Title: CHM2S1-AIntermolecular Forces Dr R. L. Johnston
1CHM2S1-A Intermolecular Forces Dr R. L. Johnston
- Handout 2 The Importance of Intermolecular
Forces - III Intermolecular Forces in Action
- Consequences of Intermolecular Forces
- Anomalous Properties of Water
- The Hydrophobic Effect
- Protein Structure
28. Consequences of Intermolecular Forces
- 8.1 Real Gases
- Ideal (or perfect) gas equation of state
- where R (the gas constant) 8.3145 J K?1 mol?1.
- Assumptions (1) atoms/molecules have no size
- (2) there are no interactions between the
atoms/molecules - Real (imperfect or non-ideal) gases dont obey
this equation, due to the failure of both
assumptions.
3p,V isotherms for an ideal gas
Note that an ideal gas can never liquify, however
low the temperature. The closest we get is He
for which TBP 4.2 K
4- van der Waals equation of state
- Introducing the molar volume, Vm V/n, this
becomes - van der Waals coefficients a, b gt 0.
- a measures strength of attractive interactions
between molecules - b measures volume of molecules
- These equations have the form peff.Veff nRT
5Ideal and Real (Non-Ideal) Gases
In real (non-ideal) gases, we allow for both
non-zero intermolecular forces and non-zero size
of molecules.
6- Pressure
- b reduction of available volume for molecules
to move in, due to non-zero size of molecules.
(Takes account of repulsive forces by modelling
molecules as hard spheres). Less volume to move
in ? more frequent collisions between molecules ?
pressure increases. - a attractive long range interactions between
molecules lead to a decrease in the frequency and
the force of collisions between molecules ?
pressure decreases. - Note
- at high T or high Vm, vdW equation ? perfect gas
equation - liquid and gas coexist when p 0 (when 2 terms
in equation balance).
7p,V Isotherms for a van der Waals Gas
vdW gases can only liquify for T ? Tc
(independent of p).
8From the vdW equation, the following expressions
can be derived Tc 8a / 27Rb Vc 3b
pc a / 27b2 i.e. the lower a (or higher
b), the lower the temperature needs to be for
liquids to form. e.g. CO2 Tc (observed)
304 K Tc (predicted by vdW) 300 K.
9Comparison of van der Waals coefficients
Gas a / Pa m6 mol?2 b / 10?5 m3 mol?1 ? / kJ mol?1 Tb / K
He 0.004 2.370 0.1 4
Ar 0.138 3.219 1.2 87
Xe 0.431 5.105 2.1 165
H2 0.025 2.661 0.3 20
N2 0.143 3.913 0.9 77
CO2 0.369 4.267 2.0 (subl.) 195
CH4 0.231 4.278 1.3 112
C6H6 1.848 11.54 3.1 353
H2O 0.561 3.049 20.0 373
- More polarisable molecules behave in a less ideal
manner, due to larger dispersion forces
(reflected in a larger van der Waals a factor). - Magnitude of b correlates to the size of the
molecule.
10- There are a number of other, more accurate
equations of state. Here, we will mention only
one other. - Virial equation of state
- B second virial coefficient
- C third virial coefficient
- B, C depend on T. B is more important than C
(B/Vm ? C/Vm2). - B has units of cm3 mol?1.
B (273 K) B (600 K)
Ar ?21.7 11.9
CO2 ?149.7 ?12.4
N2 ?10.5 21.7
Xe ?153.7 ?19.6
11- Gas Compressibility
- Ideal (perfect) gases Z 1
- Real gases
- v. low p Z ? 1 molecules far apart ? weak
interactions ? behaves like perfect gas - medium p Z lt 1 attractive forces dominate ?
easier to compress - high p Z gt 1 repulsive forces dominate ? harder
to compress
12Gas Compression Factor (Z)
13- 8.2 Non-Ideal Solutions and Mixtures
- Ideal Solutions obey Raoults Law
- pA partial vapour pressure of A in liquid
mixture - pA vapour pressure of pure liquid A
- xA mole fraction A in liquid mixture.
- Total pressure, p
- In terms of chemical potentials (?) we can write
- Raoults law implies that all interactions A?A,
B?B, A?B are the same (i.e. UAA UBB UAB). - Note does not assume no interactions, but ?mixH
0. - Raoults law is obeyed well by mixtures of
similar (shape and bonding) molecules e.g.
benzene/toluene.
14- Non-Ideal Solutions strong deviations from
ideality (positive or negative) shown by mixtures
of dissimilar liquids e.g. CS2/acetone (UAA ?
