STPM Form 6 Chemistry Liquids and Vapour pressure

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STPM Form 6 Chemistry Liquids and Vapour pressure

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STPM Form 6 Chemistry Liquids and Vapour pressure

  1. 1. Section 14.1 Chapter 3 LiquidsIntermolecular Forces and Phase Changes
  2. 2. Section 14.1Intermolecular Forces and Phase Changes
  3. 3. Section 14.1Intermolecular Forces and Phase ChangesThe Kinetic Molecular Theory of Liquids & Solids• Gas - Distance between gas molecules are so great at ordinary temperatures and pressures (25 *C and 1atm) that there is no real interaction between gas molecules.• Liquids – the molecules are so close together that there is little empty space. Allowing for a definite volume but taking the shape of it’s container.• Solids – molecules are held rigidly in a position with virtually no freedom of motion. So that they have a definite volume and shape.
  4. 4. Section 14.1Intermolecular Forces and Phase ChangesA. Intermolecular Forces• Intermolecular forces – occur between molecules• Intramolecular forces – occur inside the molecules
  5. 5. Section 14.1Intermolecular Forces and Phase ChangesA. Intermolecular ForcesDipole – dipole attraction
  6. 6. Section 14.1Intermolecular Forces and Phase ChangesA. Intermolecular ForcesHydrogen Bonding• Occurs between H and highly electronegative atom (for example N, O, F)
  7. 7. Section 14.1Intermolecular Forces and Phase ChangesIntermolecular Forces• These forces are partly responsible for the non- ideal gas law behavior discussed earlier. And these forces are why liquids and solids do not present “ideal behavior”.
  8. 8. Section 14.2Vapor Pressure and Boiling PointObjectives1. To understand the relationship among vaporization, condensation and vapor pressure2. To relate the boiling point of water to its vapor pressure
  9. 9. Section 14.2Vapor Pressure and Boiling PointMotion of Liquid Particles• Vibrational• Rotational• Translational
  10. 10. Section 14.2Vapor Pressure and Boiling PointA. Evaporation and Vapor Pressure• Liquid turns to Gas• Vaporization or evaporation – Endothermic
  11. 11. Section 14.2Vapor Pressure and Boiling PointA Vapor• A gas is a substance that is normally in the gaseous state at ordinary temperatures and pressures.• A vapor is the gaseous form of a substance that is a liquid or a solid at normal temperature and pressure – generally 25*C and 1 atm.• Heat of vaporization: the energy required to vaporize one mole of a liquid at a pressure of one atmosphere.
  12. 12. Section 14.2Vapor Pressure and Boiling PointA. Vaporization and Condensation• Amount of liquid first decreases then becomes constant• Condensation - process by which vapor molecules convert to a liquid• When no further change is visible the opposing processes balance each other - equilibrium
  13. 13. Section 14.2Vapor Pressure and Boiling PointVaporization and CondensationOnce LP have gone into the gas state (vapor) from the liquid state, what might happen? LP unstick: In general: the RP have 1. these are the enough KE fastest RP to break away (highest KE) KE > strength of 2. they move “up” interaction (away from the b/w RP surface) VAPORIZATION
  14. 14. Section 14.2 Vapor Pressure and Boiling PointWhat else is above theliquid surface ? _______) air ( So, the RP of the vapor may: 1. pass by or 2. collide with the air RP
  15. 15. Section 14.2Vapor Pressure and Boiling Point after the collision the LP of the vapor will 1. speed up or slow down 2. reverse direction What happens when the LP gets back to the surface of the liquid ?
  16. 16. Section 14.2Vapor Pressure and Boiling Point when the RP became unstuck (vaporized) the KE of the RP > strength of the interaction b/w RP up after the collision: if the RP sped up, KE goes _____ so, when the RP comes back to the liquid surface KE of RP is still > strength of interaction b/w RP ________________________________________ will stay vaporized (in gas state) so, _____________________________________ down if the RP slows down, KE goes _____ so, when the RP comes back to the liquid surface KE of RP may now be < strength of interaction b/w RP ________________________________________ the RP may get “restuck” to the surface so, _____________________________________ (back into the liquid state) __________________________________________ CONDENSATION
  17. 17. Section 14.2Vapor Pressure and Boiling Point When the container is open, observe the liquid “go away” the “escaped” RP can diffuse away so, rate of (vaporization + diffusion) > rate of condensation VAPORIZATION
  18. 18. Section 14.2Vapor Pressure and Boiling Point So, what happens if the container is closed ? doesn’t go away observe that the liquid ______________________ Can the liquid still vaporize ? Have we: - changed the T ? ______no no - changed the distribution of RP vs KE ? ______ So, are there still RP with enough energy YES ! to vaporize ? ___________________ # RP strength of interactions b/w RP Kinetic Energy (J/mol)
  19. 19. Section 14.2Vapor Pressure and Boiling Point no can the RP diffuse away ? ______ they are trapped ____________ what else ? if ALL the liquid vaporized, all would be gas P would go up gas the container would expand / explode as the RP vaporize, more RP are in the vapor (gas) state above the liquid up so, the rate of collisions goes _______ up so, the rate of condensation goes _______ we’ll model the consequence of the rate of collisions increasing…
  20. 20. Section 14.2Vapor Pressure and Boiling Point (gas) say, in a given instant at a given T, 10 of the surface RP have enough KE to vaporize (liq) # RP (liq) strength of interactions b/w RP Kinetic Energy (J/mol)
