AS-Level Chemistry Chapter 4 Chemical Bonding Bonding and Physical Properties of Substances

To view other notes related to AS-Level Chemistry. Please Click Here.

Bonding and Physical Properties of Substances

Physical state at room temperature

1) Ionic compounds

- Ionic compounds are solids at room temperature, this is because the ionic bond holding the oppositely-charged ions is very strong, a lot of energy is required to overcome the strong forces of attraction.

2) Covalent compounds

- Most simple covalent molecules(like water and ammonia) are liquids or gases at room temperature. This is because the intermolecular force between the molecules is weak, little energy is required to overcome it.

- Some simple covalent molecules(like iodine) are solids because the intermolecular force between them is strong enough.

- For giant covalent structures(like diamond and silicon dioxide), they are solids at room temperature. This is because the covalent bonds holding the atoms are very strong, a lot of energy is required to overcome it.

3) Metals

- Metals(except mercury) are solids at room temperature. This is because the metallic bond holding the metal ions is very strong, a lot of energy is required to overcome it.

Electrical conductivity

1) Ionic compounds

- Ionic compounds do not conduct electricity in the solid state, this is because the ions are not free to move.

- In the molten or aqueous state, it conducts electricity because the ions are free to move(mobile ions are present).

2) Covalent compounds

- Simple covalent molecules do not conduct electricity because the ions are not free to move.

- Some giant covalent structures(like graphite) are able to conduct electricity because the electrons are free to move(delocalized).

3) Metals

- Metals conduct electricity because the delocalized electrons are free to move.

Solubility

1) Ionic compounds

- Most ionic compounds are soluble in water(polar solvents) and insoluble in non-polar solvents.

- This is because in non-polar solvents, the molecules are held together by weak intermolecular forces. The ionic bonds in ionic compounds are much stronger and the energy needed to break them is high. If ion-solvent bonds are formed, the energy released is not enough to compensate the energy absorbed, making the entire structure unstable.

[ Note: To form ion-solvent bonds, the ion-ion(ionic bond) and solvent-solvent (van der Waal's forces) bonds must be broken first. ]

- While in polar molecules, there is an attraction between polar molecules and the ions. So, the energy released when ion-solvent bonds are formed is enough to compensate the energy needed to break the strong ionic bonds.

2) Covalent compounds

- This is because when a covalent molecule is dissolved in a non-polar solvent, the molecule-solvent attraction is strong enough to compensate the energy needed to break the weak van der Waal's forces between covalent molecules.

- While when dissolved in polar solvents, the energy needed to break the attractions in polar solvents is too high. The energy released when molecule solvent attraction set up is not enough to compensate it. This makes the structure to gain energy overall, making it less stable.

- Some covalent compounds react with water rather than dissolving in it. For example, hydrogen chloride reacts with water to form hydrogen ions and chloride ions, and the ions are soluble.

3) Metals

- Metals do not dissolve in polar and non-polar solvents.

- However, some metals like sodium and calcium can react with water.