Intermolecular forces - L6 Exam 2023/2024
what are the three intermolceular forces (in order of increasing strength)? - ANSWER-1. Van der Waals forces - act between all atoms and molecules
2. Permanent dipole-dipole forces - act between molecules with permanent dipoles (polar molecules)
3. Hyd...
Intermolecular forces - L6 Exam
2023/2024
what are the three intermolceular forces (in order of increasing strength)? - ANSWER-1. Van der Waals
forces - act between all atoms and molecules
2. Permanent dipole-dipole forces - act between molecules with permanent dipoles (polar molecules)
3. Hydrogen bonds - only when H is bonded to F, O or N
VDW forces application - ANSWER--Consider He (number of electrons = 2). He atoms move randomly &
its two e-s could be anywhere in the 1s orbital. At any point the electrons might both be towards one
end of the orbital - this creates an instantaneous dipole. One side of the atom has a positive charge, one
side has a negative charge.
-This can then induce a dipole on a neighbouring atom (a bit like the domino effect) and a VDW force
forms between the atoms. This force quickly breaks leaving each atom able to form more VDW forces.
-Because electrons are constantly moving, the dipoles are being created and destroyed all the time - but
overall effect is attractive.
Exam question: Explain how VDW forces arise (3 marks) - ANSWER--Uneven distribution of electrons
generates instantaneous dipole
-This induces a dipole on neighbouring atom
-The dipoles are attracted to one another - this is VDW force
Practice Q (VDW): State the trend in the bp of the noble gases as you go down the group. Explain this
trend. - ANSWER-The bp's increase.
As you go down the group the noble gas atoms have more electrons leading to stronger VDW forces
between the atoms.
Practice Q (VDW): As you go down the halogens group of the periodic table, there is also a trend in bp.
(a)In terms of structure and bonding, what do all the halogens have in common?
(b)Explain the trend in bpt as you go down the group. - ANSWER-(a)They are all simple molecular
structures. There are strong covalent bonds between atoms in a molecule and weak VDW forces
between molecules.
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