UBB ? UAB). - UAB gt UAA , UBB ? ?mixH lt 0 (exothermic mixing)
- negative deviation
- UAB lt UAA , UBB ? ?mixH gt 0 (endothermic mixing)
- positive deviation
- In terms of chemical potential
- where aA is the activity ( effective mole
fraction) of liquid A in the mixture. -
15Vapour Pressures of Solutions
16- 8.3 Other Consequences of IMFs
- Different phases adopted by various elements and
compounds. - Structures of solids and liquids.
- Liquid crystals unusual properties due to
anisotropic intermolecular interaction (e.g.
disk-like or cigar-shaped molecules). - Transport properties (viscosity, thermal
conductivity, diffusion). - Properties of electrolyte solutions (solvated
ions). - Supramolecular chemistry (aggregation,
self-ordering, molecular recognition, protein
folding, drug-protein interactions, DNA ).
17- 8.4 Some Experimental Techniques for
Investigating IMFs - Molecular Beams study collisions and scattering
between individual molecules. - X-ray and neutron diffraction determine long
range structures of crystalline solids and short
range structure of liquids. - Spectroscopy determine structures, binding
energies and electronic, vibrational and
rotational energies of loosely bound van der
Waals molecules. - Measurement of gas imperfection e.g. pV
isotherms, Joule-Thomson effect, compressibility. - Measurement of solution non-ideality deviations
from Raoults law and Henrys law. - Measurement of transport properties
- Atomic Force Microscopy direct measurement of
intermolecular forces between surfaces and
adsorbed molecules.
189. Anomalous Properties of Water
- 9.1 Water
- Water is the most abundant liquid on Earth.
- But it is considered to be anomalous because
it behaves differently from simple liquids (e.g.
Ar). - Differences are due to hydrogen bonding in water.
- The water molecule is small and compact, with two
H atoms and two lone pairs arranged tetrahedrally
around the O atom - The dipole moment (?) of the isolated water
molecule is 1.85 D. - Water forms hydrogen bonds the O?H bonds act as
H-bond donors and the O lone pairs act as H-bond
donors. - Each water molecule can take part in up to 4
H-bonds.
19- In the gas-phase optimal H-bond bond strength
between water molecules 23 kJ mol?1. - H-bonding in condensed phases of water is
- cooperative (non-additive) the strength of
- H-bonding increases with increasing number
- of water molecules, as this increases the
- polarization of the O?H bonds.
- This shows up in the increase in the average
dipole moment per water molecule, which increases
from 1.85 D (isolated H2O) to 2.4?2.6 D (liquid
H2O at 0?C).
20Phase Diagram for Water
21- 9.2 Ice (Solid Water)
- The structure of ice is based on tetrahedral
coordination of the water molecules, which each
take part in 4 H-bonds. - There are a number of different solid ice phases.
At 1 atm. the most stable form is hexagonal ice
Ih.
22- 9.3 Liquid Water
- For water, the liquid is more dense than the
solid (ice). This is in contrast to most
liquids. Maximum density of liquid is at around
4?C. Above 4?C, water behaves like other liquids
expanding as it gets warmer. - This is due to the disruption of the long-range
ordered tetrahedral network in liquid water. The
average number of nearest neighbours around each
H2O molecule increases from 4 to approx. 4.4 on
melting. - There is a fluctuating network of H-bonds in
liquid water. - Higher densities are favoured by increasing van
der Waals (D-D and dispersion) interactions,
though H-bonding favours lower coordination and
lower density. ? on melting, the H-bonding is
weaker but the vdW bonding is stronger. - Consequences ice-bergs burst water pipes in
winter
23- Applying pressure to ice causes melting.
According to the Clapeyron equation -
-
- ? every 133 atm. of applied pressure, decreases
the melting temperature of ice by 1 K. - This may contribute to enabling ice skating!
24- Other properties of liquid water
- Liquid water is less compressible than ice.
Compressibility decreases with T until 46?C. - Liquid water has a high dielectric constant
- (because H-bonds are polarizable) so it is
- a good solvent for ions.
- H-bonding leads to higher cohesive energies than
- for similar-sized molecules (especially compared
with - H2X molecules from the same group) ? relatively
high - boiling and melting points (same true for HF and
NH3). - The extended H-bonded network in liquid water
leads to rapid transfer of H and OH? ? changes
of pH move rapidly through aqueous solutions. - Water has a high enthalpy (40 kJ mol?1) and
entropy of vaporization (109 JK?1 mol?1),
indicating that the liquid still has quite a lot
of the order (and cohesion) of the solid ? water
has a very high liquid range (100 K). This is
critical for life on Earth!
