Ch 11 Liquids and Intermolecular forces

Ch 11 liquids and intermolecular forces

The strengths of intermolecular (between) forces vary over a wide range but are generally much weaker than intramolecular (within) forces—ionic, metallic, or covalent bonds

three types of intermolecular attractions that exist between electrically neutral molecules: dispersion forces, dipole–dipole attractions, hydrogen bonding.

electrostatic interactions get stronger as the magnitude of the charges increases and weaker as the distance between charges increases

Dispersion forces

There are electrostatic attractions between neutral molecules. These are instantaneous and called dipole

It is significant only when molecules are very close together.

The ease with which the charge distribution is distorted is called the molecule’s polarizability. We can think of the polarizability of a molecule as a measure of the “squashiness” of its electron cloud: The greater the polarizability, the more easily the electron cloud can be distorted to give an instantaneous dipole. Therefore, more polarizable molecules have larger dispersion forces.

polarizability increases as the number of electrons in an atom or molecule increases. The strength of dispersion forces therefore tends to increase with increasing atomic or molecular size. Because molecular size and mass generally parallel each other, dispersion forces tend to increase in strength with increasing molecular weight

Dipole-dipole interactions

originate from electrostatic attractions between the partially positive end of one molecule and the partially negative end of a neighboring molecule. Repulsions can also occur when the positive (or negative) ends of two molecules are in close proximity. Dipole–dipole interactions are effective only when molecules are very close together.

For molecules of approximately equal mass and size, the strength of intermolecular attractions increases with increasing polarity

Hydrogen bonding

A hydrogen bond is an attraction between a hydrogen atom attached to a highly electronegative atom (usually F, O, or N) and a nearby small electronegative atom in another molecule or chemical group.

Ion dipole attractions

An ion–dipole force exists between an ion and a polar molecule. Cations are attracted to the negative end of a dipole, and anions are attracted to the positive end. The magnitude of the attraction increases as either the ionic charge or the magnitude of the dipole moment increases. Ion–dipole forces are especially important for solutions of ionic substances in polar liquids, such as a solution of NaCl in water

1. When the molecules of two substances have comparable molecular weights and shapes, dispersion forces are approximately equal in the two substances. Differences in the magnitudes of the intermolecular forces are due to differences in the strengths of dipole–dipole attractions. The intermolecular forces get stronger as molecule polarity increases, with those molecules capable of hydrogen bonding having the strongest interactions.

2. When the molecules of two substances differ widely in molecular weights, and there is no hydrogen bonding, dispersion forces tend to determine which substance has the stronger intermolecular attractions. Intermolecular attractive forces are generally higher in the substance with higher molecular weight.

Larger principal quantum numbers correspond to the valence electrons being farther from the nucleus, therefore they are more easily distorted by an external electric field, leading to greater polarizability

Ion dipole interactions are generally stronger than dipole dipole interactions and weaker than ionic bonds. This is bc dipole means partial charge

11.3 Select Properties of liquids

The resistance of a liquid to flow is called viscosity. The greater a liquid’s viscosity, the more slowly it flows (km/m-s is its label)

Increases with molecular weight

viscosity of a substance decreases with increasing temperature

the melting point of an ionic compound can be low if the ionic charges are not too high and the cation–anion distance is sufficiently large.

Ionic liquids are nonvolatile (that is, they don’t evaporate readily) and nonflammable

The surface of water behaves almost as if it had an elastic skin, as evidenced by the ability of certain insects to “walk” on water. This behavior is due to an imbalance of intermolecular forces at the surface of the liquid.

molecules in the interior are attracted equally in all directions, but those at the surface experience a net inward force. This net force tends to pull surface molecules toward the interior, thereby reducing the surface area and making the molecules at the surface pack closely together

Because spheres have the smallest surface area for their volume, water droplets assume an almost spherical shape. This explains the tendency of water to “bead up” when it contacts a surface made of nonpolar molecules, like a lotus leaf or a newly waxed car.

