Intermolecular Forces
The intermolecular forces between molecules are important in the properties of all solid and liquid
materials. They are key to reactions that take place in biological molecules. Proteins form their secondary
and tertiary structures through hydrogen-bonding and London forces. DNA forms because of hydrogen
bonding between base pairs. Enzymes function when molecules interact with the protein active site in
these biological catalysts.
Outline
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Types of Intermolecular Forces
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Entropy Considerations
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Intermolecular Forces and DNA
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Homework
Types of Intermolecular Forces
Solutions consist of a solvent and solute. There
are gas, liquid, and solid solutions but in this unit
we are concerned with liquids.
The solvent then is a liquid phase molecular
material that makes up most of the solution.
Water is a good example of a solvent.
The solute is a smaller quantity of anything that
is dissolved in the solvent. It can be a gas (O2 in
water), a liquid (water dissolved in ethanol), or a
solid (NaCl dissolved in water).
In any solution, the molecules or ions of the solute are randomly distributed among the molecules of the
solvent.
What are the intermolecular forces? They are listed in the table below along with covalent and ionic
bonding for comparison. Notice that they have different dependence on the distance between the
attracting particles, r.
Materials dissolve in a solution when there are strong intermolecular forces between the solute and the
solvent.
London Dispersion Forces
We could discount intermolecular interactions between gas-phase molecules because these molecules are
mostly far apart and moving rapidly relative to each other. In the liquid phases, all molecules interact with
one another. The stronger the interaction between a molecule and a pure liquid, the greater will be the
solubility of the molecule in the liquid.
All molecules interact with each other through London dispersion forces, or induced dipole interactions.
In the figure below, a 2-atom molecule collides with a 3-atom molecule. The electron cloud of the first
molecule repels the electron cloud of the molecule it strikes, causing a displacement of some electron
density away from the nucleus. The nucleus is then poorly shielded by its own electrons and attracts the
electron cloud of the first molecule.
Both molecules now have a small dipole moment that was induced by molecular collision.
Dipole-Induced Dipole Interactions
When a molecule with a permanent dipole, such as HCN, collides with a molecule without a molecular
dipole, the collision itself causes a dipole to appear by changes in electron density within the molecule.
The nitrogen atom in HCN is electron rich and the molecular dipole points in the direction of this atom.
Upon collision, the electron cloud of the second atom would be repelled by the excess electron density on
nitrogen so the positively charged nucleus would be closer to N and would interact with it.
Dipole-Dipole Interactions
Molecules with permanent dipoles can interact with other polar molecules through dipole-dipole
interactions. Again this is electrostatic in nature. The molecular dipole vector points towards high electron
density.
Note that polar molecules also interact with each other through London forces. The dipolar interactions
add to this force.
Hydrogen Bonding Interactions
Hydrogen that is bonded to very electronegative elements (F, O, or N) is highly electron deficient. It acts
as a Lewis acid and interacts with basic sites in other molecules. The hydrogen bonding interaction is
stronger than dipole-dipole interactions. Again, it adds to the existing London dispersion forces to
stabilize molecules in solution.
Hydrogen bonding interactions are stronger than the other interactions that take place in solution, with an
energy of 5 to 30 kJ/mol for each interaction. It has some aspects of dipole-dipole interactions and some
aspects of covalent bonding. For example, the interaction between X and H in X---H-Y is less than the
sum of the radii of the two atoms but more than their covalent bond distance.
Entropy Considerations
What is entropy?
The easiest way to think of entropy is as a measure of disorder in a system. Alternatively, it is the
spreading and sharing of thermal energy within a system. Entropy is energy in the system that is unusable
for chemical change. Over time, entropy increases.
We use S to stand for entropy and ΔS is the change in entropy.
Free Energy
We have talked about the energy changes in chemical reactions and changes in state in terms of enthalpy.
Remember that ΔH is the change in heat energy at constant pressure.
We can classify chemical reactions as being spontaneous or non-spontaneous. In most spontaneous
reactions heat is released from the system to the surroundings and ΔH is a negative number. These are
exothermic reactions.
Some reactions proceed at a given temperature even though they are endothermic. The surroundings get
colder as heat is absorbed. How can a spontaneous reaction absorb heat when all chemical systems tend to
move to lower energy states from higher states?
The answer is entropy.
A very useful energy term is ΔG, or the Gibbs free energy. It is this that determines whether or not a
reaction will proceed spontaneously in the forward direction. If the value of ΔG is a negative number, the
reaction is spontaneous as written. If the value of ΔG is a positive number, the reaction will not occur as
written and, in fact, the reverse reaction will be spontaneous.
Δ G = Δ H - TΔ S
The enthalpy change is usually the most important factor in the Gibbs free energy because the value of
ΔH is typically much greater than the value of ΔS. However when the enthalpy change is small the
entropy change can determine the spontaneity of the reaction.
Entropy Changes in Solutions
How does this relate to solutions and intermolecular forces? Let's consider the case of water and table
salt.
Water is a highly ordered material. You made models of parts of the ice/water lattice in class showing that
each oxygen atom is connected to others around it through bridging hydrogen atoms (an extreme case of
hydrogen bonding). When something dissolves in water, some of these O-H bonds are broken. This
requires heat energy. The water molecules can then form attractive interactions to solute ions or
molecules, releasing heat energy.
A crystal of NaCl is also highly ordered. The chloride anions form a cubic close packed lattice and the
sodium cations fit into the octahedral holes in the lattice. Strong ionic bonding holds the anions and
cations together in the crystal. When NaCl dissolves in water the strong ionic bonds are broken (requiring
heat energy) and the ions interact with water molecules (releasing heat energy).
The solution of NaCl in water has much less order than the pure water and the crystalline salt. Entropy
increases every time a solute dissolves in a solvent.
Examples:
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When NaCl dissolves in water the heat required just about balances the heat released so the
temperature of the solution changes very little.
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When calcium chloride, CaCl2, dissolves in water, heat is released. This salt is used in hot packs.
•
When ammonium nitrate, [NH4][NO3], dissolves in water the solution becomes colder. This salt is
used in cold packs. Even though the enthalpy change is a positive number, the dissolution is
spontaneous because the Gibbs free energy change, G, is negative due to the entropy term.
Intermolecular Forces and DNA
Proteins
Proteins are a key part of all living things. They are
long chain polymers made of amino acids,
+NH3CH(R)C(O)O- where R stands for one of
about 20 different groups. The amino acids are
connected through covalent bonds to give the
primary structure of the protein.
Hydrogen bonds between sections of the protein
chain are responsible for the secondary structure
of the protein.
The protein, with its H-bonded sections, also folds
into a 3-dimensional structure that forms because
of hydrogen bonding, dipole-dipole interactions,
and London forces between sections of the
protein.
Enzymes
Enzymes are proteins that catalyze chemical reactions in living things. The enzyme active site is shape
selective and the non-covalent interactions between parts of the protein are responsible for the shape.
Other non-covalent interactions between the enzyme and the substrate hold the reactants in place so that
products can form.
Below you see an enzyme that binds the organic molecule camphor in its active site through a hydrogen
bond and several dipole-induced dipole and induced dipole-induced dipole interactions.
DNA
DNA is the molecule that contains the genetic code of a living thing.
It forms through a hydrogen-bonding interaction between the base
pairs adenine, guanine, cytosine, and thymine. These make up the
"steps" in the spiral staircase of DNA shown at right.
The "railings" consist of sugar and phosphate groups that connect together.
The structure of the bases are shown below. In class you'll have an activity
that focuses on the hydrogen bonding interactions between pairs of these
molecules.