CHEM 5 JUNE 12 Section A Unofficial Mark Scheme

This is Section A, obviously loads/most are wrong because I thought this was a ridiculously hard exam, and a lot of it unrelated. I still don’t think boundaries will change too much because AQA aren’t like that, 70 for an A i reckon. Everyone edit and let’s try and come up with a proper mark scheme

Question 1 is 9 marks

1a) Why is Phosporous Oxide stored in water? – Reacts violently in air
b) Why P4O10 not P2O5? – P4O10 is it in its most stable form under standard conditions
c) Equation = P4O10 + 6H2O = 4H3PO4
pH = 0
d) equation for MgO + H3PO4 – No idea
e)Why can you use excess MgO? Because H3PO4 is a strong acid
f) Why is NaOH an environmental risk? Strong alkaline harms fish?

Question 2 is 7 marks
2a) Equation – G = H – TS
b)Gradient = 0.1 Units = J K mol-1 Symbol = S
c) Why is significance of temperatures below the temp line – delta G is negative so reactions are feasible
d) Why does curve slope? – Change of state, freezing.

Question 3 is 10 marks
3a) delta H =
b)Delta S =
c) T = 700 K? Can’t remember the other 2 but they gave this temperature.
d) why would NO2 not form? – Not 700K
e) N2 + O2 = NO2
f) same number of moles so small entropy so small TdeltaS

Question 4 is 12 marks
4a) Define enthalpy of electron affinity – Enthalpy change when 1 mole of electrons added to an isolated atom in the gas phase
b) Why is F- more electronegative – more shells so more and further from nucleus
c) Why does F- have more negative enthalpy of hydration? No idea
d) Calculation – can’t remember I got 900 something I think
e) Why is it always feasible – delta G always negative because entropy positive and enthalpy change is negative?
f) Can’t remember but it was 2 marks

Question 5 (a disgrace) was 17 marks
5a) Weakest oxidising agent – CdO(OH) because it was most negative.
b) emf = 1.1? can’t remember
c) Equation – cant remember
d) Carbon anode – conduct electricity
e) Essential property of porous cover? – Inert
f) Why leakage? Ions used up
g) +3 and reverse equation
h) Environmental problem? More waste/to landfill
Two ethanol (wtf) fuel cell questions
I) Carbon neutral as no net emission into atmosphere as ethanol comes from plants that photosynthesise.

Question 6 was 10 marks
6a) Coordinate bond
b) O atom acts as Lewis base and donates electrons
Metal ion acts as Lewis Acid and accepts electrons.
c) 180 degrees
d) ratio 3:5 and calculation – no idea

Someone else do section B??

Reply 1

Yosemite80

my thoughts only, may be wrong...

i thought 2d was condensing o.o but any change in state would give a diff gradient though right?
4c Cl- has more shells than F-, just saying, so F- attracts e- more so more electronegative
5c i said greater charge density so polarises water more, not sure if right
4a did you have to say one atom or one mol? q was kinda badly worded and 1 mol electron is kinda too much for one isolated atom lol
5b emf i got .23V, as cell emf was 1V but half cell emf was 1.23V, so 1.23-X=1, X=0.23
5f i got zinc cathode is sacrificed as it is oxidised to zn ions so thickness of zn coating reduced
6d final answer 96.3 or over 100 if used given ratio
5e can you also say allows ions to pass between solutions?

glad to know it was CdO(OH) for 5a :3
oh and thx for this :3

(edited 11 years ago)

Reply 2

chill543

Two ethanol (wtf) fuel cell questions
made me LMAO :')

Reply 3

Yosemite80

one of dH or dS was 70.5 but forgot sign... might have been negative
2b units 0.1kJmol^-1K^-1 or 100JK^-1mol^-1, as the axes were in kJmol^-1
my guess for 1d is 3MgO+2H3PO4->Mg3(PO4)2+3H2O, only one which made some sense...
3e is 2NO, they asked for nitrogen monoxide i think

(edited 11 years ago)

Reply 4

Josh1993

Original post by TheBigDog

This is Section A, obviously loads/most are wrong because I thought this was a ridiculously hard exam, and a lot of it unrelated. I still don’t think boundaries will change too much because AQA aren’t like that, 70 for an A i reckon. Everyone edit and let’s try and come up with a proper mark scheme

Question 1 is 9 marks

1a) Why is Phosporous Oxide stored in water? – Reacts violently in air
b) Why P4O10 not P2O5? – P4O10 is it in its most stable form under standard conditions
c) Equation = P4O10 + 6H2O = 4H3PO4
pH = 0
d) equation for MgO + H3PO4 – No idea
e)Why can you use excess MgO? Because H3PO4 is a strong acid
f) Why is NaOH an environmental risk? Strong alkaline harms fish?

