Intermolecular forces (Van der Waals forces) Flashcards
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Define “Induced dipole-dipole”
When electrons are temporarily displaced in an atom causing one side to become more negatively charged, repelling the electrons in a neighbouring atom/molecule and this neighbouring atom has been induced.
Explain “Induced dipole-dipole”
When electrons in an atom are temporarily displaced towards one side of the atom, causing that side of the atom to have a small negative charge ( - delta). An instantaneous dipole is now produced.
This negative charge causes electrons in a neighbouring atom to be repelled to the other side of its own atom.
This causes an induced dipole where the side of the other atom closest to the original atom is now slightly positively charged.
This means that the negatively charged side of one atom and positively charged side of the neighbouring atom now attract each other.
The charges cause a force of attraction known as “Van der Waals”.
State other names for “Induced dipole-dipole interactions”
- Van der Waals forces
- London forces
- Instantaneous dipole - induced dipole
- Dispersion forces
State the forces between the induced atoms
London forces
Define “dipole”
An unequal sharing of electrons between two atoms in a molecule causes 2 different charged regions.
Define “Polar molecule”
When an unequal sharing of electrons causes dipole in a molecule, but the molecule has a neutral overall charge.
Or…
A molecule with a net dipole as a result of opposing charges
In what molecules do “London forces” occur?
Almost all covalent compounds and inert gases (Noble gases - group 0)
What factors affect “Induced dipole-dipole forces”?
- The number of electrons in the molecule
- Surface area
Are London forces strong?
They are weaker than covalent bonds and in general, are weak. They only require 5 - 10kJ of energy to overcome.
As you go down group 7, what happens to the “London forces”?
As you go down group 7, the number of electrons in the molecules increases so the London forces become stronger.
London forces in group 7 (LEARN)
In ascending order of number of electrons (Group 7)
Fluorine (g) - Lowest boiling point (Less electrons)
Chlorine (g)
Bromine (l)
Iodine (s) - Highest boiling point (Most electrons)
With more electrons, London forces are stronger so require more energy to overcome, therefore molecules with an element of a higher period have a higher boiling/melting point.
An example comparison : Why is the melting point of Iodine(s) lower than the melting point of Sodium chloride(s) ?
- Iodine has a giant molecular structure with weak London forces which requires little energy to overcome. (Melting point = 80°C)
while. .. - Sodium chloride has a giant ionic lattice structure with strong electrostatic forces so more energy is needed to overcome these forces. (Melting point = 90°C)
How does the strength of London forces change in relation to the number of electrons in a molecule?
As the number of electrons in the molecule increases, the strength of London forces increases.
How does surface area affect the strength of London forces?
As the surface area of a molecule increases, the strength of London forces also increases as there is more area for London forces to occur in so the overall net strength of London forces increases.
