AQA CHEM5 unofficial MS

The thread is a shambles so just put some solutions in order. If anyone has corrections / knows marks allocated fire away. Cheers.

Periodicity:

Covalent
P
P4O10 + 6H2O -> H3PO4

Ionic
Na
Na2O + H2O -> 2NaOH

Al2O3
Ionic
Reacts with acids and bases
3H2O + Al2O3 + 2NaOH -> 2NaAl(OH)4
Al2O3 + 6HCl -> 2AlCl3 + 3H2O
(12 marks)


Thermodynamics Calcs ****
2K(s) + ½O2(g)
2K(g) + ½O2(g)
2K(g) + O(g)
Enthalpy of Lattice dissociation = +2328kJ.mol-1
K+ ions are bigger than Na+ ions, same charge so lower charge density
Weaker attraction between K+ and O^(2-) than Na+ and O^(2-)

MgCl2 + aq --> Mg^2+(aq)+ 2Cl-(aq)
The reaction was exothermic so if temperature increased, the reaction would shift to the left so solubility would decrease
Enthalpy of solution = -155 kJ.mol-1

Enthalpy change -196
Entropy -189
G was around -135
Delta G is less than/= 0 therefore feasibleRedox Equilibria:Weakest Oxidising agent: Zn2+
Conventional cell diagram: Zn | Zn2+ || Co2+ | Co
Half equation: Zn -> Zn2+ + 2e-
Cobalt species: Co3+, oxidation product is O2
E(Au2+,Au) > E(O2, H2O) Au not oxidised by O2
Contact process V2O5 + SO2 --> V204 +SO3
V204 + 0.5 O2 --> V2O5

O2 + 4e- + 2H2O ---> 4OH- Postive electrode

H2 + 2OH- ---> 2H2O + 2e- Negative electrode

Overall: O2 + 2H2 ---> 2H2O

Increase pressure increase EMF
Graph = straight horizontal line

No effect on EMF for increase in surface area of Pt

Environmental advantage – CO2 not produced, greenhouse gas, global warming

Why H fuel cells may not be Carbon neutral?
H2 obtained by electrolysis of H2O
May use energy from combustion of fossil fuels


Cobalt Chem:
Co Atom is 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^7
Co2+ 1s2 2s2p6 3s2p6d7
Characteristic features: complex formation, coloured compounds, var. ox state. (5marks)


[Co(h2o)6]2+ + 3NH2CH2CH2NH2 --> [Co(NH2CH2CH2NH2)3]2+ + 6H2O

4 mol reactant to 7 mol product therefore deltaS positive

DeltaH roughly 0 as same type and number of bond broken

So delta G is negative and rxn is feasible

Structure of complex ion: Co central metal, 3 NH2CH2CH2NH2’s joined via 6 coord bonds to the N atoms, octahedral shape, 2+ charge aroundentire complex (7 Marks)

Chromium Chem:
P = [Cr(H2O)3(OH)3]
NaOH / any OH-[Cr(H20)6]2+ + 3OH à [Cr(H2O)3(OH)3]+ 6H20

Q = CO2
Na2CO3 / any CO3^(2-)2[Cr(H20)6]2+ + 3CO3^(2-) à 2[Cr(H2O)3(OH)3]+ 3CO2 + 3H2O

RXN 4 = [Cr(OH)6]3-
Excess NaOH / Excess of any OH-[Cr(H20)6]2+ + 6OH- à [Cr(OH)6]3-+6H2O
Zn + HCL
Blue

Redox titration
H2SO4 was the acid to tick
Reaction was slow as requires two negative ions to collide, which repulse each other, so high Ea. (I also just dropped in, little/no Mn2+formed so uncatalysed)

Calculation:Ratio of C2O4^2- to MnO4^1 was 2:5
There were 26.4cm3 of 0.02 moldm3 MnO4- = 0.000528 mols MnO4 therefore therewas 0.00132 moles C2O4^2- in 25cm3 therefore 0.0132 moles in 250cm3,

There was 3 C2O4^2- per one mole of the salt therefore there were 4.4x10-3moles salt

Mr K3(Fe(c2o4)3).3h20 = 491.1, Mass = moles x mr = 4.4x10-3 x 491.1 = 2.16g

Original mass = 2.29 therefore 2.16/2.22 * 100 = 94.4% purity( 7 Marks) – Common wrong answer of 35% is likely to be 5/7marks

Colourimetry
Make solutions with known concs of MnO4- ions Plot calibration curve of concs to lightabsorpted/transmitted
Measure absorption/transmission of light in unknown soln,
extrapolate MnO4- conc (3 Marks)

*** as pointed out by JCleggy, what I think is a reasonable end to the divideby two or not debate: “Enthalpy/Entropy calculation -

I was conscious of similarly-worded questions in the past that required you todivide to get enthalpy and entropy values for 1 mole.
However, the question asked for the 'standard enthalpy change for thereaction', with that reaction being the one given in the above equation, notthe 'standard enthalpy of formation of SO3”
The use of the word standard here applied to the conditions of the reactionbeing 100kPa and 298K, and not the creation of 1 mole of product.

This, combined with the fact that both questions were worth 2 marks rather than3 (previously released questions requiring you to do that extra division stepwere worth 3 marks) leads me to believe the values were the initial undividedones and that it's just a poorly worded question on aqa's behalf.

This said I think if you divided by 2 and gave the units kJmol^-1 you will getboth marks, and if you did not divide by 2 but gave the units kJ you willequally get both marks.”

'This said I think if you divided by 2 and gave the units kJmol^-1 you will get both marks, and if you did not divide by 2 but gave the units kJ you will equally get both marks.'
”

Ty for the shoutout but after reading what (cleverer) people have said, I retract this:

Though it is a completely understandable mistake to make, there is no logical chemical argument for dividing by 2 and I doubt aqa will award the mark for misinterpreting their question.

yo for cr3+ to cr2+, could you say h2so4 instead of hcl?
and for colourimetry could you do mno4- with some known concentrations of c2o4 and use absorbance of mn2+ to find the conc (probs wrong)

for the reaction with Na in Q1, I remember the question asking for the reaction of 'Y' which was Na with water, not the oxide as shown in the answers above?

I did that, what did you put?