Question 2 is 7 marks
2a) Equation – G = H – TS
b)Gradient = 0.1 Units = J K mol-1 Symbol = S
c) Why is significance of temperatures below the temp line – delta G is negative so reactions are feasible
d) Why does curve slope? – Change of state, freezing.

Question 3 is 10 marks
3a) delta H =
b)Delta S =
c) T = 700 K? Can’t remember the other 2 but they gave this temperature.
d) why would NO2 not form? – Not 700K
e) N2 + O2 = NO2
f) same number of moles so small entropy so small TdeltaS

Question 4 is 12 marks
4a) Define enthalpy of electron affinity – Enthalpy change when 1 mole of electrons added to an isolated atom in the gas phase
b) Why is F- more electronegative – more shells so more and further from nucleus
c) Why does F- have more negative enthalpy of hydration? No idea
d) Calculation – can’t remember I got 900 something I think
e) Why is it always feasible – delta G always negative because entropy positive and enthalpy change is negative?
f) Can’t remember but it was 2 marks

Question 5 (a disgrace) was 17 marks
5a) Weakest oxidising agent – CdO(OH) because it was most negative.
b) emf = 1.1? can’t remember
c) Equation – cant remember
d) Carbon anode – conduct electricity
e) Essential property of porous cover? – Inert
f) Why leakage? Ions used up
g) +3 and reverse equation
h) Environmental problem? More waste/to landfill
Two ethanol (wtf) fuel cell questions
I) Carbon neutral as no net emission into atmosphere as ethanol comes from plants that photosynthesise.

Question 6 was 10 marks
6a) Coordinate bond
b) O atom acts as Lewis base and donates electrons
Metal ion acts as Lewis Acid and accepts electrons.
c) 180 degrees
d) ratio 3:5 and calculation – no idea

Someone else do section B??

Have you got the QP if so I will do one if you like.

Reply 5

MissMarple1911

Section B

7(a)Cr₂O₇^2- + 3Zn = Cr³⁺ + 7H₂O + 3Zn²⁺
2Cr ³⁺ + Zn = 2Cr²⁺ + Zn²⁺
Orange = Green = Blue (5 Marks)
7(b) [Cr(H₂O)₆]³⁺ + 3OH^- = [Cr(H₂O)₃(OH)₃]+ 3H₂O +
Green solution = grey-green ppt
[Cr(H₂O)₃(OH)₃] + OH^- = [Cr(H₂O)₂(OH)₄]^- + H₂O
Grey-green ppt = green solution (4 marks)
7(c) Ligand subsitution
Different ligand makes energy gap between d orbitals different so electrons require more/less energy light from visible spectrum to excite??
7(d) overall reaction has a postive +1.67V EMF
CrCO₃
[Cr(H₂O)₃(OH)₃]
Cr³⁺ more highly charged pulls electron density from coordinate bonds more, weakening OH bond in H₂O making it more susceptible to hydrolysis/deprotonation and more acidic. Therefore it is acidic enough to react with the carbonate ion to release carbon dioxide but the Cr²⁺ is not so the metal carbonate is formed. (7 marks)
8(a) (12 marks)
8(b) 2SO3^2- + O₂ = 2SO₄^2-
4CO²⁺ +4H⁺ + O₂ = 4CO³⁺ + 2H₂O
2CO³⁺ + H₂O + SO₃^2- = 2CO²⁺ + SO₄^2- + 2H⁺
Faster with CO²⁺ as in same phase and oppositely charged so collison more likely (4 marks)

(edited 11 years ago)

Reply 6

Josh1993

Please has someone got the paper, I can be sure that my answers will be reliable and accuarate

Reply 7

Kidneyjean

7

Original post by TheBigDog

x

Thanks for starting one of these!

For 1d): 3MgO + 2H₃PO₄ → Mg₃(PO₄)₂ + 3H₂O (treated it like a normal acid-base reaction?)

For 1e): Excess MgO can be used because it's not very soluble in water, so the excess won't dissociate and dangerously lower the pH of the lake.

EDIT: Remembered what I put for those awful ethanol fuel-cell questions (can't remember the question numbers though):

Overall reaction: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Reaction at ethanol electrode (anode): C₂H₅OH + 3H₂O → 2CO₂ + 12H⁺ + 12e⁻

(edited 11 years ago)

Reply 8

Edward1994

1)d) Mg3(PO4)2 + 3H2O
e) Dissociated weakly, pH approx 8, therefore not far from neutral.

2)b) kJ mol-1 K-1 (if you got 0.1, that is)
d) Liquid, not solid? (was going from a gas initially)
c) 794K
d) High activation energy?
e) 2NO, not NO2

4)b) F- has fewer electron shells, less shielding, therefore greater tendency to attract electrons in a covalent bond.
c) Attracts a greater number of delta+ H in the H2O, hence more exothermic.

