3.1.3.7 Forces between molecules Flashcards
What is an intermolecular force?
A force between molecules.
Weaker than ionic, covalent or metallic bonds
What are the three types of intermolecular force?
From weakest to strongest:
Induced dipole-dipole (Van der Waal’s force)
Permanent dipole-dipole
Hydrogen bonding
Describe how Van der Waal’s forces arise
An uneven distribution of electrons in one molecule creates a temporary dipole. The temporary dipole induces a dipole in the adjacent molecule. There is an attraction between the partially positive charge on the induced dipole on one molecule and the partially negative charge on the molecule with temporary dipole
What factors does the strength of Van der Waal’s forces depend on?
Number of electrons
Surface area
How does the number of electrons affect Van der Waal’s force strength?
Larger molecules have higher b.p. because they have more electrons, so stronger VdW forces, so more energy to break forces
How does surface area influence VdW force strength?
The greater the surface area in contact with each other, the stronger the VdW force, so more energy to break bonds
How do permanent dipole-dipole forces arise?
Difference in electronegativity leads to bond polarity because dipoles don’t cancel so the molecule has a permanent dipole. There is attraction between delta+ on one molecule and delta- on the other molecule
Do symmetrical molecules have permanent dipole-dipole forces?
No because dipoles cancel out so no permanent dipoles
How to compare bp/mp of molecules?
If both polar/ non-polar:
Compare vdw , number of electrons. Only organic molecules with chain difference - surface area.
One polar and other non-polar:
Compare Vdw and permanent dipole-dipole and Vdw
How does hydrogen bonding arise?
Only formed between F/O/N and H.
There is a large difference in electronegativity between F/O/N and H, so form delta- (F/O/N) and H forms delta+. Lone pairs on F/O/N form weak bonds with Hdelta+
What are the rules for drawing hydrogen bonds?
Draw lone pairs and partial charges on atoms.
Dotted lines between lone pairs and Hdelta+
at 180 degree angle
Why is ice less dense than water?
Water molecules held further apart so more spaces/holes in the structure and molecules are more spread out
