If you are asked to rank molecules in order of melting point, boiling point, viscosity, surface tension or vapour pressure what they are actually asking is for you. As the intermolecular attraction increases,. • The vapor pressure (the pressure of the vapor that is in equilibrium with its liquid) decreases. • The boiling point. These forces are called intermolecular forces, and are in general much weaker than the Just how much difference one sees as a function of time is based on the polarizability Every substance also has an associated vapor pressure with it.
This is called an intramolecular force. We know how the atoms in a molecule are held together, but why do molecules in a liquid or solid stick around each other? What makes the molecules attracted to one another?
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These forces are called intermolecular forces, and are in general much weaker than the intramolecular forces. The attraction of a positive charge with a negative charge is the force that allows for the structure of the atom, causes atoms to stick together to form molecules; both ionic and covalent, and ultimately is responsible for the formation of liquids, solids and solutions.
London dispersion forces The forces that hold molecules together in the liquid, solid and solution phases are quite weak. They are generally called London dispersion forces. We already know that the electrons in the orbitals of molecules are free to move around. As such, if you would compare a "snapshots" of a molecule at an instant in time, you would see that there would be slightly different charge distributions caused by the different positions of the electrons in the orbitals.
Just how much difference one sees as a function of time is based on the polarizability of the molecule, which is a measure of how well electrons can move about in their orbitals. In general, the polarizability increases as the size of the orbital increases; since the electrons are further out from the nucleus they are less strongly bound and can move about the molecule more easily.
Given that two molecules can come close together, these variations in charge can create a situation where one end of a molecule might be slightly negative and the near end of the other molecule could be slightly positive.
This would result in a slight attraction of the two molecules until the charges moved around again but is responsible for the attractive London dispersion forces all molecules have. However, these London dispersion forces are weak, the weakest of all the intermolecular forces.
Their strength increases with increasing total electrons. Dipole-dipole attractions What would happen if we had a beaker of polar molecules, like formaldehyde, In addition to the attractive London dispersion forces, we now have a situation where the molecule is polar.
We say that the molecule has a permanent dipole. Now, the molecules line up. The positive ends end up near to another molecule's negative end: Since this dipole is permanent, the attraction is stronger. However, we only see this sort of attraction between molecules that are polar.
It is usually referred to as dipole - dipole interaction. The strength of this attraction increases with increasing total number of electrons.
Hydrogen bond Hydrogen is a special element. Because it is really just a proton, it turns out that it can form a special type intermolecular interaction called the hydrogen bond. If the hydrogen in a moleucle is bonded to a highly electronegative atom in the second row only N, O, or Fa hydrogen bond will be formed. In essence the three elements listed above will grab the electrons for itself, and leave the hydrogen atom with virtually no electron density since it had only the one.
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Now, if another molecule comes along with a lone pair, the hydrogen will try to position itself near that lone pair in order to get some electron density back. This ends up forming a partial bond, which we describe as the hydrogen bond. The strength of this interaction, while not quite as strong as a covalent bond, is the strongest of all the intermolecular forces except for the ionic bond.
Obviously from experience one would expect water to easily speed right past the honey, a fact that reveals honey has a much higher viscocity than water. One may ask the question of what is actually going on in the liquids to make one type flow faster and the other more resistant to flow such as the comparison between honey and water earlier.
Because part of a fluid moves, it forces other adjacent parts of the liquid to move along with it causing an internal friction between the molecules which ultimately leads to a reduced rate of flow. It is also important to note that the viscosity of liquids and gases are affected by temperature but in opposite ways meaning that upon heating, the viscosity of a liquid decreases rapidly, whereas gases flow more sluggishly.
Why is this the case?
As temperature increases, the average speed of molecules in a liquid also increases and as a result, they spend less time with their "neighbors.
The viscosity of a gas, however, increases as temperature increases because there is an increase in frequency of intermolecular collisions at higher temperatures.
Since the molecules are flying around in the void most of the time, any increase in the contact they have with one another will increase the intermolecular force which will ultimately lead to a disability for the whole substance to move.
Measuring Viscosity There are numerous ways to measure viscosity. One of the most elementary ways is to allow a sphere, such as a metal ball, to drop through a fluid and time the fall of the metal ball: