W4.3_Properties of Water

Question 1

Explain the importance of water in cells.

Answer 1

• Structure and function of biological molecules, cells,… are determined by interactions of atoms they are made from
• Water: 70% cell composition, thus very important

Question 2

Explain why oxygen is a very polar solvent and its unique properties.

Answer 2

• Oxygen: highly electronegative -> attracts electrons away from H atoms to create partial -ve charge on O atom/partial +ve charge on H atom
• Imbalance of charge -> creates dipole -> align to form binding environments
• ∴ very polar solvent
• Presence of polarised OH bonds and oxygen lone pairs -> formation of H-bonds -> extended H-bonding networks -> unique properties (solid less dense than liquid, high melting and boiling points, high viscosity)

Question 3

Explain the bonding interactions that can be formed with water molecules and briefly explain its insoluble regions.

Answer 3

• ∴ disrupting bonding between water molecules to solubilise another species require energy -> energy lost in bonds between water can be regained by forming new bonding interactions (directional H-bonding interactions/dipole-dipole interactions/dipole-ion interactions)
• Environments with free lone pairs/polarised X-H bonds that makes H-bonds with water
• Polar molecules (ex. -OH, C=O, other C-N/C-O/C-F bonds)
• Interactions between water and ionised species (especially strong)
• Insoluble regions: environments with low/no polarity/H-bonding capacity, C-C/C-H rich regions, larger halogens
• *Molecules can contain both polar and non-polar regions

Question 4

Explain why dissolution is usually exothermic and relate it to Gibb’s energy.

Answer 4

• Exothermic dissolution: enthalpically favourable
• Endothermic dissolution: enthalpically unfavourable
• ∵ H-bonding interactions in bulk (pure) water are very dynamic -> molecules can move freely (high entropy)
• ∵ To accommodate solute, water forms a hydration sphere/solvation shell -> move less freely (low entropy)
• ∴ ∆G decreases in dissolution

Question 5

Explain why some exothermic reaction is still disfavoured and why some endothermic dissolution is favoured (ex. CaCl2).

Answer 5

• More energy required to reduce entropy of water molecules in hydration sphere
• More energy released by increased entropy of Ca2+ and Cl- ions

Question 6

Describe all equations involving drug moving between aqueous and non-aqueous environments.

Answer 6

• Drug(aq) ⇌ Drug(org) OR Drug(s) ⇌ Drug(aq)
• Drug in GI(aq) ⇌lipophilic membrane⇌ Drug in blood(aq)
• Protein(aq) + Ligand(aq) ⇌interactions with H2O⇌ Protein-ligand complex(aq)