Body

chem 127 August 30

chapter 10 continued

Discussion-- lecture (Web notes) vs. text
- today completes loop-- about same text/notes
- if in doubt-- study text first
- more complete (notes are outline)
- text sequence tested-- it works, fits problems
- lecture may try to show where we'll end up and why

Hydrogen Bonding
- critical to the nature of water
  - really (-) .... (+) ends of a molecule
- polar bonds show major difference when H₂O (or NH₃)
  - actually it's H(+) and the lone electron pairs on Oxygen (or nitrogen)
- gives water higher boiling point (for small molecule)
- higher viscosity (passed only by much larger molecules)
- higher surface tension (surpassed only by metals)
  - Still energy<< chemical bond energy
- H-bonds also occur in larger molecules
  - C--C--C--C--OH --C--C--C--C(O)OH acids
    - often 2,3 ... groups on large molecules
    - often forces add up, quite stable
  - DNA held in helix and double helix by such
    - weaker bonding, accumulated effect = well defined structure
- Example of wood-- many C-O-H bonds on cellulose (also some protein involved)
  - many H-bonds between molecules provides rigidity
    - soak wood in ammonia (liquid, cold-- not usual ammonia in water)
    - slowly replace C-O-H by weaker C-N-H
  - can tie wood stick into knot, restore the OH as piece reacts with humidity
  - form a final hard, rigid knot or complex bend in wood

Phase Changes

freezing / melting = fusion
evaporation / condensation
sublimation
also solid₁--->solid₂....
- important in metallurgy (anneal, harden)

--------------------------------------
- in practice, the melting temperature of a species is constant
If ice is "bigger" than liquid (lower density)
- squeezing hard can force molecules closer together
  - that's more like liquid water
  - water can be melted by pressure
    - at very high pressure freezing point of water is less than 0^oC
  - effect is vastly overrated
    - need extreme pressure to see effects
      - in ocean depths or deep in the earth, yes
The next phase change is evaporation
- liquid ---> vapor (molecules break loose)
  - my wet cellar floor slowly dries out
- solid ----> vapor (sublimation, it occurs)
  - the February snow pile decreases in size long before it gets above freezing
Evaporation and Condensation are simultaneous
- both occur, we see only the net difference
- eventually we reach equilibrium
  - both still occur, but rate is same
  - net effect is no further change
- rate of evaporation depends only on temperature (and surface area exposed)
- rate of condensation depends on temp, surface area
  - but also on vapor pressure of the species
  - higher pressure... more collisions with liquid
- Initially, liquid evaporates
  - vapor pressure rises
    - too small for much condensation
  - Continues and pressure rises
    - more condensation
    - so slower NET evaporation
  - Soon pressure near maximum and no net change
- Equilibrium Vapor Pressure
  - varies, increases with temperature
- Water Vapor-- (15 torr at room temp; vs. 760 torr)
  - relative humidity (RH)
  - how close (%) to equilibrium
  - If winds (rain) remove water vapor, actual vapor pressure < equilibrium
  - -------------------------------------------------------------

Clausius Clapyron

the only computations in the chapter
how does vapor pressure vary with temperature?
- it's really the wrong question
- should ask what is temperature and what's its effect
- we can find lots of properties
  - rate of a reaction, vapor pressure, equilibrium
  - constants (later), viscosity, surface tension, ...
  - plot vs temperature
  - property rises dramatically as temp increases
- plot log (property) vs 1/T(K)
  - get a straight line
  - and slope is always (-) DH_process / (R)
    - remember D is often a delta
  - where R is the "gas law constant"
Clausius Clapyron is when property is
- equilibrium vapor pressure
- and DH refers to DH_vaporization
behind the law and graphs--
- probability of getting that energy DE (or DH)
- goes as exp (- DE /RT)
- log of probability is proportional to DE
- and inversely proportional to T
Clausius Clapyron is a useful tool
- a plot of a couple of vapor pressures at a couple of temperatures
- allows one to draw the line
  - figure out property at all other temperatures
  - true over a wide range
- allows a tiny table to tablulate lots of data
- we can compute any other points confidently
also useful: "swap hard experiment for easy one"
- ordinary species -- easy to measure pressure
- simple gauge / thermometer
  - data and plot let one compute DHvaporization
  - much harder to measure
Text shows calculations..... work through examples
- Also remember.. a graph could be simpler

<where Monday lecture ended.. material continues and will be in Wed lecture>

Supercooling and Superheating

place bottled water or pop in your freezer (plastic please)
leave it several hours, cools ... to about -8^oC
but it stays liquid, yet well below 0^oC
- Supercooled
  - unscrew the cap and it turns solid in about 10 seconds
OR heat a cup of water for 2-3 minutes in microwave
- reaches about 110^oC
- stays liquid, no boiling
- jostle the cup removing it and it boils over (can burn)
- or add a tea bag now and it might boil furiously
  - Superheated
It costs energy to build a crystal (freeze) or to form a bubble of vapor (boil)
- Tiny crystals and bubble have lots of surface for the volume
  - From an energy point of view they are expensive to form
- In that region (a few molecules to either side)
  - the energy is not available
  - so the phase change doesn't occur
  - it's unstable, could happen abruptly
- If you add a tiny ice crystal or a bubble of air
  - that's a start and it costs much less energy to enlarge
    - (flexing plastic creates pattern that starts ice)
    - (porous stone chip release trapped air, aids boiling)

Boiling is special case of evaporation
- Equilibrium vapor pressure = atmospheric pressure
  - Instead of evaporating (liquid disappearing)we see liquid "roil" -- bubbles, small waves ...
- Try at slightly lower temperatures
  - liquid tries to evaporate
  - perhaps bubble forms near bottom of the pot
    - (heat is higher there)
  - the vapor pressure < equilibrium vapor pressure
    - atmosphere pushes down on liquid surface
    - height of water also adds pressure at bottom
    - bubble gets forced back into liquid state
    - bubbles of vapor can't form, survive and grow
- evaporation all occurs at the surface
at boiling point
- with a nucleation bubble, evaporation is rapid
  - bubbles enlarge, rise
  - motion from the bottom (where the heat is)
- if superheated
  - the first real bubbles form explosively

Solutions--

interactions with a few solute molecules
- and many more solvent molecules
  - -- (could be about equal amounts of two liquids)
A material dissolves if net energy of solution
- is less (or comparable) to energy of components
Two terms play role
- Energy (often as heat since it comes from temperature unless reaction occurs)
  - enthalpy (H) is energy when P = constant
- Entropy -- the degree of disorder of the system
  - nature favors disorder
    - dilution, dispersion, evaporation, mixing
    - but only if the added energy cost is small

Compare forces between like species to unlike pairs
- water-- strong forces between water molecules
  - could only be better forces with Ions
- water is great solvent for ionic species
  - IF... the lattice forces aren't too large
- water actually surround the ions (+/- ends inward)
  - ion keeps some water molecules
  - ion reorganizes the water around it
Water also dissolves moderately polar molecules
- alcohols, acetone
- Can even dissolve a little bit of nonpolar molecules
  - N₂ and O₂, Ar, even a bit of CH₄
  - more entropy than enthalpy
  - (costs energy -- loss of some H2O...HOH )
Often cost is too high
- with hydrocarbons, cooking oils, ...
- lower energy if water bonds to water, oil to oil
- separates into layers
- only tiny trace of each dissolves in the other

return to chem 127 home page