vapour pressure

Question 1

alt the states of matter

Answer 1

In a gas the atoms/molecules move effectively independently within the container (although collisions and interactions are inevitable)

In a solid (condensed phase) the atoms/molecules motion is effectively restricted to vibrations around a mean position

A liquid is intermediate between these two states; the molecules can move but the intermolecular interactions result in volume being restricted

Question 2

how do the molecules interact in a liquid? (the diff interactions are what?)

Answer 2

Van der Waals’ / intermolecular forces (four main types in decreasing strength)

• Ion-dipole (e.g a salt in water)
• Dipole-dipole (including hydrogen bonding)
• Dipole-induced dipole
• Induced dipole-induced dipole (also called dispersion or London forces)

These tend to be quite weak, short range forces but crucially important in determining liquid properties

Question 3

what is a dipole

Answer 3

By dipole we mean an electronically asymmetric molecule – may be permanent or induced

Question 4

how do dipole dipole forces arise

Answer 4

• two permanent dipoles attract
• Important sub-class is hydrogen bonding
• This occurs when H is attached to O, N or F which are all electronegative
• These tend to be strong bonds and result in high boiling points of water, methanol and hydrogen fluoride
• This also results in a ‘structuring’ of water around solutes, resulting in a degree of order in supposedly random systems
• Entropy affected
• This interaction between water and the solute molecule is crucial for many bio applications e.g protein activity is very much related to shape
• Hydrogen bonding with water is integral to the correct conformation of proteins in the body and in medication + it holds DNA together

Question 5

Dipole-induced dipole

Answer 5

• when a polar molecule (e.g.water) induces a distortion in the electron cloud of a non-polar molecule
• Explains why non-polar molecules e.g oxygen will dissolve in water

Question 6

Induced dipole-induced dipole

Answer 6

• These are due to fluctuations in the electron clouds
• Occur with all molecules and are very weak, but explain why non-polar molecules such as pentane exist as liquids

Question 7

whats the triple point of a substance

Answer 7

The triple point of a substance is the specific temperature and pressure at which all three phases (solid, liquid, and gas) coexist in equilibrium.

Question 8

whats the concept of vapour pressure in air vs water

Answer 8

air - high kinetic energy and exerts an atmospheric pressure
water - lower kinetic energy, and exerts a vapour pressure, has h bonding

Question 9

a proportion of molecules will have enough energy to escape where

Answer 9

into the vapour phase, even below the boiling point - that proportion increases with temperature
strong bonds = low vapour pressure

Question 10

what happens to molecules when they ‘boil’

Answer 10

heat gives molecules in liquid energy
increases vapour pressure
if vapour pressure = atmospheric pressure
boil occurs

Question 11

why vapour pressure is important in pharmacy:

Answer 11

• it leads to evaporation – you do not need to boil a liquid to get it to go into the gaseous state
• Drying is one of the most important processes in preparing dosage forms
• Water is particularly difficult to remove – low vapour pressure
• Residual solvents can cause toxicity
• Avoiding heating can be useful