Vapor Pressure Notes

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Vapor Pressure:

Vaporization is an endothermic process because it requires energy to overcome the IMF's holding molecules together in a liquid

Factors that affect vapor pressure:

Temperature: higher temperature = greater kinetic energy

Surface area: number of molecules in position to enter the gas phase

Intermolecular forces: greater force requires greater kinetic energy needed for molecules to escape the surface

At any temperature, some molecules in a liquid have enough energy to escape

As the temperature rises, the fraction of molecules that have enough energy to escape increases

As molecules escape the liquid, the pressure they exert increases

The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate

Vapor Pressure of a Liquid:

As more molecules escape the liquid, the pressure they exert increases

The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate

At equilibrium, the pressure of the vapor present "over its liquid" is called the "equilibrium vapor pressure" or "vapor pressure of the liquid"

Vapor pressure is temperature-dependent

Liquids with higher vapor pressures are said to be "volatile" because they evaporate quite readily

The boiling point of a liquid is the temperature at which its vapor pressure matches the atmospheric pressure

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Phase Transitions (Changes of State):

Substances can change phase, often because of temperature change

At low temperatures, most substances are solid

As the temperature increases, they become a liquid

At even higher temperatures, they become gases

The process of a solid becoming a liquid is called melting

The reverse process is called solidification

It requires energy for a solid to melt a liquid

During melting, energy goes to changing the phase of a substance but does not go to changing the temperature of the substance

What happens when a solid becomes a liquid? In a solid, individual particles are stuck in place because the IMFs can't be overcome by the energy of the particles

When more energy is supplied, there comes a point at which particles have enough energy to move around but not to separate

The phase change between a liquid and a gas has some similarities to the phase change between a solid and a liquid

What happens when a liquid becomes a gas? The particles separate from each other, with each particle going its own way in space

This is how gases tend to fill their containers

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The Liquid State:

The state of attraction between particles can greatly affect the properties of a substance or solution

Water is the most abundant liquid

Liquids vary greatly in a few areas:

Density

Viscosity

Capillary action

Surface tension

Solubility

The Liquid State - Surface Tension:

Surface tension results from the net inward force experienced by the molecules on the surface of a liquid

Example: water "beading" on the waxy hood of a car

Water molecules prefer to interact with each other while minimizing interactions with a car's hood

Liquids with high IMF's will have high surface tensions

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The Liquid State - Viscosity:

Viscosity is the resistance of a liquid to "flowing" because of strong IMF's

Maple syrup and honey are very viscous

Viscocity is greater in substances with stronger IMF's because molecules are more attracted to each other, preventing them from freely flowing around each other

Factors:

Intermolecular forces

Molar mass

Molecular shape

Temperature

The Liquid State - Capillary Action:

Capillary action is the spontaneous rising of a liquid in a narrow tube, or the ability of a liquid to flow against gravity up a narrow tube due to adhesive and cohesive forces

Two forces involved:

Cohesive forces: the intermolecular forces among the molecules of a liquid

Adhesive forces: the attractive forces between the molecules of the liquid and the surface of the tube