omg
page 51 cs toh advanced guide
i have been learning wrong things throughout my life
cs toh say CO2 molecules have intermolecular pd-pd forces of
attraction.
the next statement below "this is possible because in the solid
state, CO2 molecules are fixed in position." does this statement
mean only CO2(s) has pdpd? what is that statement for?
i thought only molecules with (net)dipole moment have
intermolecular pd-pd.
i am now confused already. is CO2 a polar molecule? it's a
symmetrical molecule
i always thought that in order for a molecule to be polar, the
molecule must have net dipole moment
so CO2 has id-id (all molecules have) , does it have pdpd?
is CO2 a polar molecule without a net dipole moment,
or is CO2 a non-polar molecule without a net dipole moment but
has pdpd?
so in order to have pdpd IMFs, just need two(or more) different
atoms chemically combined tgt in a molecule, without a need for net
dipole moment?
I told you liao : don't blindly trust Singapore JCs or your school
lecture notes. The Distinction A grade student, will not blindly
follow or memorize school lecture notes, but intelligently
understand the Chemistry concepts in his/her own way. And the
intelligent student will also sometimes disagree with his/her
school teacher, because school teachers often disagree with each
other as well.
In CO2, the linear molecular geometry ensures that the 2
individual dipoles cancel out, hence it is an overall non-polar
molecule. Being non-polar doesn't necessarily always mean only
London dispersion van der Waals forces are present between
molecules. Intelligently apply your own chemistry understanding
case-by-case. For CO2, all 3 types of van der Waals forces are
present : Keesom, Debye and London dispersion forces.
When comparing the intermolecular interactions for 2 different
species separately, eg. when asked to explain the difference in
melting or boiling point for 2 different species, you only need to
state van der Waals forces versus hydrogen bonding or
ionic bonding, and specify which is stronger. Only when both
molecules have only van der Waals intermolecular forces, then it
will be necessary to specify which type of van der Waals
interactions are present between molecules of each species.
But be careful, contrary to what your school (ie. Singapore JCs)
may have taught you, instantaneous dipole - induced dipole London
dispersion van der Waals forces may not necessarily be weaker than
permanent dipole - induced dipole Debye van der Waals forces which
may not necessarily be weaker than permanent dipole - permanent
dipole Keesom van der Waals forces, it depends on the total no. of
electrons present and the molecular size, and hence the
polarizability of electron charge clouds, and hence the magnitudes
of partial charges and dipoles, and hence the strength of the
electrostatic van der Waals attractions, regardless of which type
of van der Waals is present.
You should be able to figure out such counter examples (which
are contrary to the oversimplifications taught in Singapore JCs),
including extreme counter examples where even instantaneous dipole
- induced dipole London dispersion van der Waals forces may be even
stronger than intermolecular hydrogen bonds! Shhhh! even if you
know such counter examples, don't post them here and spoil the fun
for others, let them figure these out for themselves! ;Þ
Hi UltimaOnline, for CO2, you mentioned that
"In
CO2, the linear molecular geometry ensures that the 2 individual
dipoles cancel out, hence it is an overall non-polar molecule.
Being non-polar doesn't necessarily always mean only London
dispersion van der Waals forces are present between molecules.
Intelligently apply your own chemistry understanding case-by-case.
For CO2, all 3 types of van der Waals forces are present : Keesom,
Debye and London dispersion forces.", is this only true for solid
state?
No, not only for solid state, but Keesom van der Waals forces
predominate moreso for the solid state > liquid state >
gaseous state, for obvious entropy reasons. If the reasons are not
obvious to you, go ask your school teacher or private tutor.
but which one is the predominant (most extensive) IMFs between
CO2 molecules?
I'll leave you with a final parting statement in reply to your
question quoted above : it depends on the state/phase of the CO2,
and besides, Cambridge won't ask which predominates for the
different states/phases, you need only state "van der Waals
forces" (ie. all 3 types of van der Waals forces) as the
intermolecular interactions between CO2 molecules, regardless of
state/phase.
