Be exam-smart and psychologically profile what Cambridge wants.
okay got it , Thanks Ultima.
For H2 syllabus , cambridge expect us to know
[CuCl4]2- Tetrahedral
[Cu(NH3)4]2+ Tetrahedral
which is special due to Jahn teller effect,
is that all? which coordination complex we should know to due to Jahn Teller effect which may be Tetrahedral or Octahedral ?
Thanks Ultima.
https://en.m.wikipedia.org/wiki/Jahn–Teller_effect
The Jahn–Teller effect is responsible for the tetragonal distortion of the hexaaquacopper(II) complex ion, [Cu(OH2)6]2+, which might otherwise possess octahedral geometry.
Initially thought that JahnTeller effect only causes tetrahedral distortion from expected square planar geometry .
Am now cognisant that JahnTeller effect causes other distortions resulting in other geometries too .
Originally posted by Flying grenade:Be exam-smart and psychologically profile what Cambridge wants.
okay got it , Thanks Ultima.
For H2 syllabus , cambridge expect us to know
[CuCl4]2- Tetrahedral
[Cu(NH3)4]2+ Tetrahedral
which is special due to Jahn teller effect,
is that all? which coordination complex we should know to due to Jahn Teller effect which may be Tetrahedral or Octahedral ?
Thanks Ultima.
And every Ah Beng and Ah Lian from Bedok to Jurong knows that [Cu(NH3)4]2+ is square planar lah (becoz of the Jahn-Teller effect lah) in regard to NH3 ligands (or square pyramidal in regard to the NH3 and H2O ligands) lah, not tetrahedral lah, u Coconaden!!!
I'm going to ignore any more posts u make on this topic.
Can help me out 1 last qn in this topic pls
square pyramidal seems strange for me becos its just considering each one of H2O ligands with the NH3 ligands right?
should we write/more correct to write Octahedral in exams instead since there are 4 NH3 ligands and 2 H2O ligands ?
pls help clarify
Thanks Ultima !
The solid state salt of tetraammine copper (II) sulfate contains the [Cu(NH3)4H2O]2+, which has a square pyramidal molecular geometry.
https://en.m.wikipedia.org/wiki/Tetraamminecopper(II)_sulfate
ok so if one H2O ligand its square pyramidal
so if two H2O ligand it's Octahedral
Hi Ultima, I have some Qs from past yr papers and prelim papers :)
2011 P2 Q4(c)(iii) What is 'inertness'? The suggested answers I have says that C does not have low-lying vacant d-orbitals to accept loe pair of electron from water. But I answered that energy released from weak VDW forces of attraction formed between ccl4 and h20 is insufficient to compensate energy required to break strong H-bonds between h20 molecules hence ccl4 is insoluble in h20 and hence inert.
2011 P2 Q4(e) Since the electrode cell potential for oxidation of (VO)2+ to (VO2)+ and (VO3)- is the same, can we say it is oxidised to (VO3)- instead of (VO2)+? Also, when do we use (VO3)-?
2012 P2 Q2(c)(iii) My answer was that from reaction 1, position of equilibrium shifts left at higher temperature as high temperature favours backwards endothermic reaction hence SO2 is more stable at high temperature when heated hence SO2 is formed. The suggested answer says that formation of SO3 is a slow process with higher Ea and is hence not formed. Is my answer wrong?
2012 P3 Q5(a) can the synthesis pathway be:
1. HCN in trace amount of NaCn, cold at 10-20 degree celsius. 2. KCN in ethanol, heat under reflux. 3. HCl(aq), heat under reflux
2014 P2 Q5(d) Will there be formation of white ppt or is phenylamine originally white?
2015 P2 Q2(c) Is it enough to mention H-bonding and permanent dipole between methanol and only weak idid between methane or do I have to compare idid between both methanol and methane as well?
NYJC 2016 Prelim P2
Q2(a) In step 2, Al(OH)3 is produced but in (b)(i), it was mentioned that excess NH3 is used. Shouldn't soluble complex Al(OH)4- be formed when excess NH3 is used?
Q2(b)(iii) Why is CO2 produced?