5)a) Cd (s) The electrons were on the CdO...whatever side, so has to be the solid.
e) Allows the flow on ions (salt bridge).

Those are my few contributions.

If someone posts up the Section B questions, I'll be happy to give remembering those a go too.

Reply 9

Edward1994

1)d) Mg3(PO4)2 + 3H2O
e) Dissociated weakly, pH approx 8, therefore not far from neutral.

2)b) kJ mol-1 K-1 (if you got 0.1, that is)
d) Liquid, not solid? (was going from a gas initially)
c) 794K
d) High activation energy?
e) 2NO, not NO2

4)b) F- has fewer electron shells, less shielding, therefore greater tendency to attract electrons in a covalent bond.
c) Attracts a greater number of delta+ H in the H2O, hence more exothermic.

5)a) Cd (s) The electrons were on the CdO...whatever side, so has to be the solid.
e) Allows the flow on ions (salt bridge).

Those are my few contributions.

If someone posts up the Section B questions, I'll be happy to give remembering those a go too.

Reply 10

Yosemite80

Original post by MissMarple1911

Section B

7(a)Cr₂O₇^2- + 3Zn = Cr³⁺ + 7H₂O + 3Zn²⁺
2Cr ³⁺ + Zn = 2Cr²⁺ + Zn²⁺
Orange = Green = Blue (5 Marks)
7(b) [Cr(H₂O)₆]³⁺ + 3OH^- = [Cr(H₂O)₃(OH)₃]+ 3H₂O +
Green solution = grey-green ppt
[Cr(H₂O)₃(OH)₃] + OH^- = [Cr(H₂O)₂(OH)₄]^- + H₂O
Grey-green ppt = green solution (4 marks)
7(c) Ligand subsitution
Different ligand makes energy gap between d orbitals different so electrons require more/less energy light from visible spectrum to excite??
7(d) overall reaction has a postive +1.67V EMF
CrCO₃
[Cr(H₂O)₃(OH)₃]
Cr³⁺ more highly charged pulls electron density from coordinate bonds more, weakening OH bond in H₂O making it more susceptible to hydrolysis/deprotonation and more acidic. Therefore it is acidic enough to react with the carbonate ion to release carbon dioxide but the Cr²⁺ is not so the metal carbonate is formed. (7 marks)
8(a) (12 marks)
8(b) 2SO3^2- + O₂ = 2SO₄^2-
4CO²⁺ +4H⁺ + O₂ = 4CO³⁺ + 2H₂O
2CO³⁺ + H₂O + SO₃^2- = 2CO²⁺ + SO₄^2- + 2H⁺
Faster with CO²⁺ as in same phase and oppositely charged so collison more likely (4 marks)

doesnt Cr dissolve in xs OH-, so would go to Cr(OH)6^3-?
going by this MS i think i got about 80/100 :/

(edited 11 years ago)

Reply 11

westham1313

4

How ccan you remember all the questions? do you have the QP? and if you do can you put it on here please :smile:

Reply 12

Polioz

14

the Zinc EMF value was the most negative not the Cd one.. so the mots weakest oxidising agent was the most powerfull reducing agent which was Zn (s)... I think anyways

Why would answer to pouros material be inert ? its stated in the question its non reactive, so we would just be repeating question.

Reply 13

kahmed

Original post by Polioz

the Zinc EMF value was the most negative not the Cd one.. so the mots weakest oxidising agent was the most powerfull reducing agent which was Zn (s)... I think anyways

Why would answer to pouros material be inert ? its stated in the question its non reactive, so we would just be repeating question.

I could be wrong but I said that it can't conduct electricity

Reply 14

m1a1tank

1

On 3c) where you lot got 794K, i think i got -794K because i think the entropy or enthalpy, can't remember which one, i got it as -70.5.

Did anyone else get this?

Reply 15

Polioz

14

Original post by m1a1tank

On 3c) where you lot got 794K, i think i got -794K because i think the entropy or enthalpy, can't remember which one, i got it as -70.5.

Did anyone else get this?

I defintely got -70.5 somewhere

Reply 16

Polioz

14

Original post by kahmed

I could be wrong but I said that it can't conduct electricity

I thought it had to be able to conduct electricity otherwise there would be no circuit ? this paper was bullsheet

Reply 17

Yosemite80

Original post by Polioz

the Zinc EMF value was the most negative not the Cd one.. so the mots weakest oxidising agent was the most powerfull reducing agent which was Zn (s)... I think anyways

Why would answer to pouros material be inert ? its stated in the question its non reactive, so we would just be repeating question.

i said it acts as a salt bridge so lets ions pass between solutions

Reply 18

authentic

1

was it define electronegativity or electron affinity? I swear it was negativity- I wrote the ability of an electron to withdraw electron density to itself in a covalent bond..