1. 2.8 - Properties of Compounds

2. Properties of Compounds
The properties of compounds are determined by the intra and intermolecular forces
Keep in mind:
Stronger intramolecular forces means the atoms are held tightly together in an individual compound
Stronger intermolecular forces means each compounds is attracted to the other

3. Melting and Boiling Points
The melting point is the temperature at which the substance changes from a solid to a liquid. The boiling point is the temperature for liquids to become gases
That means you have to put in energy to get the molecules to separate from one another
Ionic compounds have very strong attractions between the ions in the compound
Very high melting points and boiling points
Lots of energy is needed to overcome this attraction

4. Melting and boiling Points
Covalent compounds tend to have low melting points
Less attraction between molecules, less energy needed to put into the system
Polar compounds have higher melting and boiling points than non-polar compounds
More attraction = more energy needed
Non-Polar compounds will depend on their size
Large molecules = more London Forces = more attraction = higher melting/boiling points

5. Melting and boiling Points
You can predict the melting points based on the intra and intermolecular forces
In order of decreasing melting and boiling points:
Ionic Compounds
Polar Covalent Compounds
Non-Polar Covalent Compounds
High molecular weight/size non-polar compounds
Low molecular weight/size non-polar compounds

6. Surface Tension
Surface tension is the film created on the surface of a liquid.
Ionic substances tend not to be considered because the are solid at room temperatures
The stronger the attraction between molecules, the greater the surface tension
H-bonding leads to greater surface tension
Dipole interactions have some surface tension
Non-polar compounds have low surface tension

7. Surface Tension
Water has a large amount of hydrogen bonding attracting molecules towards each other
This makes water have an extremely high surface tension

8. Solubility Like dissolves like
This means that polar compounds will dissolve in other polar compounds
Non-polar compounds will dissolve in non-polar compounds
Since ionic compounds have charges, they only dissolve in polar compounds

9. Viscosity
The stronger the intermolecular forces, the higher the viscosity of a substance
More intermolecular forces mean a stronger attraction
This means the molecules do not want to flow past each other easily
That leads to a high viscosity
Long (chain-like) molecules also have high viscosity because the molecules can get tangled

10. Vapour Pressure
Vapour pressure is the pressure of a vapour above a liquid
The higher the vapour pressure, the faster something evaporates
The lower the vapour pressure, the slower something evaporates
The higher the intermolecular forces, the lower the vapour pressure
Molecules are held more tightly and will not evaporate as quickly

11. Vapour Pressure
Substances with high vapour pressure are sometimes called volatile
They evaporate very quickly
These tend to be non-polar compounds
Many non-polar compounds are hydrocarbons
Hydrocarbons tend to be flammable