Intermolecular Forces

Question 1

What are intermolecular forces?

Answer 1

Weak interactions between dipole of different molecules.
London forces
Permanent dipole dipole interactions
Hydrogen bonding

Question 2

What properties do intermolecular forces control?

Answer 2

Physical properties

Question 3

What properties do the covalent bonds control?

Answer 3

Identity and chemical reactions of molecules

Question 4

What is the bond enthalpy of London forces?

Answer 4

1-10kj/mol

Question 5

What is the bond enthalpy for permanent dipole dipole interactions?

Answer 5

3-25kj/mol

Question 6

What is the bond enthalpy for hydrogen bonds?

Answer 6

10-40kj/mol

Question 7

What are London forces?

Answer 7

Weak intermolecular forces that exist between all molecules. Act between induced dipoles

Question 8

How do London forces work?

Answer 8

Movement of electrons produces a changing dipole in a molecule.
At any point, an instantaneous dipole will exist, but the position constantly changes.
The instantaneous dipole induces a dipole on the neighbouring molecule
The induced dipole induces further dipoles on the neighbouring molecules, which then attract one another.
Induced dipoles are only temporary

Question 9

How strong are London forces?

Answer 9

The weakest forces.
The more electrons in each molecule, the larger the instantaneous and induced dipoles, the greater the induced dipole dipole interaction, the stronger the attractive forces between the molecules.

Question 10

What is the relationship between relative strength of London forces and the boiling point?

Answer 10

The stronger the London forces, the higher the boiling point

Question 11

What are permanent dipole dipole interactions?

Answer 11

Intermolecular forces that act between the permanent dipoles in different polar molecules.

Question 12

What is the relationship between the boiling point and permanent dipole dipole forces?

Answer 12

If there are permanent dipole dipole interactions between the molecules, more energy to required to break the permanent dipole dipole interactions between the molecules.

Question 13

What is a simple molecular substance?

Answer 13

Made up of small units containing a definite number of atoms with a definite molecular formula

Question 14

What is a simple molecular lattice?

Answer 14

A regular structure for simple molecules, where the molecules are held in place by weak intermolecular forces and the atoms within the molecules are strongly covalently bonded

Question 15

Properties of simple molecular substances

Answer 15

Low melting and boiling point
Non polar solutes are soluble in non polar solvent
Polar solutes are soluble in polar solvents
Non polar solutes are insoluble in polar solvents.
Insulators of electricity.

Question 16

Why do simple molecular substances have low melting points

Answer 16

Weak intermolecular forces can be broken by a small amount of energy
Covalent bonds do not break.

Question 17

Why are non polar substances soluble in non polar solvents?

Answer 17

Intermolecular forces form between molecules and solvent

Interactions weaken intermolecular forces in simple molecular lattice which break and compound dissolves

Question 18

Why are non polar substances insoluble in polar solvents?

Answer 18

There is little interaction between lattice molecules and solvent molecules
Polar solvent intermolecular bonding is too strong to be broken

Question 19

Why are polar simple molecular substances dissolve in polar solvents?

Answer 19

Polar compounds attract and bond with polar water molecules.
Polar solvent surrounds simple polar molecular lattice and the attractive forces between solvent and solute are stronger than forces between molecular lattice and between solvent

Question 20

When is it hard to predict the solubility of polar molecular substances?

Answer 20

Depends on dipole strength

Some molecules have hydrophobic and hydrophilic parts, which are polar and non polar

Question 21

Why are simple molecular substances insulators of electricity?

Answer 21

No mobile charged particles, cannot move, there is nothing to complete an electrical circuit.