SCH 4U - Intermolecular Forces

First, let’s be clear about the distinction between INTERmolecular and INTRAmolecular forces:

Intermolecular Forces – forces of attraction between molecules

Intramolecular Forces – forces of attraction within a molecule (ie, between the atoms or ions in a molecule).

In this unit, we will mainly focus on intermolecular forces, as they are responsible for most molecular properties.  These properties include melting point (mp), boiling point (bp), volatility, state under normal conditions, etc.

 

Dipole-Dipole Forces

Dipole-dipole forces (dip-dip or DD) occur between neutral polar molecules.

Polar molecules are attracted to one another when the  ẟ⁺ end of one molecule is near the ẟ^- end of another molecule (ex. HCl)

 ẟ⁺ H-Cl ẟ^- ┃┃┃┃┃┃ ẟ⁺ H-Cl ẟ^-

For molecules of approximately equal mass and size, the strength of attraction increases with increasing polarity.  Thus, mp or bp increases as μ (the variable for polarity) increases.

 

Hydrogen Bonding

Hydrogen bonding (H-bonding or HB) is a special type of dipole-dipole force that occurs when H is bonded to a very small, very electronegative atom (F, O, N).

The ẟ⁺ H is attracted to the electron pair on a nearby electronegative atom (F, O, N).  Because the atoms are small, the ẟ^- charge is very concentrated.

Take a look at the "Boiling Point vs Period" graph.  Notice that the boiling points of the hydrides in group 14 follow a predictable pattern.  This pattern is also apparent in group 15, 16 and 17, except for the hydrides that contain F, O or N.  Those molecules (HF, H₂O and NH₃) have vastly higher boiling points due to their ability to hydrogen bond.

 

 

 

London Forces/Dispersion Forces

London forces operates between non-polar substances.

Typically, the electrons around the nucleus of an atom are fairly evenly distributed in three dimensions (see Figure #1).

The motion of electrons in an atom or molecule can create an instantaneous dipole - at any given instant, most of the electron density could be on one side of the nucleus, rather than evenly distributed (see Figure #2).

Since electrons repel each other, the movement of electrons in one atom will cause electron movement in neighbouring atoms.  This is called an induced dipole (see Figure #3).

Thus, the very slightly positive region of one particle is attracted to the very slightly negative region of its neighbor.  This force of attraction is the London force. 

LF increase with increasing molar mass, since larger atoms have more easily distorted electron clouds.  Take a look at the data table, which shows that boiling point increases as the number of electrons/molar mass of the compounds increase.

 

 

TryIts!

ex. Of Br₂, Ne, HCl and N₂, which is likely to have

(a)   the largest LF forces?

(b)   the highest bp?

 

 

Answers: Br₂ (non-polar, highest M), HCl (able to DD)

 

Homework # 18-26