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Aqa chem 4/ chem 5 june 2016 thread

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Just a bit of an answer dump, missing loads if someone gives me question topics I'll fill if in.
Q1.
Covalent
P
P4O10 + 6H2O -> H3PO4
Ionic
Na
Na2O + H2O -> 2NaOH
Al2O3
Reacts with an acid and a base
3H2O + Al2O3 + 2NaOH -> 2NaAl(OH)4
Al2O3 + 6HCl -> 2AlCl3 + 3H2O

Q2. Standard Born Haber stuff
Lattice dissociation = +2328? Something around this value

Q7.
Enthalpy change -196
Entropy ~-160
G was around -185
Reaction is feasible as G is less than or equal to zero

Q8. First equation was just reacting it with 6 water molecules
P was Cr(H2O)3(OH)3
Reagent was anything with OH- ions

Q was CO2
Reagent was anything with CO3 2- ions

Reaction 4 product was [Cr(OH)6]3-
Excess reagent containing OH-

Not sure what the Reagent was for the last part, final thing was a blue solution.
Q9.
Calculation was 94.4%
Colorimetry talk about MnO4- ions being coloured use the intensity of the colour to measure the concentration etc.

Unassigned:
Hydrogen-Oxygen fuel cell Q:
Overall equation 2H2 + O2 -> 2H2O or multiples

Posted from TSR Mobile

G is -134 i think.
The first question you had to say it was P4O10 not just P because it said X is an oxide . there was the two questions increase in pressure how it affect emf . and how surface area of platinum will affect emf if anyone can plz confirm what the answers for these two questions?

Reply 2721

jonnypiercy

Original post by jjsnyder

Classic jjsnyder
Reagent would be Zn and H2SO4
wouldnt have to be an excess of OH- as youve got a precipitate there already so even adding small amounts of Oh would cause the reaction
Could have NH3 as the reagent aswell for the first one

Reply 2722

YoloMed

Original post by jjsnyder

Zinc (in HCl) to reduce chromium down to 2+. Note: Fe2+ etc, would not be strong enough to reduce beyond Cr3+ so this would not be acceptable.

Reply 2723

jonnypiercy

Original post by koolgurl14

It asked for element X which would have just been P

Reply 2724

Parallex

Original post by Aerosmith

I'm afraid to say guys you did have to divide by 2. It said standard enthalpy change, which is defined as forming 1 mol. So all of you please calm your tits

No you didn't...

Reply 2725

Bailey2j09

Original post by justinamarina

I picked sulfur . Phosphorus or sulfur was fine

No because it said pick something from Na - P and sulfur is after P

Reply 2726

YoloMed

Original post by koolgurl14

Incorrect. It said Element X forms an oxide.....

Identify X

Answer : X= P

Reply 2727

hi-zen-berg

Original post by jjsnyder

Thanks a lot;
For last part of Chromium Q, Zn + Hcl were reagents

I'll just add on some stuff Beginning of section B:

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 (5 marks)

[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 is attached photo with 2+ charge around entire complex in brackets

Reply 2728

Aerosmith

Original post by jjsnyder

Enthalpy/ entropy question is incorrect - should be -98, -94.5 and -65.1

Reply 2729

chzm

what did everyone get for the tick box acid that should be used? i put H2SO4 and for the reducing agent for chromium??

i reckon 88/89 for an A* (maybe even 90)
82/83 for an A
70 something for a B

Reply 2730

Aerosmith

Original post by Parallex

No you didn't...

Can you not read?

Reply 2731

26december

I think AQA have messed up a bit there if they asked "standard enthalpy change for this reaction"
Standard enthalpy change would mean dividing by 2
But the "this reaction" would suggest not
However the reaction can be halved so I think the mark scheme will say that you have to divide by 2

Posted from TSR Mobile

Reply 2732

j5994

Original post by chzm

what did everyone get for the tick box acid that should be used? i put H2SO4 and for the reducing agent for chromium??

i reckon 88/89 for an A* (maybe even 90)
82/83 for an A
70 something for a B

doubt it, the highest its ever been for an A is 76%, it was good but like not overly good haha, 93% for full ums

Reply 2733

Kay Fearn

Original post by chzm

what did everyone get for the tick box acid that should be used? i put H2SO4 and for the reducing agent for chromium??

i reckon 88/89 for an A* (maybe even 90)
82/83 for an A
70 something for a B

Lol has it ever been that high? The paper was easy but that's just how we found it.
A lot of people will much lower which always brings the grade boundaries down. I reckon 86 for an A* max

Reply 2734

26december

Original post by Aerosmith

Enthalpy/ entropy question is incorrect - should be -98, -94.5 and -65.1

Agree with that

The colorimeter question should be about using known concentrations and absorption rates surely?

Posted from TSR Mobile

Reply 2735

thehollowcrown

For the Cr2+ do you get the mark for saying H2SO4 as the acid? I bloody put HCl originally then changed it for some reason...

Reply 2736

_newbe

Was formation as there was this symbol

Reply 2737

EverydayHell

Oh I've seen something in the 1st question about Na the equation asked for the element reacting with water not the oxide of the element I think? What did everyone else write? I did 2Na + H2O ---> Na2O + H2.
The other equation asked for the equation with oxide which is why I did that.

Reply 2738

username2280751

Original post by Suits101

That's what I said except I explained it the other way around e.g Au+ reduces water to oxygen hence oxygen doesn't react with gold? :smile:

Regardless of what's correct, only one mark would be deducted from each incorrect answer so -2 max.

I said exactly that lol?

I just went into more detail.

The question said to calculate the standard enthalpy change for this reaction:

"The standard enthalpy of reaction (denoted ΔH_r^⊖) is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions."

Maybe you're right, but my gut feeling was to divide by 2.

No you didn't? The question didn't ask about Au+ ions the question asked about the reaction between gold solid and water because they don't react at all under standard conditions- that was the nature of the question. If it asked about what would happen if Au+ ions were placed in water then you'd be right, but it didn't so you're wrong... lol.

The question didn't say anything about standard enthalpy of formation of SO3, it asked for the enthapy change of the reaction given which was the formation of 2 moles of SO3 from 2 moles of SO2 and one mole of O2.