NYJC 2016 Prelim P3
Q2(a) Should the bond angle be 109.5 degree of slightly less than that due to electronegative O atoms around S? For such Qs, sometimes answer keys take into consideration the electronegativity of atoms in the molecule but sometimes they don't. Do we always consider during the exam?
Thanks :)
2016 Prelim RI P2 Q4(a)(iii) Can the cloudiness be due to white fumes of HBr instead of insoluble white ppt 2,4,6-tribromophenol?
Originally posted by Ephemeral:Hi Ultima, I have some Qs from past yr papers and prelim papers :)
2011 P2 Q4(c)(iii) What is 'inertness'? The suggested answers I have says that C does not have low-lying vacant d-orbitals to accept loe pair of electron from water. But I answered that energy released from weak VDW forces of attraction formed between ccl4 and h20 is insufficient to compensate energy required to break strong H-bonds between h20 molecules hence ccl4 is insoluble in h20 and hence inert.
Originally posted by Ephemeral:2011 P2 Q4(e) Since the electrode cell potential for oxidation of (VO)2+ to (VO2)+ and (VO3)- is the same, can we say it is oxidised to (VO3)- instead of (VO2)+? Also, when do we use (VO3)-?
Originally posted by Ephemeral:2012 P2 Q2(c)(iii) My answer was that from reaction 1, position of equilibrium shifts left at higher temperature as high temperature favours backwards endothermic reaction hence SO2 is more stable at high temperature when heated hence SO2 is formed. The suggested answer says that formation of SO3 is a slow process with higher Ea and is hence not formed. Is my answer wrong?
Originally posted by Ephemeral:2012 P3 Q5(a) can the synthesis pathway be:
1. HCN in trace amount of NaCn, cold at 10-20 degree celsius. 2. KCN in ethanol, heat under reflux. 3. HCl(aq), heat under reflux
Originally posted by Ephemeral:2014 P2 Q5(d) Will there be formation of white ppt or is phenylamine originally white?
Originally posted by Ephemeral:p>2015 P2 Q2(c) Is it enough to mention H-bonding and permanent dipole between methanol and only weak idid between methane or do I have to compare idid between both methanol and methane as well?
The 1st reason and more important reason is, as you've mentioned, hydrogen bonding exists between methanol molecules only (and not methane molecules).
The 2nd reason is, the van der Waals forces between methanol molecules, are stronger than the van der Waals forces between methane molecules, for 2 reasons (yes, 2 sub-reasons within the 2nd reason) : the presence of a permanent dipole for the polar methanol molecule involving an electronegative O atom, as well as the greater number of electrons present in methanol from the additional O atom.
But no worries, Cambridge is lenient when it comes to the (somewhat) erroneous way Singapore JCs teach this, so your answer should be fine.
Originally posted by Ephemeral:NYJC 2016 Prelim P2
Q2(a) In step 2, Al(OH)3 is produced but in (b)(i), it was mentioned that excess NH3 is used. Shouldn't soluble complex Al(OH)4- be formed when excess NH3 is used?
Q2(b)(iii) Why is CO2 produced?
It isn't. H2 is produced, not CO2.
Originally posted by Ephemeral:NYJC 2016 Prelim P3
Q2(a) Should the bond angle be 109.5 degree of slightly less than that due to electronegative O atoms around S? For such Qs, sometimes answer keys take into consideration the electronegativity of atoms in the molecule but sometimes they don't. Do we always consider during the exam?
Thanks :)
The correct reasoning for any observed deviation (eg. in H2S) from the basic geometries as predicted by period 2 VSEPR (eg. in H2O), is moreso because S is in period 3 (more diffused orbitals), hence reducing the degree (pun intended) of hybridization, compared to period 2 VSEPR geometries.
Only when you're comparing across the same period (ie. period 2 vs period 2), then you look at other factors such as electronegativity. Note that electronegativity features primarily as the reason for angular deviation from VSEPR geometry, specifically when electronegative F atoms are present.
