Chapter 11 Liquids, Solids, and IMFs: Sections 11.4-11.5 Sofia Quon-Youle!!!

IMF’s in Action Many properties of liquids and solids are determined by the strength and types of intermolecular forces present surface tension viscosity capillary action boiling point vapor pressure

Surface Tension A paper clip floats because the water’s surface acts like an elastic skin allowing the paperclip to “float” even though steel is denser than water. This is due to the phenomenon: surface tension. Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. How does it work? The layer of molecules on the surface behave differently than molecules in the interior. Surface molecules have a higher potential energy and are less stable than those in the interior (which have stronger intermolecular forces) because they have fewer neighbors to attract them

Viscosity Another manifestation of intermolecular forces is viscosity: the resistance of a liquid to flow. Is Viscosity greater or smaller in substances with strong intermolecular forces? Why? Viscosity is greater in substances with stronger intermolecular forces because if molecules are more strongly attracted to each other, they do not flow around each other as freely. Viscosity also depends on molecular shape, increasing in longer molecules that can interact over a greater area and possibly become entangled.

Viscosity Notice the graph, do you see a trend in viscosity? Viscosity increases with increasing molar mass (and therefore increasing magnitude of dispersion forces) and with increasing length (and therefore increasing potential for molecular entanglement)

Capillary Action Capillary Action: the ability of a liquid to flow against gravity up a narrow tube. It results from a combination of two forces: cohesive and adhesive. Cohesive forces: the attraction between molecules in a liquid. Cause the liquid to stay together. Adhesive forces: the attraction between these molecules and the surface of the tube. Cause the liquid to spread out over the surface of the tube.

Capillary Action What do you observe about this picture? What could it mean? If the adhesive forces are greater than the cohesive forces (as in water in a glass tube), the attraction to the surface draws the liquid up the tube and the cohesive forces pull along those molecules not in direct contact with the tube walls. The water rises up the tube until the force of gravity balances the capillary action-- the thinner the tube, the higher the rise. If cohesive forces are greater than adhesive forces (mercury), the liquid will not rise. If the meniscus of water is concave, adhesive forces are greater than the cohesive forces

Process of Vaporization Vaporization: transition from liquid to gas Condensation: transition from gas to liquid (opposite of vaporization Weaker intermolecular forces allow more molecules to evaporate at a given temperature, increasing the rate of vaporization. Volatile: liquids that vaporize easily Nonvolatile: liquids that do not vaporize easily. Why is acetone more volatile than water? In water, intermolecular hydrogen bonding takes place due to the presence of highly polar O-H bond in the H2O molecule. In acetone (CH3COCH3) hydrogen bonding is not possible, as there is no existence of polar O-H bond in the acetone molecule, unlike in the water molecule. So, acetone is much more volatile than water.

Boiling Boiling: a liquid boils when thermal energy is high enough to cause molecules in the interior of the liquid to break free of their neighbors and enter the gas state. Boiling point: the temperature at which the liquid’s vapor pressure equals the external pressure. When a liquid reaches its boiling point, the thermal energy is enough for molecules to break free of their neighbors and enter the gas state. Normal boiling point: the temperature at which its vapor pressure equals 1 atm. Normal boiling point of pure water is 100 degrees Celcius. However, at a lower pressure, water boils at a lower temperature. Once the boiling point of a liquid is reached, additional heating only causes more rapid boiling; it does not raise the temperature of the liquid above its boiling point As long as liquid water is present, its temperature cannot rise above its boiling point.

Summarizing the Process of Vaporization The rate of vaporization increases with increasing temperature The rate of vaporization increases with increasing surface area The rate of vaporization increases with decreasing strength of intermolecular forces

Heat of Vaporization The amount of heat energy required to vaporize one mole of the liquid is called the heat of vaporization, ∆Hvap. Sometimes called the enthalpy of vaporization It is always endothermic; therefore, ∆Hvap is + It is somewhat temperature dependent ∆Hcondensation = -∆Hvaporization

Dynamic equilibrium When water is in a sealed container, water molecules begin to vaporize As water molecules build up in the gas state, they begin to recondense into the liquid When the rate of evaporation equals the rate of condensation, dynamic equilibrium is reached When a system in dynamic equilibrium is disturbed, the system responds so as to minimize the disturbance and return to a state of equilibrium

Vapor Pressure Vapor Pressure: the pressure of a gas in dynamic equilibrium with its liquid is called its vapor pressure The vapor pressure of a particular liquid depends on the intermolecular forces present in the liquid and the temperature How do you think weak and strong intermolecular forces would affect vapor pressure? Weak intermolecular forces result in volatile substances with high vapor pressure because the intermolecular forces are easily overcome by thermal energy. Strong intermolecular forces result in nonvolatile substances with low vapor pressures.

Vapor Pressure At higher temperatures, more molecules have enough thermal energy to escape into the gas state, so vapor pressure increases with increasing temperature The lower the external pressure, the lower the boiling point of the liquid

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