D3 Medicines; Physiochemical properties 1 (solutions + solubility)

Question 1

What is a solution?

Answer 1

A mixture of two or more components that form a homogenous molecular dispersion (no lumps left)

Question 2

Define: single phase system

Answer 2

A solution; not another phase dispersed in another, where you can’t distinguish between phases.

Question 3

Define: solvent.

Define: solute.

Answer 3

Solvent: the component present in greater amount (usually the liquid).

Solute: the component present in smaller amount (usually a solid or a liquid)

Question 4

What is meant by a saturated solution?

Answer 4

Where the solute is at the limit of its solubility at a given temp. and pressure.

Question 5

Can a suspension become a solution? (if so how)

Answer 5

Heating a suspension can turn it into a clear solution; where a previously translucent solution can turn transparent due to saturation (too much solute)

Question 6

What are the two types of liquid in liquid solution? Explain.

Answer 6

• Completely miscible; e.g. ethanol and water (where addition of more alcohol won’t result in phase separation)
• Partially miscible; e.g. water and ether (will phase separate when too much solute is added; affected by temperature)

Question 7

What is meant by miscibility?

Answer 7

The mutual solubility of components in liquid in liquid systems.

Question 8

What are the two types of solid in liquid solution? Explain.

Answer 8

• Non-electrolytes (non-ionic); e.g. organic molecules (phenol, sucrose)
• Electrolytes (forms ions in solution); strong electrolytes and weak electrolytes. |

> Strong; fully ionised over a wide pH range (e.g. NaCl)
Weak; ionisation dependent on pH (many drugs)

Question 9

What are the effects of solute structure on solution?

Answer 9

A small change in molecular structure can have a massive effect on solubility; where benzene is just miscible in water, phenol (benzene + -OH) is v. miscible.

Question 10

What changes in solute structure increases solubility?

Answer 10

Hydroxylation/esterification. (-OH, -COO- and -NH3 are dece; hydrophilic substituents)

Question 11

Outline the steps in the dissolution of a solid solvent.

Answer 11

1. ) Solvent molecule leaves particle (the drug/tablet etc.)
2. ) Creation of a cavity in the solvent
3. ) Solute molecule enters the cavity

Question 12

What is the relationship between solute surface area and aqueous solubility?

Answer 12

They are inversely correlated; the smaller the solute surface area, the greater the aqueous solubility, where a larger number of solvent molecules can be arranged around the solute.

Question 13

What is solvation?

Answer 13

The process of binding of solvent to solute molecules.

Question 14

What is special about water as a solvent?

Answer 14

Hydration is what water does; unique due to the H-O-H bond angle and its ability to form H-bonds.

Question 15

Describe the ‘flickering cluster’ model for liquid water.

Answer 15

Involves clusters of H-bonded ‘ice-like’ water within pools of unbound (i.e. no H-bonding) water. Cliques of ‘ice-like’ H-bonded water circle dancing whilst surrounded by fluid, non-H-bonded water.

Question 16

How is the flickering cluster liquid water model affects by ions in solution?

Answer 16

Ionic species are hydrated, all ions are bound by an inner circle of tightly bound water, the number of which corresponding to their ‘hydration no.’; monoatomic atoms normally have 4.

Question 17

How are water molecules arranged and orientated around ions in solution?

Answer 17

Cations (+) have the - oxygen bit of water pointing towards them in the inner circle, whilst anions have the + hydrogen groups pointing towards them.

• Inner circle of tightly bound orientated water
• Structure-breaking zone; loose randos milling around just outside lacking normal structure (like no H-bonded)
• Then outer zone of ordinary ‘ice-like’/H-bonded water