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OCR Chemistry A F321- May 23rd Mark Scheme

MARK SCHEME MAY 23RD 2011

Q1) Proton = +1 charge, mass =1, in NUCLEUS
Neutron = 0 charge, mass = 1 in nucleus
Electron = _1 charge, mass 1/2000, on outer shell
TOTAL MARKS = 1

Q2) The first ionisation energy is the energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
TOTAL MARKS = 3

Q3) 2p orbital + 2 electrons
3s subshell = 2 electrons
4th shell = 32 electrons
TOTAL MARKS = 3

Q4) periodicity = The trends/patterns shown be elements across a period
TOTAL MARKS = 1

Q5) X, 14, 6 was C
ion with -3 charge with structure of neon was nitrogen
The metal which forms the strongest metallic bond: Al
Lowest 3rd ionisation energy was Aluminium
The last bit with the 6 ionisation energies shown below was Mg
TOTAL MARKS = 5

Q7A) MgCO3 -------> MgO + CO2
TOTAL MARKS = 1

Q7B) PRODUCT OF TEST TUBE A = MgO + CO2
PRODUCT OF TEST TUBE B = MgCl2 (aq) + CO2 (g) + H20(l)
TOTAL MARKS = 2

Q7C) SIMILARITIES = Effervescence seen as CO2 given off in both reactions
DIFFERENCES = The MgCO3 dissolves in Tube B
TOTAL MARKS = 2

Q8A) Sodium Chlorate (V)
TOTAL MARKS = 1

Q8B)Cl2(aq) + 2NaOH(aq)---------> NaClO(aq) + NaCl(aq) +H2O(l)
TOTAL MARKS = 1

Q8C) Disproportionation of chlorine, chlorine reduced +5 to -1. Chlorine oxidised +5 to+7 .
TOTAL MARKS = 2

Q8D) RAM of substance given = 203.3
MARKS=1

Q9a) Question about how you know a salt is produced:
The H+ ion in the HCl acid is replaced by the metal ion (Zn) to make ZnCl2
TOTAL MARKS = 1

Q9b) Product formed is Zn3(PO4)2
TOTAL MARKS = 1

Q10) Why not use Chlorination:
Many answers from:
- chlorine forms chlorinated hydrocarbons which are carcinogenic
- chlorine can form toxins
Why use chlorination?
Kills microorganisms/bacteria in water.
TOTAL MARKS = 2

Q11) Diagram showing Ionic Bonding between MgCl2:
ANSWER: 1 Mg ion with no outer electrons surrounded by a bracket with the charge 2+
2 chloride ions with full outer shells and each having a 1- charge outside the bracket/ OR 2 Cl ions within a bracket and -2 outside the bracket.
TOTAL MARKS = 2

Q12) How would you know that Brine has chlorine ions Include an equation for the reaction that occurs.:
1: Add dilute HNO3 to get rid of any organic ions which may interfere with reaction
2: Add aqueous silver nitrate solution
3: Would see a solid white precipitate of AgCl
4: Add a few drops dilute aqueous ammonia solution, if it dissolves instantly (goes colourless) then presence of Cl- is confirmed.
5. Equation used Ag+ + Cl- --------> AgCl
TOTAL MARKS = 4

Q13A) Diagram showing the bonding of CH3Cl:

1 central carbon atoms covalently bonded to 3 hydrogen atoms and 1 covalently bonded chlorine atom
TOTAL Marks =1

Q13B) Shape of molecule = Tetrahedral
TOTAL MARKS = 1

Q14) Titration question
a). Moles of HCL = 0.016 mol
b) 2 moles of HCL = 1 mole of the Mg(OH)2, so moles of Mg(OH)2 = 0.008 mol

TOTAL MARKS = 2 MARKS

Q14 PART 2) The one with the calculations finding out about the percentage of that chemical in that drug of 500mg.
0.008*58.3 (RAM of Mg(OH)2) = mass of Mg(OH)2. Mass of Mg(OH)2/ 0.5*100 = ANSWER.
-The percentage at the end was something like 93-94% - can't remeber what it was exactly.
TOTAL MARKS = 3 MARKS

Q15)Question on the the difference in electrical conductivity between solid and molten compound, conductivity in giant ionic lattice and metallic:
Ans: In the solid state, all the ions are held together in an ionic lattice
-The ions are held strongly by the ionic bonds and so cannot move;
but in the dissolved state, the ionic forces can be overcome enough to make the ions mobile so they are then free to move and can carry an electric current/charge with them;
Ions can not move in a solid;
Ions mobilised in liquid;
Metallic bonding holds delocalised electrons(which are mobile).

