Flashcards in Water Deck (5):
Define the term hydrogen bond
Water is a polar molecule, due to its electronegative oxygen atom. This allows it to form hydrogen bonds between any other electronegative atom (usually N or O) and an electropositive H atom.
Describe the interactions between water molecules and solutes that define solubility.
Molecules that form H bonds are water soluble. In water, the solute-solute and water-water H bonding is replaced with water-solute H bonding, which is more favourable.
This also works for charged molecules.
Uncharged molecules are not soluble. They rearrange themselves in water so as to minimise the disruption of the surrounding water molecules, forming “cages”.
Define the term amphipathic and give an example of an amphipathic molecule.
Amphipathic molecules contain both hydrophobic and hydrophilic parts. For example, phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails. In water, they minimise disruption by forming bilayers or micelles.
Define the term pH and derive an expression that relates the degree of proton dissociation of a weak acid to its ionisation constant and the pH.
pH is a measure of proton concentration. pH = -log[H+]
pH=pKa + log [A-]/[HA]