Chapter 11 part 2 Properties of Liquids Viscosity Surface Tension Capillary Action Phase Changes (energy of phase changes) Dynamic Equilibrium Vapor pressure Phase diagram 1
Structure Affects Function Functional Group Boiling point Structure Hydrocarbon MW = 72amu Aldehyde MW = 72amu Ketone MW = 72amu amine MW = 73amu ether MW = 74amu Alcohol MW = 74amu carboxylic acid MW = 74amu 36 C CH 3 CH 2 CH 2 CH 2 CH 3 75 C 79 C O CH 3 CH 2 CH 2 C H O CH 3 CH 2 C CH 3 78 C CH 3 CH 2 CH 2 CH 2 NH 2 34 C CH 3 CH 2 O CH 2 CH 3 117 C CH 3 CH 2 CH 2 CH 2 OH 141 C O CH 3 CH 2 C OH 2
Kinetic Molecular Description of Liquids and Solids gas: Kinetic energy >> intermolecular forces Liquid: Kinetic energy intermolecular forces Solid: Kinetic energy << intermolecular forces Kinetic Energy T Heating: T, KE solid liquid gas 3
Properties of Liquids Intermolecular forces play an important role in the properties of liquids Cohesive Forces: forces within liquid Adhesive forces: forces between the liquid and a surface. There is a competition. 4
Properties of Liquids Viscosity: resistance to flow cohesive forces, viscosity For pure compound: as T viscosity Motor oil: SAE 10 has lower viscosity than SAE 40. Problem: Need low viscosity at low T and high viscosity at high T Multi-grade motor oils: e.g. 10w30 5
Properties of Liquids Surface Tension: energy needed to increase surface area cohesive IM forces, surface tension Surface Tension Surface molecules have fewer interactions. Energy is minimized by minimizing the surface area. Intermolecular interactions are favorable (heat is required to break them) The more interactions, the better. 6
Properties of Liquids Capillary Action: result of adhesion and surface tension Mechanism for ground water motion fluid movement in plants, animals wicking (sponges, candles, paper towels, chromatography) capillary rise The height depends on weight of water that can be supported by surface tension 7
Energy of phase changes Endothermic It requires energy to disrupt intermolecular forces. vaporization sublimation melting (fusion) condensation deposition freezing Exothermic Energy is released when intermolecular interactions are formed 8
CALORIMETRY Experimental measure of heat flow q = heat flow C = specific heat (heat capacity per gram) m = mass ΔT= T final T initial q = C m ΔT For H 2 O: Molar heat capacity C = 4.184 J/g C = 75.2 J/mole C q = C m ΔT = amount of heat given off ( ) or absorbed (+) as temperature changes 9
CALORIMETRY ΔH = Heat change = amount of heat given off (-) or absorbed (+) when a change occurs Examples: ΔH fusion = amount of heat needed to freeze a mole of substance ΔH vap = amount of heat needed to vaporize a mole of substance 10
HEATING THE SAMPLE As heat is added, two types of changes take place: 1. Within single phase (in red) changes are continuous q = n C p ΔT Since T : kinetic energy Energy, molecular motion separation between molecules molecular attractions, and order. 11
HEATING THE SAMPLE As heat is added, two types of changes take place: 2. Between phases, (phase transition in blue) the changes are abrupt, from one physical state to another q = n ΔH x (x = melting, vaporization) Since T is constant, kinetic energy is constant but energy, molecular separation, molecular attractions, order. 12
EXAMPLE PROBLEM 2 moles of ice at 25 C are heated to 125 C. How much energy is needed? C p (ice) = 37.6 J/mol K ΔH fusion = 6.02 kj/mol C p (water) = 75.3 J/mol K ΔH vapor = 40.67 kj/mol C p (steam) = 33.1 J/mol K 13
1. ice 25 o C 0 o C Break problem into steps 2. ice 0 o C water 0 o C (phase transition) 3. water 0 o C 100 o C 4. water 100 o C steam 100 o C (phase transition) 5. steam 100 o C 125 o C 14
Vapor Pressure vapor pressure (v.p.): the pressure exerted by a vapor in equilibrium with its liquid or solid phase. Dynamic Equilibrium forward rate = backward rate evaporation = condensation No net change, but change is occurring on molecular level. 15
As T, what happens to vapor pressure? 1 increases 2 decreases 3 stays the same WHY? 16
Using Kinetic Molecular Theory As I.M. forces, what happens to vapor pressure? 17
Vapor pressure and boiling point boiling point: T at which v.p. = P ext As P ext, what happens to boiling point? 1. increases 2. decreases 3. stays the same normal boiling point: boiling point of a liquid when P ext = 1 atm T at which the v.p. of a liquid is 1 atm. 18
Phase Diagrams Plot of pressure vs. temperature of the system showing the boundaries between the phases. Find these on a phase diagram normal melting point pressure dependence of melting point normal boiling point pressure dependence of boiling point critical point triple point supercritical fluid coexistence curves What happens when the pressure and/or temperature of the system is changed? 19
Phase diagrams 20
Phase diagrams of H 2 O and CO 2 21