CO2 is simple molecular - talk about covalent bonds requiring more energy to overcome than london forces
4. name was 4-chloro, 3,5-dimethylphenol, 5 NMR peaks
Ka of acid is 1.71 x 10^-10
E/Z because of 2 different groups attached to each C of C=C double bond, Z isomer due to CIP rules, only one because of cyclic nature of compound
to test alkene - use Ni and H2 to hydrogenate, for alcohol there are a few - I put ammonia for alkylation
5. enthalpy change is -65.7 kJmol^-1, the temperature change was 1 degree
the entropy is 156.1
structure is tetrahedral with 109.5 bond angle
dimer is connect the S together
6. transition metal question: chlorine act as oxidising agent to make into Fe3+ so ppt is Fe(OH)3 and the other thing is Ag2S
CDEFG were Fe(NO3)3.9H2O, Fe(NO3)3, Fe2O3, NO2 and O2

(edited 4 years ago)

The water of crystallisation in the last 6 marker was 9 waters not 7 waters

Reply 5

Original post by radishcoffee

Here are my answers - not sure if all correct so sorry about this
1. CF4 is not polar because all the dipoles cancel out in each direction - each bond is polar because of F being more electronegative
water may have 20 MR because of deuterium
partial pressure was 21000Pa
231000 g of CO2 produced from something
rate when pH 3 is used was 2.4 x 10^-5
number of O2 atoms in P2O5 was 9.03x10^22
2. Carbonate half equation just need to make OH- on RHS
acid added to protonate the anion formed in alkaline conditions
percentage yield 33.8%
recrystallisation - standard mark scheme answer? (dissolve in minimal hot solvent, hot filtration etc.)
3. Question about lead - reduction to Pb equation had 8 Pb on RHS (can't remember the rest)
safety - not too sure, something about avoid heat by heat proof mat/gloves
modify experiment - use gas syringe and pass gas over until constant volume/take mass readings until 2 are the same
the empirical formula was Pb3O4
SiO2 is giant covalent lattice, CO2 is simple molecular - talk about covalent bonds requiring more energy to overcome than london forces
4. name was 4-chloro, 3,5-dimethylphenol, 5 NMR peaks
Ka of acid is 1.71 x 10^-10
E/Z because of 2 different groups attached to each C of C=C double bond, Z isomer due to CIP rules, only one because of cyclic nature of compound
to test alkene - use Ni and H2 to hydrogenate, for alcohol there are a few - I put ammonia for alkylation
5. enthalpy change is -65.7 kJmol^-1, the temperature change was 1 degree
the entropy is 156.1
structure is tetrahedral with 109.5 bond angle
dimer is connect the S together
6. transition metal question: chlorine act as oxidising agent to make into Fe3+ so ppt is Fe(OH)3 and the other thing is Ag2S
CDEFG were Fe(NO3)3.9H2O, Fe(NO3)3, Fe2O3, NO2 and O2

4 peaks

Reply 6

RedGiant

22

Original post by georgew9828

Question 1 - Mixed Chemistry Question

Why does CF₄ have no overall dipole?
Each C-F bond is polarised due to F electronegativity, but all bonds have this dipole, so in every direction, the dipoles cancel out, leaving a symmetrical molecule

Water as a 20gmol^-1 mass
Small percentage of water made up of isotopes of Hydrogen (Deuterium) which have mass of 2g rather than 1g for H. Therefore, small percentage of water exists as D₂O, hence 20gmol^-1

Partial Pressure
Just multiply the percentage (as a decimal) by the total pressure for the partial of O₂ - can't remember the answer

Mass of CO₂formed
Write out the combustion equation, work out the moles of propane (using n=v/24), then moles of CO2, and work out the mass by x by Mr of CO2

New rate of reaction
Can either:

•

Work out rate constant of first reaction, then work out the new rate from the concentration of H+ in second mixture

•

Use proportion to work out the proportional difference in rate, as a proportion of the change of H+ (new rate was x10^{-5 /}100th of the value given in question)

Atoms of oxygen
Work out moles of substance, then * by Avogadro's number, then * by number of oxygens in the formula (can't remember the number)

Question 2 - Organic Practical (Benzoic Acid)

•

Equation for alkaline conditions: CO₃^2- + H₂O --> CO₂ + 2OH^-

•

Need to react with an HCl for the conjugate base (sodium benzoate) to accept a proton and form the acid (alkaline hydrolysis makes the salt). Also the salt is ionic which is soluble in water, whereas benzoic acid isn't (even though it can form hydrogen bonds), hence the crystals will precipitate out)

•

Percentage yield = 33% (I think)

•

Recrystallisation - Dissolve in min amount of hot solvent, filter insoluble impurities from the hot mixture, allow to cool. Crystal will form on cooling, filter under reduced pressure. Then wash in cold solvent and dry for pure product.

