When an ion and a polar molecule (a molecule that has a dipole moment) get near one another, the
charged species (the ion) will interact with the partial charged species (the polar molecule) such that the
oppositely charged and partial charged ends will attract one another. The most common and probably
the most important example is the dissolving of an ionic compound (ions) in water (a polar molecule).
The ionic compound will break down into ions which will be attracted to either the partially negative
oxygen or the partially positive hydrogens in the water molecule.
When polar molecules lie near one another, they will orient themselves in such a manner that their
oppositely charged ends are near one another. In a solid, the molecules will lock into place with their
oppositely charged ends attracting one another. In a liquid, again the oppositely charged ends will
attract one another but the molecules are allowed to move and flip. They are not locked into place as in a
solid, instead, it is like a square dance. They are “partnered up” but constantly flip and roll around,
finding new molecules to interact with.
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Dipole-dipole forces are very important for explaining some common physical properties – such as
boiling point of molecules. For molecules of similar molecular weight, the greater the dipole moment,
the larger the disparity in the partial charges (the more “ionic in nature the bond becomes), the greater
the forces between the molecules and the more energy it takes to overcome those attractions. Thus, the
greater the dipole moment, the more energy is needed to boil the compound (the higher the boiling
point). This means, that when given the dipole moments, you can predict the order of when the
compounds will boil!
Concept Test
Put the following species in order of increasing boiling point based
on their dipole moments, put the lowest boiling point on the LEFT
and the highest boiling point on the RIGHT
Molecule
CH3CH2CH3
CH3Cl
CH3OCH3
CH3CN
CH3CHO
Molecular Wt (g/mole)
44.09
50.48
46.07
41.05
44.05
Dipole Moment (D)
0.1
1.9
1.3
3.9
2.7
The special ability of a molecule to form a hydrogen bond only occurs when a hydrogen is bonded to a
highly electronegative atom that has lone pairs – meaning an oxygen, nitrogen, or fluorine. ONLY!!!
The N-H, O-H, and F-H bonds are very polar, so the electron density is drawn away from the hydrogen
(if we calculated oxidation number on the H – what would it be for every situation like this??) As a
result, this partially positive hydrogen of one molecule is attracted to the partially negative lone pair on
a N, O, or F on another molecule. The hydrogen bond is shown with a dashed or dotted line between the
H and the other atom (O, N, or F). It is not a complete bond – it is the interaction and attraction of
oppositely partially charged species!
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As long as the N, O, or F have lone pair electrons and there is a
partially positive H, H bonds can form.
Concept Test
Which of the following molecules will form H-bonds?
H2N-C≡N
CH2=N=N
N = N
CH2
H bonding is another force that must be overcome in order to change phases. It keeps the molecules
together, specifically in the liquid state where molecules are free to move around and interact.
When comparing boiling points, if a molecule can H-bond, this extra force that must be overcome will
cause an enormous deviation because the H bonds require additional energy to break before the
molecules can separate from one another and enter the gas phase.
The strength of the H bond is only about 5% of a typical covalent single bond, but when
combined, the overall effect of H bonding can be enormous. H bonding helps to hold two
strands of DNA together, but is weak enough to allow the chains to separate for protein synthesis
and cell reproduction.
Surface Tension
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In a sample of liquid, intermolecular forces affect the surface of a sample differently than they affect the
interior of the liquid. Interior molecules are attracted by others on all sides. Molecules on the surface are
only attracted by the molecules on the side and the molecules below it. As a result, the molecules on the
surface are pulled inward and downward which causes the surface to behave like a skin. In order for a
molecule to be on the surface, or to increase the surface area of the liquid, the molecule must overcome
the attractions within the interior of the liquid which requires energy. The stronger the forces are
between the particles in a liquid, the greater the surface tension.
