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Chem5 June 2014 Unofficial Mark Scheme

This sums up our exam :P :
https://www.youtube.com/watch?v=2tjVAjHFDio

Add below and I will edit it all in with question and Marks.

Write the equation for the electron affinity:
1. Cl + e- ----> Cl- (1 mark)

Why Ea (Electron affinity) exothermic: (2 mark)
(Adding electron- requires energy for sticking of e-) - Debated - need people to suggest their idea.
Weak electrostatic attraction - net gain of e- compared to + proton.

Born haber cycle I got : (3 mark)
Ag(S) + 1/2F2 (g)
Ag(g) + 1/2 F2 (g)
Ag+ (g) + 1/2 F2 (g) + e-
Ag+ (g) + F (g) + e-

Didn't do the bond enthalpy ( 2 mark)
Apparently 158 KJ mol-1

Enthalpy of hydration: ( 2 mark)
Enthalpy change when 1 mole of gaseous ions form 1 mol of aq ions (when bound to water)

Why is enthalpy of dissociation experimental and theoretical are different: (2 mark)
Theroetical takes into account fully ionic
Experimental takes into account covalent character as values are different

AgF and AgCl why is AgF got a higher dissocation value: (2 mark)

Fluoride is smaller ion
Stronger Ionic bond - greater electrostatic attraction- requires larger amount of energy to break bond.

Something about why is the value positive or something like that? (2 mark)
Endothermic-break bonds
Between H delta+ of water and Cl- ion.

Enthalpy of solution: (3 marks)
Got something like -364 KJmol-1

T=2333k (4 marks)
temp is significant because water boils at 373k (ish), so the reaction cannot happen (u can't dissolve something in steam)

Can't remember what I got but I did: I think it was -20.1 ( 3marks)
Delta H-Delta G / T
Temp 25+273=298K
Delta G = -17
Delta H - Can't remember value, possibly

The Mg with water I got wrong: (3 mark)
But the correct answer is MgO
H2
Mg + H20 -----> MgO + H2

Difference between Na and Mg in terms of bonding: (2 mark)
Mg ion-smaller radius/smaller ion
Mg ion 2+ charge and Na +1 charge
Therefore Mg has stronger metallic bonding.

P (V) Oxide question: (3 mark)

P4O10
Covalent bond between O and P with in the molecule
Molecules held by van der waals due to electrons

SiO2 (4 mark)
structure Macromolecule(1)
bonding Covalent(1)
and two properties of sio2 :frown:

2)
-Insoluble
-

SO3^2- + H20 ----> HSO3^- + OH- (1 mark)

P4O10 + 6MgO----> 2Mg3(PO4)2 (1 mark)

Hydrogen electrode and Fe2+ / Fe: (2 marks)

Pt|H2(g)|H+(ag)||Fe2+(aq)|Fe(s)

What's an Reducing agent: (1 mark)
Electron donor

Weakest reducing agent from column and why (2 mark)
Cl2
And which atom was reduced in HOCl (the tick box question) (2 mark)
Cl Oxygen state goes from +1 ---> O From HOCl to HCl

Hearing aid cell:

(1 mark)
EMF = 1.60 V / 1.65V
Rhs-Lhs

(2 marks)
A- steel lid:
Reduction occurs at the anode

(2 marks)
2H+ + 2e- + O2 ----> H2O
H2 -----> 2H+ + 2e-

Overall equation for H2O2 and ZnO (2 marks)
2Zn + 202---> 2Zno2

Why Fuel cell is better than internal combustion engine (2 marks)

Hazard of using fuel cells (1 mark)
H2 is flammable could lead to explosion

Cis Platin: (3 mark)
90 degrees
Was a square planar with 2 NH3 at top and 2 Cl at bottom
No charge

(1 mark)
[Pt(NH3)2(Cl)2] + H20 -----> [Pt(NH3)2 (H2O) (Cl)]^+ + Cl-

(1 mark)
Administrate a smaller dose reduce toxic effects

(5 Marks): Reaction between 2 aminoethane with [Co(H2O)6]2+

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

4:7 molar ratio
Entropy increase
TDeltaS> Delta H

Delta S = +Ve
Therefore Delta G<O
Therefore reaction is feasible.