Question 3 - Group 4 Chemistry

•

Equation for reaction: Lead oxide + methane --> Lead + CO₂ + H₂O (needs to be balanced - cant remember the numbers - could have worked it out using redox method)

•

Safety precaution - Carry out in a fume cupboard because Pb is a toxic metal

•

Methods for improvement - Heat until constant mass is obtained, heat for longer (unsure of these)

•

Empirical formula - Pb₃O₄

•

SiO2 is giant covalent between ATOMS, other molecules was simple molecular, with London forces and permanent dipoles between MOLECULES. Giant covalent need more energy to break because they're stronger, hence a higher MP

Question 4 - Isomers and related things

•

1 chiral centre at the bottom of the hexagon

•

E/Z could happen because of the double bond, the molecule was a Z isomer, and it could never be E because it had a cyclic ring. The ring cannot be formed if the molecule was an E isomer as 2 points would never join to each other

•

Tests for each functional group - Alkene = Bromine water, would decolourise and form a new compound with Br attached at both sides of double bond. Alcohol = Cr2O72-/H+, was tertiary alcohol, so no colour change, the molecule/product would be the same as the original

•

New molecule with new functional groups - had Phenol on it - had to test for a phenol (Bromine water, decolourised upon adding, forms a white ppt. - unsure about the ppt because there were limited areas for substitution)

•

Detol Ka Calculation - 1.71x10^-10 I think (can't remember the exact number)

Question 5 - Enthalpy and Entropy stuff

•

6 Marker - Use Q=mcdeltaT for the first equation, with the mass found by subtraction of the table on the left (think it was 50.7g), calc moles of copper sulphate and divide Q by the answer to get enthalpy change of solution. Use Hess's law cycle for the enthalpy of reaction - was exothermic, around -60kJmol^-1 (cant remember exact)

•

Standard entropy calculation - +156

•

Can't remember any of the other questions in this section

Question 6 - Transition Elements & Reactions

•

Flow diagram - A: [Fe(H₂O)₆]³⁺B: Ag₂S

•

Equation to form A: 2[Fe(H₂O)₆]2⁺+ Cl₂ --> 2[Fe(H₂O)₆]³⁺ + 2Cl^-

•

Equation for Manganese Reaction - Standard redox reaction of MnO4- to Mn2+ combined with H2S to S (Overall = 2MnO₄^- + H₂S + 6H⁺ --> 2Mn²⁺ + 5S + 8H₂O

6 Marker -

•

Work out the mass of water lost, moles of water lost, and then divide this by the moles of the whole crystallised compound to get the number of waters of crystallisation (7H₂O), overall compound was Fe(NO₃)₃

•

When burnt, the oxide has to be Fe₂O₃as the Iron is definitely Iron(III). Overall gas volume was 270cm³, but one of the gases remained a gas afterwards, which allows you to identify the amounts of each gas in this 270.

•

Can work out the moles of the gas that formed a solid (n=v/24), and then work out this gases Mr (Mr=m/n), which gave 46. This corresponds to NO₂

•

The other information given allowed you to identify O₂. However, this could be deduced from what was burning, as this was the only other possible gas from the burning of the salt. It could also be calculated (cant remember how I did it).

Feel like I've missed a few questions, but I cant remember them
If I've missed anything/got anything wrong, let me know :smile:

Edit:

•

Thiosulphate question - Tetrahedral Shape, Bond angle 109.5^o

•

Dimer: flip the molecule around, and attach sulphur to sulphur with a single bond

For Q4, it asked for what reagent would react only with that group, no need for a test/observation. So the alcohol one should be add a carboxylic acid and acid catalyst, to make an ester.

(edited 4 years ago)

Reply 7

jason0597

16

Also for testing for a phenol, I used a pH meter first to check if it's less than 7 (it should be), then add Na2CO3 to see if it fizzes (it doesn't), that's what was in MaChemGuy's video

Reply 8

Would I get the marks if I wrote the molecular formula as Fe N3O9, instead of having them in brackets as Fe(NO3)3?

Reply 9

jason0597

16

Original post by JJJJJAAAAMES

4 peaks

I got 5

Reply 10

Original post by jason0597

I got 5

its symettrical so 4

Reply 11

Chocolate Hobnob

9

DAMN that means I won't be getting the mark for acidified sodium bromide then because HBr adds across the double bond :angry:

Original post by RedGiant

For Q4, it asked for what reagent would react only with that group, no need for a test/observation. So the alcohol one should be add a carboxylic acid and acid catalyst, to make an ester.

Reply 12

jason0597

16

Original post by JJJJJAAAAMES

its symettrical so 4

did you include the 2 methyl groups that stick out?

Reply 13

georgew9828

OP

Original post by jason0597

The water of crystallisation in the last 6 marker was 9 waters not 7 waters

thanks i've changed it now

Reply 14

Original post by jason0597

did you include the 2 methyl groups that stick out?

ffs

Reply 15

Chocolate Hobnob

9

Bloody hell that's another mark down the pan ffs

Original post by jason0597

did you include the 2 methyl groups that stick out?

Reply 16

username4012464