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 :)
Again, you've been erroneously brainwashed by your school (ie.
Singapore JCs), which teach wrongly for this subtopic.
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).