Brown's Chemistry Advanced A Level Notes - Theoretical–Physical
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 8
Part 8.1 Phase Equilibrium – Vapour pressure – Boiling
brown - comments - query?
Index of ALL my chemical equilibrium
context revision notes Index
ALL my advanced A
chemistry revision study notes
mobile phone or ipad etc. in 'landscape' style
This is a BIG
website, you need to take time to explore it [SEARCH
The liquid–gas phase equilibria situation
is discussed and the vapour pressures of selected liquids are compared
and discussed and the relationship between boiling point and ambient
Phase equilibria – vapour pressure – boiling point
- e.g. liquid water
vapour or H2O(l)
- The co–existence of water in two
different phases is a clear example of a phase equilibrium.
- If water (or an aqueous solution) is
poured into a dry bottle of 'dry' air and then stoppered, some of
the water will then evaporate to create a vapour pressure, so water
vapour and liquid water co–exist.
- However, some of the water vapour may
condense if the gas molecules hit the liquid surface but others can
escape the intermolecular forces at the surface (which are
imbalanced and incidentally cause the net inward pull of surface
- When the rate of evaporation = rate
of condensation, a dynamic physical equilibrium exists
between liquid water and water vapour.
- Note, that at a certain temperature
and pressure (called the triple point), liquid water, water vapour
and ice can all co–exist!
- The saturated vapour pressure
is the maximum pressure the vapour from a liquid (or solid/solution)
can exert at a given temperature if the substance alone occupied a
container in which the liquid/solution/solid is in equilibrium with
- The maximum or saturated vapour
pressure that liquid water exerts at a particular temperature
can be measured and it increases exponentially with increase in
temperature, i.e. the equilibrium is shifted more to the right,
see graph below.
- The same shaped graph line (examples
below for pure liquids) is obtained with any liquid, though the
relative position of the curve depends on the volatility of the
liquid, which in turn depends on the strength of the inter–molecular
attractive forces (see later paragraph).
Liquids containing dissolved substances
i.e. mixtures of miscible liquids or solutions of solids, will show
a reduced vapour pressure compared to the pure liquid (not covered
saturated vapour pressure curves (max. pvap versus
- The vapour pressure curves are shown
in the graph above for
- tetrachloromethane CCl4, ethanol
benzene C6H6, water H2O
and ethanoic acid CH3COOH
- The increase in vapour pressure of
any liquid with rise in temperature is due to the increasing average
kinetic energy of the liquid molecules, so more molecules with
sufficient KE can 'escape' the intermolecular attractive forces of
the liquid molecules, increasing the rate of evaporation until it
matches the rate of condensation i.e. a dynamic equilibrium point is
reached and the liquid exerts its maximum or saturated vapour
- When the vapour pressure of a
liquid matches the ambient pressure, the liquid boils as bubbles
of vapour can now form in the bulk liquid.
- At 100oC, water vapour
pressure = normal atmospheric pressure, and so water boils at normal
atmospheric pressure (1 atm/760 mmHg, see dotted lines on diagram
- This has consequences for brewing a
cup of tea as you climb a high mountain. It gets more and more
difficult to brew a good pot of tea because the boiling temperature
gets lower and lower as the pressure falls with increasing height!
- Note on some applications of vapour
- A knowledge of vapour pressures from
data tables is important is understanding and using the technique
steam distillation, described
in section 8.5, which is a method of extracting thermally
unstable compounds from a mixture by a distillation process
operating at a far lower temperature than its boiling point.
- You can also distil thermally
unstable compounds at a more lower temperature than its normal
boiling point at normal atmospheric pressure by means of reduced
pressure distillation ('vacuum distillation'). If you reduce the
pressure in the flask and condenser system, you then reduce the
vapour pressure needed for the liquid to boil i.e. lower vapour
pressure means a lower boiling–distillation temperature. This is
actually a much cleaner and efficient process than steam
- The greater the
intermolecular forces between molecules,
the lower the saturated (maximum pvap) vapour pressure at
a given temperature, given in Pa/mm Hg 20oC (sat'd pvap)
in section 8.2.
- Collectively 'intermolecular forces'
are also referred to as 'Van der Waals Forces' and you must
be able to clearly distinguish these forces from ionic or covalent
bonds, even for the unfortunately misnamed 'hydrogen bond'.
- All intermolecular forces are
essentially electrostatic in origin arising from 'Coulombic'
attraction between positive and negative 'centres' in the atoms or
molecules. A 'centre' might quite simply be a region more positive
or more negative than another due to asymmetry in the distribution
of the electron charge (clouds).
- Details of these intermolecular
forces are further discussed later in the context of comparative
studies of particular series of molecules.
- The greater the intermolecular
forces the lower the vapour pressure, the higher the boiling point
of the liquid
and to some extent enthalpies of vaporisation, the latter does not
give as clearer a pattern as you might expect.
- You also see, not surprisingly, higher enthalpies of vaporisation (ΔHvap/kJ
mol–1) BUT not a good trend if you study the summary
- However you get a more clearer trend with increasingly higher boiling point (bpt at 101325Pa / 1atm / 760mmHg) as more kinetic energy is needed by the molecules to
overcome the increase in intermolecular attractive forces.
- These intermolecular
forces are strongly influenced by the structure of the molecule and
the extent of bond polarity can have a major effect on the boiling
point of an organic liquid, but most of the attractive force for
most molecules originates from transient (temporary) dipole –
induced dipole interactions (explained further in the ALKANE
8.2.1 and 8.2.2 compares eight
of similar molecular mass (58 to 60 and 64.5 for the halogenoalkane) and total number of electrons
(32 to 34) whose physical
properties illustrate these trends.
- All eight organic molecules, apart
are based on combinations of 4 C/N/O atoms plus H atoms.
- They were the best match and range I could come up with to do a
comparative paper based study!
8 molecules are discussed in order of
increasing boiling point (bpt).
Part 8 sub–index:
8.1 Vapour pressure origin and examples * 8.2.1
Introduction to Intermolecular Forces * 8.2.2
Detailed comparative discussion of boiling points of 8 organic molecules * 8.3
Boiling point plots for six
homologous series * 8.4 Other case studies of
boiling points related to intermolecular forces * 8.5
distillation – theory and practice * 8.6 Evidence and theory
for hydrogen bonding in simple covalent hydrides *
Solubility of covalent compounds, miscible and
Advanced Equilibrium Chemistry Notes Part 1. Equilibrium,
Le Chatelier's Principle–rules * Part 2. Kc and Kp equilibrium expressions and
calculations * Part 3.
Equilibrium and industrial processes * Part 4.
solubility product and ion–exchange * Part 5.
pH, weak–strong acid–base theory and
calculations * Part 6. Salt hydrolysis,
Acid–base titrations–indicators, pH curves and buffers *
Redox equilibria, half–cell electrode potentials,
electrolysis and electrochemical series
TOP OF PAGE
|Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision summaries & references to science course specifications