# Vapor pressure and intermolecular forces relationship quiz

### Vapor Pressure and Changes of States - AP Chemistry

Vapor pressure is defined as the pressure exerted by a gas in equilibrium with its liquid. Which of the following has the highest vapor pressure?. The boiling point of a liquid is the temperature at which the vapour pressure a) Is equal to the internal pressure b) Is equal to the external. As pressure decreases, the boiling point of a substance {increases, data to study the relationship that exists between vapor pressure and.

Ion-ion interactions are very strong so they will dominate. Problem IF5 solid, BrF5 liquid, ClF5 gas The compound with the strongest intermolecular interactions is expected to be the solid, the one with the weakest interactions the gas, and the intermediate case the liquid. Intermolecular interactions are discussed in detail below in Problem For these molecules, the applicable intermolecular interactions are dipole and induced dipole interactions.

The induced dipole or dispersion interactions increase with molecular weight. Dipole-dipole interactions are also important for these interhalogen compounds; their bond dipoles do not cancel thereby giving rise to a permanent dipole.

Considering the electronegativity differences in the compounds and hence the magnitude of the bond dipoles, IF5 is expected to have the strongest dipole-dipole interactions, followed by BrF5, and the weakest interactions are expected with ClF5. Ordering of boiling points: The greater the attractive interactions, the higher the boiling point.

Before discussing intermolecular interactions, it is first necessary to introduce the idea of a dipole. A dipole can be represented as two charges, equal in magnitude but opposite in sign, at a fixed distance from one and other.

The positive end of one dipole interacts attractively with the negative end of another dipole and vice versa. The electron density of atoms is continuously fluctuating. Thus, at any given instant the electron density about an atom may be asymmetrically distributed thus giving rise to a transient dipole see Figure below.

### Does water's boiling point change with altitude? Americans aren't sure | Pew Research Center

A snapshot in time of two Ar atoms. The shading represents the instantaneous electron density about the atoms. The deltas indicate the partial charge distribution. The arrow with a plus symbol on its tail is a symbol for a dipole note the orientation relative to the specifed partial charges. It is important to realize that these are not permanent dipoles, they are transient since the electron density is continually fluctuating. Despite this continual fluctuation, the net result is an attractive interaction.

The more volatile it is, the more rapidly it will evaporate.

### Determine Boiling Point from Vapor Pressure

It is also the case that raising the temperature of the liquid will cause it to evaporate more rapidly. At any given temperature the molecules in a liquid have a broad range of kinetic energies — some are very high, some are very low, but most are close to the average.

To escape from the liquid, a surface molecule must have sufficient kinetic energy to overcome the intermolecular bonds holding it to its neighbors. The higher the temperature, the greater the fraction of surface molecules that will have this amount of energy and the greater the rate of evaporation. Further, at any given temperature, in a liquid whose molecules are held together by weak intermolecular bonds, a relatively larger fraction of the surface molecules will have sufficient kinetic energy to overcome those bonds than in a liquid whose molecules are held together by strong intermolecular bonds.

As the liquid evaporates, gas begins to accumulate in the empty volume above the liquid, and this gas exerts a pressure against the walls of the container. As more liquid evaporates, the pressure increases.

Recall from the ideal gas law that the pressure is determined by the volume, temperature, and number of moles of gas present. As the liquid slowly evaporates, the volume of the space above the liquid remains the same as does the temperature.

But the number of moles of gas slowly increases, increasing the pressure. The gas above the liquid can condense back into the liquid. The rate at which this happens depends on how much gas there is — in other words, the more gas, and the higher the pressure, the greater the rate at which the gas condenses back to the liquid phase.

The molecules now in the gas phase above the liquid are in constant motion and strike each other, the walls of the container, and the surface of the liquid.

The rate at which this happens depends mostly on how many such collisions take place, which in turn depends on the number of gas molecules there are above the liquid. This number also determines the pressure the gas exerts, hence, the rate of condensation increases as the pressure increases.

As liquid evaporates, and the amount of gas present increases, the rate of condensation also increases. Eventually, the rate at which the gas condenses is the same as the rate at which the liquid evaporates.

## States of matter and intermolecular forces

When this happens, the pressure of the gas stops changing. This constant pressure is called the vapor pressure of the liquid. But this equilibrium is dynamic because it results from two ongoing but opposing actions that happen to exactly balance each other out.

Intermolecular Forces

It is the same idea as if you were running 10 miles per hour on top of a long train of flat cars moving north at 10 miles per hour. You would, in effect, be stationary, even though you were moving. What determines the vapor pressure of a particular liquid?

One factor is its volatility. A more volatile liquid evaporates more rapidly, so more gas must be present above the liquid, exerting a greater vapor pressure, for the rates of evaporation and condensation to be the same. At this point we are, of course, describing the same factors that influence volatility. As we have seen, the weaker the intermolecular bonds, the greater the volatility and, consequently, the greater the vapor pressure.

At room temperature, the vapor pressure of water, for example, is just little over 20 torr, not very high. Yet, if water is left in an open container, where the pressure cannot build up, the rate of condensation will never equal the rate of evaporation and the water will eventually all evaporate. The other factor is the temperature. The higher the temperature, the faster a liquid will evaporate and therefore the higher the pressure must be for the rate of condensation to equal the rate of evaporation.

Raising the temperature of water, for example, by just ten degrees Celsius nearly doubles the vapor pressure. And, of course, if the water is in an open container, it will evaporate nearly twice as fast.

This is the end of Lesson 2. Be sure that you complete and hand in the Problem Set and the Experiment for this lesson.