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 ion (usually N or O) and an electropositive H atoms
• i.e. bonds from H to N, O or H

Question 2

Describe the interactions between water molecules and solutes that define solubility

Answer 2

• molecules that form H bonds are soluble
• in water, solute-solute and water-water H bonding is replaces with water-solute H bonding which is more favourable
• also works for charged moles
• uncharged molecules are not suitable, they rearrange themselves in water so as to minimise the disruption of the surrounding water moles

Question 3

Define amphipathic

Answer 3

A molecule which contains both hydrophobic and hydrophilic parts

Question 4

Give an example of an amphipathic molecule

Answer 4

Phospholipids - hydrophobic fatty acid tails, hydrophilic phosphate heads

Question 5

Define pH

Answer 5

a measure of proton concentration

- pH = - log [H+]

Question 6

What expression relates the degree of proton dissociation of a weak acid to its ionisation constant and the pH?

Answer 6

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

Question 7

What is a buffer?

Answer 7

Solution of a weak acid which resists changes in pH

Question 8

How does a buffer work?

Answer 8

In solution, acids dissociate to a conjugate acid-base pair

• weak acids do not dissociate very much
• in solution, if OH- is added, it combines with free H+ to form H2O
• if H+ is added if combines with the free OH- -> H2O
• these ions are replaced by the dissociation of the acid and the water, so the pH is kept constant