Chemistry Lect 5

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Chemistry Lect 5
2010-12-23 03:02:41
Chem Heat Phase Colligative

Heat Capacity, Phase Change, and Colligative Properties
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  1. "Phase" Definition
    any part of a system that is homogenous
  2. "Heat Capacity" Definition
    a measure of the energy chance needed to change the temp of a substance --> think of it as internal energy capacity

  3. Heat Capacity Equation

    • C: heat capacity
    • q: heat
    • T: temp
  4. "Specific Heat" Definition
    simply the heat capacity per unit mass
  5. Specific Heat Equation
    q = m c ΔT

    • q: heat
    • m: mass
    • c: specific heat
    • T: change in temp
  6. Coffee Cup Calorimeter
    • - measures the Δenergy at atmospheric pressure (∴ a constant pressure system)
    • -used to measure heats of a reaction (@ constant pressure q=ΔH)

    • i.e. we mix H+ + OH- --> H2O
    • we can solve for "q" in q=mcΔT and since @ const. P, q=ΔH, we have the heat of the rxn
  7. Bomb Calorimeter
    • - measures Δenergy at constant volume
    • - used to measure internal energy change (@ constant volume q=ΔU)

    i.e. use known heat capacity (C) of the container and the q=CΔT to deduce the internal energy (U)
  8. Phase Changes
    • (1) Melting - Freezing
    • (2) Vaporization - Condensation
    • (3) Sublimation - Deposition
  9. When does evaporation occur?
    when the partial pressure above the liquid is less than (<) the vapour pressure of the liquid

    BUT atmospheric P. > vapour P.

    *this allows the liquid to evaporate rather than boil
  10. Boiling Point
    Atmos. P = Vapour P. of a liquid
  11. Melting Point
    Vapour Psolid = Vapour Pliquid
  12. Compare the amount of heat released when moving from liquid --> solid to the amount of heat absorbed when moving from a solid --> liquid
    Exactly the same amount of heat that is absorbed when melting is released when freezing
  13. Identify where bonds are broken during phase changes
    solid --> liquid (melting)

    liquid --> gas (vaporization)
  14. Temp v. Energy Phase Change Graph
  15. Phase Change Diagram: triple pt, critical pt
  16. Critical Point
    - critical temp: above which a substance cannot be liquified regardless of the pressure applied

    - critical pressure: required to produce liquification while the substance is at the critical temp

    critical pressure, critical temp = critical pt
  17. What happens to the temperature of a system during a phase change?

    When there is not a phase change?
    There is no change in temperature during a phase change.

    Otherwise, energy increases molecular movement which increases temperature.
  18. What is the slope of a temp v. energy graph when the phase is not changing?
    slope = 1 / mc

    • m: mass
    • c: specific heat
  19. Colligative Properties
    - properties that depend only on the number of particles and not the type

    • (1) vapour pressure
    • (2) boiling point
    • (3) freezing point
    • (4) osmotic pressure
  20. Explain Boiling Point Elevation
    - with the addition of a nonvolatile solute there is a decrease in vapour pressure

    - boiling point occurs when V.P. = Atmos. P.

    - ∴ when V.P. decreases the b.p. increases
  21. Boiling Point Elevation Equation
    ΔT = kbmi

    • kb: a constant
    • m: molality
    • i: # of ions after dissociation
    • (i.e. NaCl --> Na + Cl ∴i = 2)
    • (i.e. MgCl2 --> Mg+2 + 2Cl ∴ i = 2)
  22. Explain Freezing Point Depression
    • - the addition of a nonvolatile solute can interrupt the crystal lattice
    • - this will lower the freezing point

    *be careful: eventually you get to a point where the solvent becomes the impurity preventing the solute from freezing --> ∴ the freezing point lowers at first and then rises again after it has hit this point
  23. Freezing Point Depression Equation
    ΔT = kfmi

    • kf: a constant
    • m: molality
    • i: # of ions after dissociation
  24. Osmotic Pressure Concept
    - only relevant when comparing one solution to another

    • - think of a selectively permeable membrane
    • if one side is concentrated, the water will move to wards that side in order to maintain the same dilution factor
  25. Osmotic Pressure Equation
    Π = iMRT

    • Π: osmotic pressure
    • i: # of ions after dissociation
    • M: molarity
    • T: temp

    - this gives you the pressure on one side of the membrane (the total pressure is the difference between both sides)

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