Water

Question 1

Define the term hydrogen bond

Answer 1

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.

Question 2

Describe the interactions between water molecules and solutes that define solubility.

Answer 2

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”.

Question 3

Define the term amphipathic and give an example of an amphipathic molecule.

Answer 3

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.

Question 4

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.

Answer 4

pH is a measure of proton concentration. pH = -log[H+]

pH=pKa + log [A-]/[HA]

Question 5

Describe what is meant by the term “buffer” and describe how a buffer works.

Answer 5

Buffers are solutions of weak acids. In solution, acids dissociate to a conjugate acid-base pair. Weak acids do not dissociate very much. In solution, 2 reactions are occurring at once.

H2O H+ + OH-

HA H+ + A-

Both these equilibriums have to maintain Ka and Kw.
If OH- is added, it combines with free H+ to form H2O.
If H+ is added, it combines with the free OH- to form H2O.
These ions are replaced by the dissociation of the acid and the water, so the pH is kept constant.