In this case of H2SO4, the deviation is primarily due to greater electron repulsion between doubly bonded S=O groups and the greater magnitude of partial negative charge on these O atoms in the resonance hybrid, compared to moderate electron repulsion between the OH groups (due to steric strain), compared to least electron repulsion between the OH and S=O groups (favorable electrostatic interaction between partial positive H atom of OH groups and partial negative O atom of S=O groups).
Be that as it may, for A level purposes, be BedokFunland JC exam-smart, and always write the basic H2 syllabus basic VSEPR geometry first, with qualification (ie. "assuming identical geometry as a period 2 central atom"), before writing the additional Uni level considerations stated above (if you suspect Cambridge is asking you for it, eg. "The angles actually deviate from the expected geometry, suggest reasons why", or if you've the time and you're bored and just feel like showing off your elite Uni level understanding, then go ahead and add these points in your A level paper answer script like I do, just for fun).
You're welcome, Ephemeral, but in future pls post 1 qn at a time, and wait for my reply, before posting the next qn, instead of posting 10 qns at 1 shot (which may be more convenient for u, but more troublesome for me to ans so many qns at 1 shot).
Al2O3 has lower melting point than MgO due to partial covalent character.
Al2O3 does not react with water due to strong ionic lattice.
But doesn't induced covalent character means weaker ionic lattice?
Why isit that for di-substituted alkenes, they are energetically more stable than mono substituted alkene?
ie cis-but-2-ene and trans-but-2-ene more energetically stable than but-1-ene.
For reaction of ROH and H-X gas, why is this reaction limited to only H-Cl and H-Br? What about H-I?
and why the guidebook says "alkenes may also be formed due to competition from elimination reactions'?
please help me Ultima!! ):
Hi Ultima, why is benzene ring a weak electron donating group? I thought it always 'withdraw' electrons since lone pair delocalise into benzene?
When Cu2+ and NH3(aq) are mixed, will it form Cu(NH3)4(H20)2 ligand or Cu(OH)2 or both?
and how will we know its precipation of Cu(OH)2 and not form complex of Cu(OH)6?
and Cu(OH)2 consist of Cu in =2 oxidation state, but I thought only Cu+ can form solid?
Originally posted by Ien:Al2O3 has lower melting point than MgO due to partial covalent character.
Al2O3 does not react with water due to strong ionic lattice.
But doesn't induced covalent character means weaker ionic lattice?
From the different melting points, you are expected to deduce the Ea required to melt MgO is higher than to melt Al2O3. This is due to the greater degree of covalency in Al2O3. Remember that compared to ionic lattice structures, covalent simple molecules have stronger intramolecular covalent bonds, but weaker intermolecular van der Waals forces. This explains why the greater the degree of covalency, the more unequal the distribution of bond strengths within the lattice structure, hence the lower the melting point.
From the different solubilities, you are expected to deduce the Ea required to hydrolyze Al2O3 is higher than the Ea required to hydrolyze MgO. This is due to the lone pairs on the O atoms in Al2O3 being 'locked up' in the form of dative covalent bonds, and less available for hydrolysis.
Originally posted by Ien:Why isit that for di-substituted alkenes, they are energetically more stable than mono substituted alkene?
ie cis-but-2-ene and trans-but-2-ene more energetically stable than but-1-ene.
Originally posted by Ien:For reaction of ROH and H-X gas, why is this reaction limited to only H-Cl and H-Br? What about H-I?
and why the guidebook says "alkenes may also be formed due to competition from elimination reactions'?
please help me Ultima!! ):
Upon protonation of the OH group (which is the Bronsted-Lowry acidic role of HX), E1 vs E2 vs SN1 vs SN2 all become possible (the 1st order reactions favoring tertiary alcohols, the 2nd order reactions favoring primary alcohols). Hence, "alkenes may also be formed due to competition from elimination reactions".
Originally posted by Ien:Hi Ultima, why is benzene ring a weak electron donating group? I thought it always 'withdraw' electrons since lone pair delocalise into benzene?
While Singapore JCs do not teach their H2 students to be this explicit, but BedokFunland JC students and all A grade students are encouraged to specify the modality of electron donation or withdrawal.