Remember Chapter 12.1 Introduction to Kinetic Molecular Theory and Intermolecular forces

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1 Remember Chapter 12.1 Introduction to Kinetic Molecular Theory and Intermolecular forces 1

Start with states of matter Kinetic Molecular Theory is used to explain what is

2 To understand properties, we want to connect what we see to what is happening on a molecular level. Start with states of matter Kinetic Molecular Theory is used to explain what is happening on a molecular level. Key postulate of this theory is that molecules have kinetic energy: they are in constant motion in all states. Mary J. Bojan Chem 110 2

entities: The kinetic energy of the particles The

3 The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: The kinetic energy of the particles The strength of the attractions between the particles gas à liquid à solid. 3

4 If we know the relative strengths of these two interactions we could predict the state of a substance. gas: Kinetic energy >> intermolecular forces Liquid: Kinetic energy intermolecular forces Solid: Kinetic energy << intermolecular forces solid liquid gas! Kinetic Energy T Heating: T é, KE é 4

5 IM Forces: 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 C O CH 3 CH 2 CH 2 C H O CH 3 CH 2 C CH 3 78 CH 3 CH 2 CH 2 CH 2 NH 2 34 C CH 3 CH 2 O CH 2 CH C CH 3 CH 2 CH 2 CH 2 OH 141 C O CH 3 CH 2 C OH 5

6 Chapter 12 part 2 Properties of Liquids Viscosity Surface Tension Capillary Action Phase Changes (energy of phase changes) Dynamic Equilibrium Vapor pressure Phase diagram 6

7 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. H 2 O Hg 7

Viscosity is the measure of the resistance to

Problem: Need low viscosity at low T and high

8 Viscosity is the measure of the resistance to flow As cohesive forces é, viscosity é For a pure compound: as T é, viscosity ê 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 8

Energy is minimized by minimizing the surface area.

9 Surface Tension is the energy needed to increase surface area é cohesive IM forces, é 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. Mary J. Bojan Chem 110 9

10 Capillary Action is the 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 10

vaporization sublimation melting (fusion) condensation

11 Phase changes involve changes in energy. Endothermic It requires energy to disrupt intermolecular forces. vaporization sublimation melting (fusion) condensation deposition freezing Exothermic Energy is released when intermolecular interactions are formed 11

We use CALORIMETRY to measure the energy changes involved in chemical processes. Calorimetry is the experimental measure of heat flow (section 10.

12 We use CALORIMETRY to measure the energy changes involved in chemical processes. Calorimetry is the experimental measure of heat flow (section 10.4) q = C s m ΔT q = heat flow C s = specific heat (heat capacity per gram) m = mass ΔT= T final T initial For H 2 O: C s = J/g C Molar heat capacity = C m = 75.2 J/mole C q = C s m ΔT (= C m n ΔT) = amount of heat given off ( ) or absorbed (+) as temperature changes 12

13 ΔH = Heat change (at constant P) = amount of heat given off ( ) or absorbed (+) when a change occurs ΔH vap ΔH fus Examples: (as defined these are positive, the reverse process will be negative) ΔH fusion = amount of heat needed to melt a mole of substance ΔH vap = amount of heat needed to vaporize a mole of substance 13

14 As heat is added, two types of changes take place. 1. Within single phase (in red) changes are continuous q = n C m ΔT Since T é : kinetic energy é Energy é, molecular motion é separation between molecules é molecular attractions ê, and order ê. 14

15 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 é, molecular separationé, molecular attractions ê, order ê. 15

16 2 moles of ice at 25 C are heated to 125 C. How much energy is needed? C m (ice) = 37.6 J/mol K ΔH fusion = 6.02 kj/mol C m (water) = 75.3 J/mol K ΔH vapor = kj/mol C m (steam) = 33.1 J/mol K Temperature C q added (kj) 16

17 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 17

18 Vapor Pressure (vp) is 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. 18

19 Kinetic Molecular Theory can be used to explain the trends in vapor pressure Use this theory to predict: As T é, what happens to vapor pressure? As I.M. forces é, what happens to vapor pressure? 19

20 These plots of vapor pressure as a function of temperature can be used to illustrate. As T é, vp As I.M. forces é, vp WHY? 20

There is a relationship between the vapor pressure of a substance and its boiling point boiling point: temperature at which v.p. = P ext As P ext é, what happens to boiling point?

21 There is a relationship between the vapor pressure of a substance and its boiling point boiling point: temperature at which v.p. = P ext As P ext é, 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. 21

22 A phase diagram is a plot of pressure vs. temperature of a system showing the boundaries between the phases. Label these on a phase diagram Gas, liquid and solid regions triple point coexistence curves normal boiling point normal melting point 22

What happens when the pressure and/or temperature of the system is

23 A phase diagram is a plot of pressure vs. temperature of a system showing the boundaries between the phases. What happens when the pressure and/or temperature of the system is changed? What is the pressure dependence of melting point? pressure dependence of boiling point? 23

24 These are the phase diagrams of H 2 O and CO 2 water Carbon dioxide What is the critical point? What is a supercritical fluid? 24

25 Heat these at constant P = 1atm. What phase changes will occur? Which one does not have a normal boiling point? Which one might melt as pressure increases? 25

Chapter 11 part 2. Properties of Liquids Viscosity Surface Tension Capillary Action. Phase Changes (energy of phase changes)

Chapter 11 part 2. Properties of Liquids Viscosity Surface Tension Capillary Action. Phase Changes (energy of phase changes) 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