12 marks in total for the following:
1) NH3
Cu(H20)4(OH)2
Equation to form it

2) NH3 in excess
[Cu(NH3)4(H2O)2]^2+

3) Na2CO3
CuCO3

4) HCl
[Cu(Cl)4]^2-

6 marker X^2+ and X^3+ one:
Couldn't answer the first part because I don't how they can have the same electron config :frown:

they had the same electron arrangement (and the same oxidation states) but one had a Cl- ligand (oxidation state -1) so overall it had charge +2 (3-1 = 2), tho other only had water (oxidation state 0) and 3±0 = 3 so overall charge 3

Different d - electrons in different D sub shells.
Visible light absorbed, excites electron is different - different Delta E values
Therefore different wavelength of lights are absorbed.

(4 marks)
Overall: SO2+1/2O2 ----> SO3

SO2 + V2O5 -----> SO3 + V2O4
V2O4 + 1/2O2 ----> V2O5

V2O5 provides alternative mechanism with lower Ea. It's a heterogeneous catalyst and is not used up or changed in the reaction.

(5 marks)
5.34g for redox titration

(2 marks)
Last question: Why calculated value is higher than actual. I put impurity X can reduce Cr2O72- therefore more dichromate is needed giving a higher amount of Fe2+ than there actually was.

(edited 9 years ago)

Scroll to see replies

Reply 1

pandabun

Do you remember the marks given?

Posted from TSR Mobile

Reply 2

mariam.therese

****..............i must've been seriously blind with the first bit... but i did it for fluorine not chlorine :'(

Reply 3

VigneshSB

Hey, I swear the question was why the enthalpy of hydration of flouride ions is exothermic? delta h+ on water is attracted to the negative charge on flourine? So the thing is exothermic?

Reply 4

VigneshSB

Original post by mariam.therese

****..............i must've been seriously blind with the first bit... but i did it for fluorine not chlorine :'(

It was flourine. And I can't remember a question asking for why the hydration enthalpy of flourine was endothermic, only exothermic :frown:

Reply 5

mariam.therese

also the first question asked for electron affinity and youve put bond dissociation??

Reply 6

flamegoblin0

for the first question the born haber was definitely for fluorine not chlorine

Reply 7

tigerboobs

I thought the first one was Cl(g) + e- -> Cl-(g) .... It can't be 1/2 Cl(g) because it has to be atomised first.

I may be remembering incorrectly, but I thought it asked why the hydration enthalpy of Chloride ions was EXOthermic?? Net attraction between positive dipole on H and Cl-

4 to 7 moles change 3 [Cu(H20)6]2+ + 3(H2NCH2CH2NH2) to form 7

Reply 8

mariam.therese

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

2:7 molar ratio

you need to balance the diamine so the molar ratio is 4:7

Reply 9

propagation

Original post by pandabun

Do you remember the marks given?

Posted from TSR Mobile

Yes I do

Reply 10

azsxdcvf95

Oh noooooo, I thought the born haber cycle was for AgF !!!!! Damn :frown:

Reply 11

VigneshSB

Original post by tigerboobs

yes to all, plus it was flourine not chlorine

Reply 12

propagation

Original post by mariam.therese

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

2:7 molar ratio

you need to balance the diamine so the molar ratio is 4:7

Ahh cock, I lost to many marks now :frown:

Reply 13

Seth James

Ratio with diaminoethane is 4-7
Colour changes :
1) NH3
Cu(H20)4(OH)2
Equation to form it

2) NH3 in excess
[Cu(NH3)4(H2O)2]^2+

3) Na2CO3
CuCO3

4) HCl
[Cu(Cl)4]^2-

Reply 14

danny2014

The question about chromium:

Both had the configuration [Ar] 3d3 because both had the oxidation state +3. The complex ions, however, had different charges due to the Cl- ligand on the second complex ion. The different ligands alters the ΔEso different frequencies of light are absorbed. This means that different frequencies of light are reflected, and so, two different colours are observed.

Reply 15

tigerboobs

Further questions to add answers too are:

Weakest reducing agent from column
And which atom was reduced in HOCl (the tick box question)

Last question: Why calculated value is higher than actual. I put impurity X can reduce Cr2O72- therefore more dichromate is needed giving a higher amount of Fe2+ than there actually was.