Intermolecular Attractions and the properties of liquids and Solids Chapter 12

12.3 Intermolecular Forces and properties of liquids and solids Learning Objectives: To learn some general properties of liquids and solids and how these properties are related to the closeness of packing of molecules and to intermolecular attractive forces.

Physical properties of liquids and solids depends 1.On how Tightly Molecules Pack 2.On strength of intermolecular attractions. Physical properties and intermolecular forces

Physical properties that depends how tightly molecules pack Compressibility Diffusion Physical properties and intermolecular forces

5 Compressibility Measure of ability of substance to be forced into smaller volume Determined by strength of intermolecular forces Gases highly compressible Molecules far apart Weak intermolecular forces Solids and liquids nearly incompressible Molecules very close together Stronger intermolecular forces

The incompressibility of liquids is also the foundation of the engineering science of hydraulics, which uses fluids to transmit forces that lift or move heavy objects. Application of incompressibility of liquids Pascal’s law — states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container.

7 Diffusion Movement that spreads one gas though another gas to occupy space uniformly Spontaneous intermingling of molecules of one gas with molecules of another gas

 Occurs more rapidly in gases than in liquids  Hardly at all in solids Diffusion

9 In Gases Molecules travel long distances between collisions Diffusion rapid In Liquids Molecules closer Encounter more collisions Takes a long time to move from place to place In Solids Diffusion close to zero at room temperature Will increase at high temperature

Properties that depend on strength of intermolecular attractions. Retention of volume and shape Surface Tension Wetting of a surface by a liquid Viscosity Physical properties and intermolecular forces

Retention of volume and shape Solids retain both volume and shape Strongest intermolecular attractions; Molecules closest Liquids retain volume, but not shape Attractions intermediate Gases, expand to fill their containers Weakest intermolecular attractions; Molecules farthest apart

Surface Tension Surface tension is a physical property equal to the amount of force per unit area necessary to expand the surface of a liquid. Surface tension forces are due to intermolecular forces between the liquid's molecules at the liquid's outer boundaries.

Inside body of liquid Intermolecular forces are the same in all directions Molecules at surface Potential energy increases when removing neighbors Molecules move together to reduce surface area and potential energy Surface Tension

Causes a liquid to take the shape (a sphere) that minimizes its surface area Molecules at surface have higher potential energy than those in bulk of liquid and move to reduce the potential energy  Wax = nonpolar  H 2 O = polar  Water beads in order to reduce potential energy by reducing surface area Why does H 2 O bead up on a freshly waxed car instead of forming a layer?

Surface Tension Liquids containing molecules with strong intermolecular forces have high surface tension Allows us to fill glass above rim Gives surface rounded appearance Surface acts as “skin” that lets water pile up Surface resists expansion and pushes back  Surface tension increases as intermolecular forces increase  Surface tension decreases as temperature increases

Walking on water

Wetting of a surface by a liquid Wetting: ability of liquid to spread across surface to form thin film Greater similarity in attractive forces between liquid and surface, yields greater wetting effect Occurs only if intermolecular attractive force between surface and liquid about as strong as within liquid itself

18 Wetting Ex. H 2 O wets clean glass surface as it forms H–bonds to SiO 2 surface Does not wet greasy glass, because grease is nonpolar and water is very polar Only London forces Forms beads instead Surfactants Added to detergents to lower surface tension of H 2 O Now water can spread out on greasy glass

19 Surfactants (Detergents) Substances that have both polar and non-polar characteristics Long chain hydrocarbons with polar tail  Nonpolar end dissolves in nonpolar grease  Polar end dissolves in polar H 2 O  Thus increasing solubility of grease in water

20 Solubility “Like dissolves like” To dissolve polar substance, use polar solvent To dissolve nonpolar substance, use nonpolar solvent Compare relative polarity Similar polarity means greater ability to dissolve in each other Differing polarity means that they don’t dissolve, they are insoluble Surfactants Both polar and non-polar characteristics Used to increase solubility

Surface Tension  Surfactant decrease surface tension

22 Your Turn! Which of the following are not expected to be soluble in water? A. HF B. CH 4 C. CH 3 OH D. All are soluble

23 Viscosity Viscosity is resistance to flow Measure of fluid’s resistance to flow or changing form Related to intermolecular attractive forces

 Also called internal friction  Depends on intermolecular attractions Viscosity

25 Viscosity Viscosity decreases when temperature increases Most people associate liquids with viscosity Syrup more viscous than water Gases have viscosity Respond almost instantly to form-changing forces Viscosity in gases arises from the molecular diffusion that transports momentum between layers of flow. The kinetic theory of gases allows accurate prediction of the behavior of gaseous viscosity.

Solids, such as rocks and glass have viscosity Normally respond very slowly to forces acting to change their shape Indeed, some scientists have claimed that amorphous solids, such as glass or many polymers, are actually liquids with a very high viscosity Debatable by scientist? Viscosity of solids

27 Amorphous Solids (Glass) Have little order, thus referred to as “super cooled liquids” Edges are not clean, but ragged due to the lack of order

Your Turn! For each pair given, which is has more viscosity? CH 3 CH 2 CH 2 CH 2 OH, CH 3 CH 2 CH 2 CHO C 6 H 14, C 12 H 26 NH 3 (l ), PH 3 (l ) A. CH 3 CH 2 CH 2 CH 2 OH C 6 H 14 NH 3 (l ) B. CH 3 CH 2 CH 2 CH 2 OH C 12 H 26 NH 3 (l ) C. CH 3 CH 2 CH 2 CHO C 6 H 14 PH 3 (l ) D. CH 3 CH 2 CH 2 CHO C 12 H 26 NH 3 (l ) E. CH 3 CH 2 CH 2 CH 2 OH C 12 H 26 PH 3 (l ) 28

29 Effect of Intermolecular Forces on Viscosity Acetone Polar molecule Dipole-dipole London forces Ethylene glycol Polar molecule Hydrogen-bonding Dipole-dipole and London forces water Polar molecule Hydrogen-bonding Dipole-dipole and London forces

Lab: Molar mass of CO 2 Experimentally determine the molar mass of CO 2. Molar mass= g/mole We need grams and we need number of moles of a CO 2 sample Dry ice is CO 2 grams by weighing How do we get the number of moles

For moles of CO 2. how about using PV=nRT P= atm. Pressure in lab V = can be measured Using water to determine the volume R= gas constant T= can be measured (room temperature we are working at ) Lab: Molar mass of CO 2