Water is an amazing little molecule. Oxygen, O2, has a molecular mass of 32 amu or 32 g/mole and we
know that O2 is a gas at room temperature. Water, H2O, has a molecular mass of 18 amu or 18 g/mole and it
is a liquid at room temperature. We take these simple things for granted. But if O2 was not a gas and
H2O was not a liquid at standard temperatures and pressures, WE would NOT be here!! So water is
almost half the mass of oxygen – yet it is a liquid. WHY aren’t these two things reversed?? Because of
intermolecular forces! These small, very small, attractions between molecules keep our planet – and our
life (remember DNA!!) in working order. O2 is a nonpolar molecule – therefore the only intermolecular
force that it is subjected to are dispersion forces or other induced dipole forces. This keeps those oxygen
molecules AWAY from one another – making oxygen a gas. Water has a dipole moment. It is a polar
molecule – it is subjected to London dispersion forces (as are all molecules) but it has that oxygen there
with lone pairs and a dipole moment – which means the oxygen is partially negatively charged and the
hydrogens are partially positively charged and this makes water an ideal molecule to participate in H
bonding. Water has a tetrahedral VSEPR geometry and a bent/angular molecular geometry. This shape
and the dipole moment allow for water to engage in 4 hydrogen bonds per 1 molecule of water! That is a
LOT of extra attraction/interaction that must be overcome giving rise to water’s unique physical
properties.
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H is represented by the white spheres, O by the red spheres
And the lone pairs on O by the purple spheres
Water is often called the universal solvent. This is due to its polarity and its ability to H-bond. Water
dissolves ionic compounds through ion-dipole interactions that separate the ions in the solid yet keep
them in solution. The partially negative O will surround the positive cations while the partially positive
H’s will surround the negative anions from the ionic solid. Water can solubilize polar species like
alcohols and sugars by hydrogen bonding with them. Water can even solubilize gases through dipoledipole and London dispersion forces.
Water has a very high – completely unpredicted – specific heat capacity. Heat capacity is defined as the
measure of heat absorbed by a substance to cause a given temperature rise. When energy is put into a
system of water, some of that energy causes molecules to vibrate, some to rotate, and some to increase
the average kinetic energy or speed of the molecules. It is the increase in the average kinetic energy that
we measure by temperature. However, for a given amount of energy put into a system of water, the
overall temperature does not rise all that much meaning water stays liquid and does not evaporate into a
gas! Remember, all those H bonds have to be broken first – before H2O(l) is converted to H2O(g) meaning
that the energy put into the system disrupts the H bonds first. With water covering 70% of our planet
and being one of the most important components for life on this planet – it would not be a good thing for
the oceans – or our bodies! – if a little input of heat caused water to boil.
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H bonding also gives rise to the amazingly high surface tension that water has. You can make a pin float
on top of water due to its surface tension. It may take some practice – but you can try it. Except for
some metals and molten salts, water has the highest surface tension of many liquid. Pretty amazing for
such a small molecule! Water also has high capillary action which allows plants to stay hydrated
especially during the dryer seasons.
When water solidifies, the H bonds become frozen in space. In the liquids, the H bonds are free moving,
rotating and flipping and changing as the molecules move and rotate through space. When the
temperature drops, the water molecules and the H bonds become locked in a hexagonal form.
Snowflakes and ice crystals arise because of this hexagonal open structure of the ice.
The large spaces in the ice crystals give rise to the fact that ice actually has a lower density than does
liquid water. Ice floats on water. If the solid were denser than the liquid – which is true for nearly every
other substance, the surface of lakes would freeze and sink, icebergs would sink, then the surface would
freeze again, and then sink, and then the surface would freeze again and sink. Slowly, our water would
turn into the solid state for the winter months and aquatic life would not survive. Water density changes
slightly as temperature changes and this allows for water to cycle in the system, keeping it all
oxygenated and distributing the nutrients. The water cycle (given in the beginning) helps to maintain
life on earth as well. Water helps with erosion – which can be both a good and bad thing! – it feeds our
plants, and us too! It keeps us cool, and gives us electricity just to name a few things!
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