Surface tension is the energy required to increase the surface area of a liquid by a unit amount.

Intermolecular forces that bind similar molecules to one another, such as the hydrogen bonding in water, are called cohesive forces. Intermolecular forces that bind a substance to a surface are called adhesive forces.

Water placed in a glass tube adheres to the glass because the adhesive forces between the water and the glass are greater than the cohesive forces between water molecules; glass is SiO2 which is polar. This causes meniscus

The rise of liquids up very narrow tubes is called capillary action

The adhesive forces between the liquid and the walls of the tube tend to increase the surface area of the liquid. The surface tension of the liquid tends to reduce the area, thereby pulling the liquid up the tube. The liquid climbs until the force of gravity on the liquid balances the adhesive and cohesive forces.

By forming beads, water minimizes its contact with the surface, which tells us that the cohesive forces between water molecules are stronger than the adhesive forces between water molecules and the surface.

11.4 Phase Changes

Melting is called (somewhat confusingly) fusion. The increased freedom of motion of the particles requires energy

vapor pressure increases with increasing temperature until it equals the external pressure above the liquid, typically atmospheric pressure. At this point the liquid boils—bubbles of the vapor form within the liquid. The energy required to cause the transition of a given quantity of the liquid to the vapor is called either the heat of vaporization

The particles of a solid can move directly into the gaseous state. The enthalpy change required for this transition is called the heat of sublimation

Just as for a given substance the heat of condensation is equal in magnitude to the heat of vaporization and has the opposite sign, so too the heat of deposition for a given substance is exothermic to the same degree that the heat of sublimation is endothermic; the heat of freezing is exothermic to the same degree that the heat of fusion is endothermic

Because melting is an endothermic process, the heat we add at 0°C is used to convert ice to liquid water, and the temperature remains constant until all the ice has melted.

The highest temperature at which a distinct liquid phase can form is called the critical temperature. The critical pressure is the pressure required to bring about liquefaction at this critical temperature. The greater the intermolecular forces, the higher the critical temperature of a substance.

Notice that nonpolar, low-molecular-weight substances, which have weak intermolecular attractions, have lower critical temperatures and pressures than substances that are polar or of higher molecular weight.

When the temperature exceeds the critical temperature and the pressure exceeds the critical pressure, the liquid and gas phases are indistinguishable from each other, and the substance is in a state called a supercritical fluid.

11.5 vapor pressure

The ethanol quickly begins to evaporate. As a result, the pressure exerted by the vapor in the space above the liquid increases. After a short time, the pressure of the vapor attains a constant value, which we call the vapor pressure. The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor are in dynamic equilibrium.

The condition in which two opposing processes occur simultaneously at equal rates is called dynamic equilibrium (or simply equilibrium)

Point T, where the three curves intersect, is the triple point, and here all three phases are in equilibrium.

H2o phase diagram above

11.7 liquid crystals

Reinitzer’s work represents the first systematic report of what we call a liquid crystal, the term we use today for the viscous, milky state that some substances exhibit between the liquid and solid states.

Today liquid crystals are used as pressure and temperature sensors and as liquid crystals displays (LCDs) in such devices as digital watches, televisions, and computers. They can be used for these applications because the weak intermolecular forces that hold the molecules together in the liquid crystalline phase are easily affected by changes in temperature, pressure, and electric fields.

In a nematic liquid crystal, the molecules are aligned so that their long axes tend to point in the same direction but the ends are not aligned with one another.

In smectic A and smectic C liquid crystals, the molecules maintain the long-axis alignment seen in nematic crystals, but in addition they pack into layers.

In a cholesteric liquid crystal, the molecules are arranged in layers, with their long axes parallel to the other molecules within the same layer.37 Upon moving from one layer to the next, the orientation of the molecules rotates by a fixed angle, resulting in a spiral pattern.