• Intermolecular attractions determine how tightly liquids and solids pack

• Two important properties that depend on packing are compressibility and diffusion

• Compressibility is a measure of the ability of a substance to be forced into a smaller volume

• Solids and liquids are nearly incompressible because they contain very little space between particles

Question 11.125

The ‘rotten-egg’ odor is caused by hydrogen sulfide, H2S.Most people can detect it at a concentration of 0.15 ppb (parts

per billion), meaning 0.15 L of H2S in 109 L of space. A typical lab is 40 x 20 x 8 ft (1m3 = 1,000L; 1 ft = 0.30 m)

a) How many liters of H2S could be present in a typical lab to have a concentration of 0.15 ppb?

b) At STP, how many milliliters of 0.100 M Na2S would be needed to generate the amount of H2S in “part a” by the following reaction with hydrochloric acid?

N2S(aq) + 2HCl(aq) H2S(g) + 2NaCl(aq)

• Diffusion occurs more rapidly in gases than in liquids and solids

Diffusion in a gas (a) and liquid (b). Gas molecules move a much greater distance than liquid molecules between collisions. As a result, diffusion occurs more rapidly in the gas.

• The strength of intermolecular attractions determine many physical properties– Volume and shape

• Attractions in gases are not strong enough to retain either volume or shape

• Attractions in liquids and solids are strong enough so they retain their volume

• Attractions in solids are stronger than for liquids so that solids also retain shape

– Surface tension is the tendency of a liquid to take a shape with minimum surface area

– Molecules at the surface have higher potential energy than those in the bulk of the liquid

– The surface tension of a liquid is proportional to the energy needed to expand its surface area

– In general, liquids with strong intermolecular attractions have large surface tensions

Surface tension holds moist particles of sand together. Separation is resisted because the surface area of the water would increase.

– Wetting is the spreading of a liquid across a surface to form a thin film

– For wetting to occur, the intermolecular attractive force between the surface and the liquid must be about as strong as within the liquid itself

– Surfactants are added to detergents to lower the surface tension of water

– The “wetter” water can then gets better access to the surface to be cleaned

– Viscosity is the resistance to changing the form of a sample

• Gases have viscosity, but respond almost instantly to form-changing forces

• Solids, such as rocks, normally yield to forces acting to change their shape very slowly

• Liquids are what most people associate with viscosity

– Viscosity is also called internal friction because it depends on intermolecular attractions and molecular shape

WHICH IS MORE VISCOSE?

• A change in state is called a phase change

• Evaporation is the change in state from liquid to gas

• Sublimation is the change from solid to gas

• Both deal with the motion of molecules

• You have also probably noticed that the evaporation of liquids produce a cooling effect

• The rate of evaporation depends on the temperature, surface area, and strength of the intermolecular attractions

Molecules that are able to escape from the liquid have kinetic energies larger than the average. When they leave, the average kinetic energy of the remaining molecules is less, so the temperature is lower.

• For a given liquid, the rate of evaporation per unit surface area is greater at a higher temperature

At higher temperature, the total fraction of molecules with kinetic energy large enough to escape is larger so the rate of evaporation is larger.