TOTAL MARKS = 3

Q16) Draw a diagram to show the hydrogen bonding between 2 ammonia molecules. Show any lone pairs and delta pos and delta neg:

ANSWER: A labelled diagram showing the hydrogen bonding between 2 ammonia molecules, from a lone pair of electrons from nitrogen to the delta + on Hydrogen... H must have delta + N must have delta - for both marks.
TOTAL MARKS = 2

Q17) Why ice melts at a higher temperature than ammonia:
Ans: Ice is made of a lattice of water molecules;
these are held together by hydrogen bonding;
In ice, the hydrogen bonding is stronger and there are more hydrogen bonds than in solid ammonia;
Ice therefore requires more energy to break the strong hydrogen bonds than the amount of energy needed to break the hydrogen bonds in solid ammonia;
Stronger intermolecular forces in ice than in ammonia;
( H2O has 2BP 2 LP, NH3 3BP 1LP. LP repel more than BP);

TOTAL MARKS = 2

Q18) Empirical formula of MgSiO:
Work out moles of Mg, Si , O. Find common ratio.
Answer: Mg2SiO4
TOTAL MARKS = 2 (1 for the working method and 1 for correct answer)

Q20) Explain the order of reactivity when adding group 2 elements to HCl in descending order:
Down the group reactivity increases;
Shielding increases down the group;
atomic radius increases down the group;
shielding more than compensates for the extra protons;
resulting in less effective nuclear charge;
Outermost electron therefore given off more easily resulting in quicker reactions down the group.
TOTAL MARKS = 5 MARKS (QWC)

Q21) The very last question on the very last page about the SiO2 and SiCl4.
Si02 has a giant covalent structure;
SiCl4 has a simple covalent structure;
Si02 has stronger intermolecular forces/VdW higher in Si02 than in SiCl4;
therefore more energy needed to overcome bonds in Si02;
SiCl4 has weak VdW;
Si02 has strong covalent bonds/ Si02 double bonded to 2 O atoms;
Double bonds > single bonds;
Possible mark for describing VdW.
TOTAL MARKS = 5 MARKS (QWC)

Gone through the other thread and picked out as much as I can, there are 60 marks there and all the answers should be right! If you think that there are other answers that could be added please say as the mark scheme always has additional options. However I have listed as many as I could think of, and the answers here are all okay and ready to compare with the paper today. If you have any problems please post, thanks! :biggrin:

(edited 12 years ago)

Scroll to see replies

Reply 1

WALMO

Expected grade boundaries:

A 45
B 38
C 31
D 24

Reply 2

George-R

Thats a U for me then :smile:

Ha ha george lol!!!!

I duno, the 2 hours of revision might have paid of and got me an E :biggrin:

Reply 5

Jankenstein

WALMO, when you steal someone's mark scheme you could at least go through it and make sure it's correct and doesn't contain comments like "not true...". Cheerio.

Reply 6

WALMO

I was only trying to make one so that it was clearer to people as to where to look to find one, and I edited several questions so that the marks for each question are correct and questions have also been filled in with lots more detail so don't start having a go when all I am doing is trying to help people know what grade they got?

Reply 7

diegos

Walmo are you sure Si02 hasnt got strong covalent bonds? thas why it had a melting point of 2000??

Reply 8

lovetoparty

Did anybody find the last question of the paper quite difficult??

Reply 9

WALMO

Original post by diegos

Walmo are you sure Si02 hasnt got strong covalent bonds? thas why it had a melting point of 2000??

I'll put it in.

cheers :smile:

Reply 10

sajaali

Some of these answers are wrong...did you make this?

Reply 11

WALMO

which answers are wrong please state... I have taken these answers from what everyone has said on the F321 OCR A May 23rd thread and from the books. Most of it should be right so please state where its wrong if any

(edited 12 years ago)

Reply 12

lovetoparty

WALMO - Thank you for the markscheme :smile:

It has helped reassure my mind a little bit! :smile:

Reply 13

diegos

Yeah thanks for going into all that effort! :biggrin:

Reply 14

sohaib34

Where did you get the predicted grade boundries from?

Reply 15

WALMO

Normal grade boundaries for standard F321 papers are 48/60 .
The paper is hard and the grade boundaries will be similar to those in January so it will be around 44-46. Doubt it'll be lower than that as there are still tons of people who would have found this exam easy. Only difference is there are a few harder questions in this paper that 'genius's' wouldn't have got right therefore the boundaries are lower.
There is usually about 7 marks difference between each grade, I put them up as people wanted some sort of guide, can't guarentee they will be perfect but should be close enough to the real thing.
A 44-46
B 37-39
C 30-32
D 23-25

Those regions would be pretty accurate :smile:

(edited 12 years ago)

Reply 16

jaez121

OMG 3 marks away from a B sooooo sad! :frown:

Reply 17

jaez121

Original post by sajaali

Some of these answers are wrong...did you make this?

Excually specially with the sub shell electrons
2p is it not 6, 2 than 32

Reply 18

sajaali

You've corrected the silicon dioxide question

For the differences and similarities question on reaction A and B: Isn't effervescence the difference because heating solid magnesium carbonate can't produce the release of gass through bubbling (theres no liquid involved) and so only happens in the reaction of acid with the magnesium carbonate.

And ice question: Isnt it because oxygen(the lone pair atom in hydrogen bonding)
has a higher electronegativity than nitrogen and so forms stronger permanent dipole dipole interactions with the electron deficient hydrogen which therefore requires greater energy to overcome.

Wasn't the disproportionation question out of three marks?

Reply 19

diegos

When you decompose magnesium you still produced co2 therefore for the similarities it's effervescence and for the differences Is that in test tube A the solid dissolves as the mgo is added to hcl which produces hydrogen and an aqueous solution?? that's what I think I may be wrong! :s