Liquid Forces

measured in several ways. For example, it can be measured as the time it takes for a certain quantity of liquid to flow through a narrow vertical tube. It can also be measured as the time it takes for a solid to fall through a given volume of the liquid. This is measured in poise units. The higher the number, the higher the viscosity. Strong intermolecular forces will increase the viscosity. Long flexible molecules will have higher viscosities. This is why long-chain hydrocarbons like certain motor oils have high viscosity. Hot liquids will have lower viscosity because of increased kinetic energy.

The vapor pressure of a liquid is the pressure of a vapor when it is in equilibrium with its more condensed phases at a given temperature and in a closed system. The vapor pressure of a liquid will increase with temperature until the liquid boils. As mentioned, the normal boiling point of a liquid is that at 1 atmosphere of pressure. This is exactly 100 degrees Celsius with water. Of course, the actual boiling point is pressuredependent. Molecules of a liquid need a greater kinetic energy (higher temperature) to escape under higher pressure. This is how cooking in pressure cookers can be so successful.

LIQUID FORCES

Liquid forces are more similar to that of solids than they are to gases. It’s the intermolecular force between the molecules that affects the properties of the liquid. These forces will be less than that seen in covalent bonding. Using water as an example, it takes 927 kJ of energy to break the covalent bond between a hydrogen ion and the hydroxyl ion but only about 41 kJ of energy to overcome intermolecular forces in order to convert liquid water to gaseous water. The melting/freezing points of solids and the boiling points of liquids are determined by intermolecular forces.

These forces are primarily electrostatic in nature—between positively-charged and negatively-charged molecules. These forces fall off rapidly as intermolecular distance falls so they become more important between molecules in liquid and solid form versus those in gaseous form (except, of course, at high pressures).

Ionic forces are referred to as Coulombic forces because they involve the phenomena of “like repels like” and “opposites attract”. These forces are very strong—even stronger

than covalent bonding so that it takes greater energy to break apart sodium and chloride ions than it does to break two hydrogen atoms together.

As we’ve talked about, water is a polar molecule, having a dipole moment across the entire molecule with a partial negative charge assigned to the oxygen atom and a partial positive charge assigned to each hydrogen atom. This polarity affects the ability of ions to dissolve in a liquid, leading to hydrated ions. The strength of the ion-dipole attraction depends on the magnitude of the dipole moment and on the charge density of the ion. The charge density of the ion is its charge divided by its volume. This means that smaller positive ions have larger charge densities than negative ions. The largest charge density belongs to Hydrogen ions, which is why it exclusively found as H3O+ (hydrated hydrogen ions) in water.

Ion-dipole interactions always are negative (attractive) because an ion will line up to the aspect of the molecule that is attractive to it. The potential of the attraction in ion-dipole interactions drops off faster than is the case with ion-ion interactions. There is the inverse square law with ion-dipole reactions and a linear relationship in ion-ion interactions.

There are also dipole-dipole interactions, in which polar molecules that have a partial positive charge on one end and a partial negative charge on the other end will have an interaction with themselves. While there are attractive and repulsive forces in polar liquids, the overall forces are positive in liquids that have molecules which can move freely to align themselves in whatever direction they want to. These are weaker forces than are seen in any other type of bonding (such as ionic bonding). The interaction decreases as the distance r between the two molecules to the sixth power or interaction equals 1/r6 .

Being liquid doesn’t just apply to polar molecules. There are interactions between nonpolar molecules like benzene and hexane, which are liquid at room temperature. Depending on the pressure and temperature, even noble gases can be liquid. But what types of interactions are involved in this? These involve what are called London dispersion forces, which involve transient distributions of electrons within the atoms themselves, producing attractive forces between molecules.

These London dispersion forces involve non-equal distribution of electrons at any given point in time, creating an instantaneous dipole moment within the atom itself. These are weak forces that fall off rapidly with increasing distance and are much less than any ionion force, ion-dipole force, or dipole-dipole force. London dispersion forces get stronger with larger molecules because these molecules can shift their electrons to a greater degree than smaller molecules.

The London dispersion forces explain why there is a general trend toward higher boiling points with liquids that have a greater molecular mass and a greater surface area. The shape of the molecule will determine how much of a molecule can interact with its neighboring molecules at any given period of time as well. Long and thin molecules have a greater surface area to interact with other molecules than short and fat molecules. Molecules that are polar can have London dispersion forces as well. Dipole-dipole interactions in small polar molecules are much greater than London dispersion forces. These two forces are collectively referred to as van der Waals forces, although the term can apply to any weak intermolecular force.

We’ve talked about hydrogen bonding briefly but will talk about it here as it represents a type of intermolecular bonding force—a strong dipole-dipole force between hydrogen bond donors and hydrogen bond acceptors in two different molecules. Hydrogen bonding accounts for why HF, H2O, and NH3, have such high boiling points compared to less electronegative molecules.

The large difference in electronegativity causes a partial positive charge on the hydrogen atom and a partial negative charge on the electronegative atom (oxygen, fluorine, or nitrogen). This leads to a strong dipole-dipole interaction between these molecules and intermolecular forces between the two molecules. Figure 42 shows hydrogen bonding in an H2O molecular situation:

Figure 42.

Hydrogen bonding accounts for the high boiling temperature of water as well as its high surface tension and high heat of vaporization. Water has a high viscosity compared to similar liquids because of its intermolecular forces. Its cohesion is greater because of its polarity and it can dissolve many things because of its polarity.