Relationship between vapour pressure and intermolecular forces

Vapor Pressure - Chemistry LibreTexts

relationship between vapour pressure and intermolecular forces

To understand that the relationship between pressure, enthalpy of is needed to overcome the intermolecular attractive forces that hold a. Intermolecular (or interparticle) forces are weak interactions between The boiling point (the temperature at which the vapor pressure becomes equal to. Up until now, we have just discussed attractions between molecules in the area of the These forces are called intermolecular forces, and are in general much weaker Just how much difference one sees as a function of time is based on the The vapor pressure is defined to be the amount of gas of a compound that is in.

As the number of molecules in the vapor phase increases, the number of collisions between vapor-phase molecules and the surface will also increase. Eventually, a steady state will be reached in which exactly as many molecules per unit time leave the surface of the liquid vaporize as collide with it condense. At this point, the pressure over the liquid stops increasing and remains constant at a particular value that is characteristic of the liquid at a given temperature.

relationship between vapour pressure and intermolecular forces

The rate of evaporation depends only on the surface area of the liquid and is essentially constant. The rate of condensation depends on the number of molecules in the vapor phase and increases steadily until it equals the rate of evaporation.

relationship between vapour pressure and intermolecular forces

Equilibrium Vapor Pressure Two opposing processes such as evaporation and condensation that occur at the same rate and thus produce no net change in a system, constitute a dynamic equilibrium.

In the case of a liquid enclosed in a chamber, the molecules continuously evaporate and condense, but the amounts of liquid and vapor do not change with time. The pressure exerted by a vapor in dynamic equilibrium with a liquid is the equilibrium vapor pressure of the liquid. If a liquid is in an open container, however, most of the molecules that escape into the vapor phase will not collide with the surface of the liquid and return to the liquid phase.

Instead, they will diffuse through the gas phase away from the container, and an equilibrium will never be established.

How do intermolecular forces affect vapor pressure?

Volatile liquids have relatively high vapor pressures and tend to evaporate readily; nonvolatile liquids have low vapor pressures and evaporate more slowly. Thus diethyl ether ethyl etheracetone, and gasoline are volatile, but mercury, ethylene glycol, and motor oil are nonvolatile. The equilibrium vapor pressure of a substance at a particular temperature is a characteristic of the material, like its molecular mass, melting point, and boiling point Table It does not depend on the amount of liquid as long as at least a tiny amount of liquid is present in equilibrium with the vapor.

Molecules that can hydrogen bond, such as ethylene glycol, have a much lower equilibrium vapor pressure than those that cannot, such as octane. The nonlinear increase in vapor pressure with increasing temperature is much steeper than the increase in pressure expected for an ideal gas over the corresponding temperature range.

How do intermolecular forces affect vapor pressure? | Socratic

The temperature dependence is so strong because the vapor pressure depends on the fraction of molecules that have a kinetic energy greater than that needed to escape from the liquid, and this fraction increases exponentially with temperature.

What makes the molecules attracted to one another? 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.

How does vapor pressure relate to intermolecular forces?

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.

relationship between vapour pressure and intermolecular forces

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.

relationship between vapour pressure and intermolecular forces

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. 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.

A diagram of the hydrogen bond is here: Could the CH2O molecule exhibit hydrogen bonding?

relationship between vapour pressure and intermolecular forces