Intermolecular Forces

From MyMCAT

Introduction

When a substance is in its liquid state, a variety of forces act to keep the molecules together, without these forces, the molecules would all just simply spread out into a gas, expanding to fill their container. Liquids with strong intermolecular forces have a high boiling point because more energy is required to overcome these forces. Conversely, if intermolecular attractions are weak, the boiling point will be low.

Dipoles

When a molecule has an uneven distribution of electrical charge, it is said to be a dipole. There are numerous ways for this to occur, but the net result is always a molecule in which there is both a slightly negative and positive charged region.

Consider, hydrogen Cyanide, HCN. In an HCN molecule, the nitrogen is much more elctronegative than the rest of the molecule, thus it is capable of pulling eletrons towards itself through the C-N triple bond. The much weaker hydrogen on the other end can not fight this pull and in turns loses a bit of its own electron density. The overall result then, is a slightly positive charged "patch" or region on the hydrogen and a negative region on the nitrogen.

The HCN example above is considered a permanent dipole because the molecule is like this all the time. Induced dipoles however can also occur, when a different molecule (such as an ion or another dipole) interact causing electrons to rearrange or be pulled over thus turning the current molecule, which did not have any dipole, into a dipole. This action is called an induced dipole.

As a result of the fact that electrons are always moving and at any given time they could be anywhere around a molecule, instantaneous dipoles exist all the time regardless of the molecules in question. The results of these instantaneous dipoles however are very weak as the dipole is only transiently present at any given time.

Types of Intermolecular Forces

The nature of a liquids intermolecular forces depends entirely on the structure of the molecules present in the liquid. Ions, molecules which are polar (have dipoles) or can be induced to be a dipole, and nonpolar molecules all behave and interact differently resulting in a variety of forces.

Ion-Dipole

The strongest type of attraction is the ion-dipole. This is a result of a charged ion being attracted to the dipole of a polar molecule. Examples include the dissolution of salt in water; the negatively-charged Cl ions will be attracted to the positive dipole near the hydrogens, while the positively-charged sodium ions will seek the negative dipole of the oxygen atom. These attractions are powerful enough to tear apart the NaCl crystal when it enters water, meaning that salt dissolves. If the crystal structure is too strong to be broken by attractions between a solvent and the ionic solid, then the solid will not dissolve. In general, this force will be seen in any polar solvent capable of dissolving an ionic compound.

Dipole-Dipole

The next type of attraction is the dipole-dipole force, in which the dipoles of two molecules are mutually attracted. For example, the molecule FI has a permanent dipole because fluorine "wants" electrons more than iodine, which leads to a positive charge on the iodine and a negative charge on the fluorine. These molecules will attract each other, because the negatively-charged fluorine will be drawn to the positive iodine atom of another molecule.

Hydrogen Bonding (A Special Dipole-Dipole)

A subtype of dipole-dipole interactions is hydrogen bonding, which only occurs when a nitrogen, oxygen, or fluorine atom is bonded a hydrogen. Since each of these atoms "wants" electrons more than hydrogen and because the hydrogen is relatively small, a dipole will result causing unusually strong attractions between molecules. The intermolecular attractions in water (H2O), menthanol (CH3OH), ammonia (NH3), and hydrogen fluoride (HF) are all examples of hydrogen bonding. All of these substances have high boiling points, due to the unusual strength of hydrogen bonding.

Dipole-Induced Dipole (A Weak Dipole-Dipole)

This occurs when a polar molecule, such as water, is attracted to a non-polar molecule, such as hexane(C₆H₁₄). This type of attraction occurs because the charge on the water molecule distorts the electron cloud on hexane, causing them to be either attracted to the positive dipole or repelled by the negative dipole. These interactions are very weak, but explain the solubility of non-polar compounds in polar solvents, such as oxygen dissolved in water.

Note that the first three types of forces generally decrease with size; the larger the atoms or molecules, the less attractive the intermolecular forces will be.

Dispersion forces, London forces, van der Waals forces, Induced Dipole-Induced Dipole

Known by a lot of different names, the dispersion forces or london forces, are really just the low level effect of the randomness of electrons. Because electrons around molecules randomly move, at any given time the electron cloud around a molecule must be unevenly distributed and thus can interact.

These forces are usually weak, but increase with the molecule's size, and can become fairly strong in large molecules or atoms (because there are more opportunities for electron cloud distortion). Under standard conditions, from CH4 to C4H10 alkanes are gaseous; from C5H12 to C17H36 they are liquids; and after C18H38 they are solids, it is the dispersion forces which cause this phenomena as no other forces are acting. These forces are also the only reason for rare-gas atoms to condense at low temperature.

As a general rule, the larger the molecule the more dispersion forces possible, however one should also keep in mind surface area. A long chain alkane has a much larger surface area than an alkane with lots of branches but of the same molecular weight, thus the long chain alkane has more area to under go interactions and will have the larger boiling point/stronger forces.

Solubility Law for Liquids

These interactions lead us to postulate a solubility law for liquids: like dissolves like. For example, polar liquids will dissolve other polar liquids because of dipole-dipole interactions, and non-polar liquids will also usually dissolve non-polar liquids due to dispersion forces. However, non-polar liquids are not sufficiently attracted to polar liquids to break the strong dipole-dipole interactions between solvent molecules. Therefore, liquids with like polarities will dissolve, whereas a non-polar liquid will not dissolve in a polar liquid.

Other Effects

Intermolecular attractions have some other effects on liquids. Viscosity is a measure of how fluid, or "runny," a liquid is. For example, water has low viscosity and runs easily. Cold maple syrup flows slowly, so it has high viscosity. Intermolecular forces play some role in viscosity, because stronger attractions between molecules cause them to resist flow more strongly. Molecule size is also an important factor in viscosity; longer molecules can become tangled and flow slowly. Surface tension is also a result of intermolecular forces. Molecules at the surface of a liquid are attracted to the molecules beneath and beside them, leading to an inward force on the liquid and a kind of skin on the surface. This tension also causes drops of water to contract into spheres, minimizing surface area. Vapor pressure is also affected by intermolecular attractions.