25Comparison of boiling points of group 16 and
group 18 hydrides
2610. The Hydrophobic Effect
- 10.1 Definitions
- Hydrophobic Effect The low solubility of
hydrocarbons and other non-polar molecules in
water and their increased tendency to aggregate. -
- Hydrophobic Interaction Enhanced effective
attractions between hydrocarbon molecules etc.,
when in water. - Simple enthalpy explanation immiscibility (lack
of solubility) of solute B in solvent A occurs
when the A-B interactions are weaker than the A-A
and B-B interactions (UAB lt UAA, UBB). - This might be expected to be the case for B
hydrocarbon (quite strong dispersion forces
between long chain hydrocarbons) and A water
(strong H-bonds), with A-B interactions being
primarily dipole-induced dipole in nature
(relatively weak). - BUT this does not explain why solubility of oil
in water, as a function of T, goes through a
minimum at T ? 25?C. (Normally expect solubility
? as T ?).
27- 10.2 Origin of the Hydrophobic Effect
- Note the overall enthalpy of interaction of a
non-polar solute with water is not particularly
unfavourable (?H ? 0) because the non-polar
molecules induce cage-like ordering of the first
shell of water molecules, strengthening their
H-bonding. - The origin of the hydrophobic effect is mostly
entropic. - The ordering of the shell of water molecules
around the hydrocarbon solute (so as to minimise
dangling H-bonds), causes a significant
decrease in the entropy of the water (?S lt 0). - Typically, the total change in entropy in
dissolving small hydrocarbon molecules in water
(at 298 K) - ?S ? ?100 J K?1 mol?1
- For T lt 25?C, entropy term dominates and becomes
more unfavourable with increasing T ? solubility
decreases as T rises. - For T ? 25?C, the water cages start to break up
(weakening H-bonds) so ?H, ?S increase ?
solubility increases as T rises (enthalpy starts
to dominate).
28- 10.3 Clathrates single hydrocarbon or other
non-polar molecules (even small ones, such as CH4
and CO2) surrounded by a polyhedral cage of water
molecules. - At high P, low T, these clathrates can
precipitate out as solids. - Examples CH4-H2O clathrates in oil pipelines.
- CO2-H2O clathrates in deep ocean sites.
29- 10.4 Micelles (examples of colloids)
pseudo-spherical clusters of surfactant molecules
consisting of hydrophilic heads (polar or
charged groups) and hydrophobic tails
(hydrocarbon chains) dispersed in water. - Hydrophobic tails aggregate together (dispersion
forces) this also minimizes the unfavourable
hydrophobic entropy effect on the solvent
(water). Centre of micelle is oil-like. - Hydrophilic heads form a close-packed shell and
have strong intermolecular interactions with the
water molecules. - Sizes range from 100s (charged heads) to 1000s
of molecules. - Used to solubilize hydrocarbons in aqueous
solution - e.g. detergents, drug carriers, organic
synthesis, petroleum recovery. - Analogous to biological membranes.
30Clathrates and Micelles
3111. Protein Structure
- 11.1 Proteins natural polymers polypeptides
chains of amino acids (H2NCHRCO2H) joined by
peptide links ?CO-NH?. - Protein folding the folding up of the
polypeptide chains under the influence of
intermolecular forces. - Note the chemical function of the protein is
dependent on its 3D structure which depends on
its folding. -
- Primary structure sequence of amino acids.
- Secondary structure coiling into ?-helices or
folding into ?-sheets, due to N?H?OC
hydrogen-bonding between peptide groups (which
are close in the sequence). - Tertiary structure folding of the polypeptide
chain by forming interactions (e.g. covalent
disulfide ?S?S? links, ionic interactions,
H-bonds) between side chains (R) of amino acids
which are relatively far apart in the sequence.
In aqueous solution, the hydrophobic effect may
also be important.
32- Quaternary structure aggregation of more than
one polypeptide chain due to similar interactions
to those responsible for tertiary structure. - Protein aggregation
- sometimes beneficial (e.g. for the function of
haemoglobin an aggregate of 4 polypeptide
chains) - sometimes harmful (e.g. in protein misfolding
diseases such as BSE, CJD).
33Secondary Protein Structure
34Secondary and Tertiary Protein Structure
35- 11.2 The hydrophobic effect in protein folding
- Globular proteins in aqueous solution have
pseudo-spherical shapes - cores rich in hydrophobic residues (amino acids
with non-polar alkyl or aryl side-chains, R) - outer shell rich in hydrophilic residues (polar
side chains). - Protein folding is partially driven by the
hydrophobic effect burying ? ½ of hydrophobic
residues reduces the unfavourable decrease in
entropy of the surrounding water molecules.