Unit 1 - Structure and Bonding: Physical Properties Flashcards
Physical Properties:
Melting and Boiling Points
The melting and boiling points of a substance depends on the strength of the intermolecular forces between its molecules - Stronger forces = higher mp/bp.
When predicting melting and boiling points, the factors to consider are the
* type of van der Waals bonding present (LDF, PD-PD, H-bonding)
* total number of electrons in the molecule
The bp increases down a group due to the increases strenght of the LDF/increased no. of electrons.
Melting and Boiling Points & Hydrogen Bonding
Substances with hydrogen bonding show higher than expected boiling points. The higher boiling points of ammonia, water and hydrogen fluoride are due to hydrogen bonding.
Viscosity
Viscosity is a measure of how thick a liquid is. Thick liquids have high viscosities, runny liquids have low viscosities.
Viscosities can be compared by timing how long it takes a ball bearing to fall through the liquid (the longer the time, the more viscouse the liquid).
There is a greater degree of hydrogen bonding in propane-1,2,3-triol compared to propan-1-ol. This results in a greater force of attraction between the molecules and so the viscosity increases.
Solubility
Ionic compounds and polar molecular compounds tend to be soluble in polar substances, such as water, and insoluble in non-polar solvents (alkanes e.g heptane). The polar nature of the water molecules means they are attracted to both positive and negative ions.
Non-polar molecular substances tend to be soluble in non-polar solvents and insoluble in polar substances. However, some small molecules are soluble because they can form hydrogen bonds with water molecules e.g ethanol
Solubility - Predicting Solubility
When predicting solubility, the factors to consider are the
- presence in molecules of O-H or N-H bonds, which implies hydrogen bonding
- spatial arrangement of polar covalent bonds, which could result in a molecule possessing a permanent dipole
