15. Water

What types of bonding are found in and between water molecules?

Polar covalent bonds between H and O atoms. H bonding between 𝛿+ charge on H and 𝛿- charge on O. Dipole-dipole forces and London dispersion forces as well.

Why is H bonding strong in water?

The partial charges on H and O atoms are strong due to a large electronegativity difference.

Why does water have a higher melting and boiling point compared to other group 16 hyrides?

The strong H bonds between molecules require large amounts of energy to disrupt.

Why is ice less dense than liquid water?

When water freezes, each molecule forms 4 H bonds with neighbouring molecules, creating a fixed hexagonal lattice with empty space between molecules, decreasing the density. In liquid water, molecules can fill empty spaces because the bonds break quickly.

Surface tension definition

A measure of the resistance of a liquid to increasing its surface area.

Explain how surface tension occurs in water.

Water molecules on the surface are packed tighter together bc they have no particles above them to form H bonds with. When something is placed on the surface, the molecules are so strongly attracted to each other that the object can float.

Heat capacity definition

A measure of a substance’s capacity to absorb and store heat energy.

Latent heat definition.

The energy absorbed by a fixed amount of substance as it changes state from solid to liquid or liquid to gas at its MP/BP.

Latent heat of fusion/vaporisation definition.

Fusion: heat needed to change 1 mole of a substance from solid to liquid at its MP.

Vaporisation: heat needed to change 1 mole of a substance from liquid to gas at its BP.

What must happen for a substance to dissolve?

The attractive forces between the solvent and solute particles must be sufficient to overcome the solute-solute and solvent-solvent forces.

Describe the process of ionisation.

Dipole-dipole attraction between water molecules and the ions in a ionic substance lead to the breaking of the polar covalent bonds between the cation and anion. The ions are surrounded by polar water molecules and held in solution by ion-dipole attractions (hydrated).

Disassociation meaning.

The process of dissolving a solid ionic compound in water and separating the positive and negative ions to form hydrated ions.

Which ions are always soluble in water?

Sodium, nitrate, ammonium, potassium, ethanoate.

Solubility definition

The maximum amount of a solute that can be dissolved in a given quantity of a solvent at certain temperatures.

Why does increasing temp increase solubility?

It has more energy to overcome the forces of attraction which hold a substance together.