  21. 21. Section 14.2Vapor Pressure and Boiling Point (gas) say, in a given instant at a given T, 10 of the surface RP have enough KE to vaporize (liq) (liq) (gas) (gas) at this concentration in the vapor state, the rate of collisions is such that 1 in 10 RP(g) (liq) condense (liq)
  22. 22. Section 14.2Vapor Pressure and Boiling Point In the next “instant” (still at the same temperature) 19 RP(g) 10 more surface RP vaporize 9 + 10 = 19 the concentration in vapor doubles phase nearly _________ doubles so, # of collisions (nearly) _________ doubles so, rate of condensation (nearly) _________ (liq) 15 RP(g) now, 2 in 10 condense (2/10) 19 = 3.8 (liq)
  23. 23. Section 14.2 Vapor Pressure and Boiling PointIn the next “instant” (still at the same temperature) 10 more surface RP vaporize 21 + 10 = 31 31 RP(g) now, 3.1 in 10 condense 21 RP(g) (3.1/10) 31 = 9.6 (liq) (liq) the number of RP going into the vapor state each “instant” (10) is equal to the number of RP condensing (10) so, the number of RP in the vapor state (above the liquid) stays constant at equilibrium (dynamic)
  24. 24. Section 14.2 Vapor Pressure and Boiling Point Define vapor pressure The pressure exerted by the RP of the substance in the gas state (vapor) above the liquid when, rate of condensation = rate of vaporization (at equilibrium)**property of the liquid**can only be established in a closed container
  25. 25. Section 14.2Vapor Pressure and Boiling Point Saturated vapor pressure
  26. 26. Section 14.2Vapor Pressure and Boiling PointMeasuring vapour pressure
  27. 27. Section 14.2 Vapor Pressure and Boiling PointEquilibrium • At this point no further NET change occurs in the amount of liquid or vapor because the two opposite processes exactly balance each other. • This process is dynamic.
  28. 28. Section 14.2B. Factor Pressure Vapor Pressure Vapor affecting and Boiling Point
  29. 29. Section 14.2B. Factor Pressure Vapor Pressure Vapor affecting and Boiling Point Vapor Pressure depends on Temperature #RP vs KE distribution curves for a sample of one substance at various Temperatures as Temperature goes up #RP with enough KE to vaporize goes ____ up up so, the rate of vaporization goes _____ to establish vapor pressure (rate of vaporization = rate of condensation) up the rate of condensation has to go _____ the rate of condensation  #RP in vapor up strength of so, for the rate of condensation to go _____ # RP interaction(s) the number of RP in the vapor must go _____ up between RP up so, the vapor pressure goes ______ Kinetic Energy (J/mol)
  30. 30. Section 14.2Vapor Pressure and Boiling PointB. Factor affecting Vapor Pressure – Vapor pressures vary widely - relates to intermolecular forces vapor substance pressure at 25oC diethyl ether 0.7 atm bromine 0.3 atm ethyl alcohol 0.08 atm water 0.03 atm
  31. 31. Section 14.2B. Factor Pressure Vapor Pressure Vapor affecting and Boiling Point Vapor Pressure depends on the substance #RP vs KE distribution curves for samples of various substances (P Q and A) at one Temperature at one T (say RT ~ 20oC) all substances have the same KE distribution curve regardless of the state the substances are in as the strength of the interaction(s) between RP goes down the #RP with enough KE# RP SOIBRP up to vaporize goes ______ SOIBRP for Q up for A so, the rate of vaporization goes _____ SOIBRP then….. for P Kinetic Energy (J/mol)
  32. 32. Section 14.2Vapor Pressure and Boiling Point Ether Ethanol
  33. 33. Section 14.2Vapor Pressure and Boiling PointBoiling PointRelations with VaporPressure andTemperature
  34. 34. Section 14.2 Vapor Pressure and Boiling PointAt the boiling point,saturated vapor pressureequals atmospheric pressure.
  35. 35. Section 14.2B. Factor Pressure Vapor Pressure Vapor affecting and Boiling PointVolatile liquid>> weak intermolecular forces of attraction>> Easy to escape>> Low saturated vapor pressure>> Low boiling point
  36. 36. Section 14.2 Vapor PressureVapor Pressure and OH CH 3 Boiling Point Curves CS 2 CH CH OHVapor pressure, mmHg 3 2 H O 2 Normal boiling point: Temp at which P = 1 atm vapor Which of these liquids must have the weakest C H NH 6 5 2 forces of attraction between its molecules? How can you tell? Temperature, oC
  37. 37. Section 14.2Vapor Pressure and Boiling PointB. Boiling Point and Vapor Pressure
  38. 38. Section 14.2Vapor Pressure and Boiling PointB. Boiling Point and Vapor Pressure
  39. 39. Section 14.2Vapor Pressure and Boiling Point
  40. 40. Section 14.2Vapor Pressure of Solution Vapor Pressure and Boiling Point A solution is a _______________ mixture of 2 or more substances in a single phase. One constituent is usually regarded as the SOLVENT and the others as SOLUTES.
  41. 41. Section 14.2Vapor Pressure and Boiling Point Because of solute- solvent intermolecular attraction, higher concentrations of nonvolatile solutes make it harder for solvent to escape to the vapor phase. © 2009, Prentice-Hall, Inc.
  42. 42. Section 14.2Vapor Pressure and Boiling Point Therefore, the vapor pressure of a solution is lower than that of the pure solvent. © 2009, Prentice-Hall, Inc.
  43. 43. Section 14.2Vapor Pressure and Boiling PointVapor Pressure Reduction • Molecules of nonvolatile solute take up space, thus preventing some solvent molecules to vaporize. • Condensation continues at the same rate, but vaporization slows down. • Since condensation > vaporization, vapor pressure is reduced.
  44. 44. Section 14.2Vapor Pressure and Boiling Point Addition of solutes causes

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