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Review: Gases
Indefinite shape
Indefinite volume
Take the shape and volume of
container
Particles are far apart
Particles move fast
Low Density
Easy to expand and compress
Review: Solids
Definite shape
Definite volume
Particles close together, fixed
Particles move very slowly
High density
Hard to expand/compress
Liquids: in between
Closer to properties of solidsSlow diffusionHigh attraction between particlesMedium amount of energy
Forces of Attraction
Intramolecular forces: Hold atoms together within a moleculecovalent and ionic bonds
Intermolecular forces: Hold molecules to each other3 types
Dipole-Dipole Attraction
Dipole: molecule with a separation of charge (polar covalent)
Due to differences in electronegativity~1% as strong as a covalent bond
Hydrogen Bond
Very strong dipole-dipole attraction (10% of a covalent bond’s strength)
Occurs when H is bonded to O, N, F in a very polar bond
O H
3.5 2.1Gives water its unusual properties
H-Bonding Affects Boiling Points
Strong attraction requires much energy to overcome, so water is a liquid at normal temperatures
London Dispersion Forces
Occur in all substances-polar and nonpolar
Due to formation of instantaneous dipoles as electrons moving around nucleus concentrate on 1 side of molecule or atom
This induces a dipole in neighboring atoms or molecules
These are the weakest intermolecular forces
These are the only forces of attraction in nonpolar substances
Importance of Water
Covers 70% earth’s surfaceNecessary for reactions in living cellsModerates earth’s temperatureCoolant for engines & nuclear power
plantsTransportationGrowth medium for many organisms
Properties of Water
ColorlessTastelessAt 1 atm, water freezes at 0°C and
vaporizes completely at 100°C Liquid phase occurs from 0-100°C
Special Properties of Water
Surface TensionLiquids tend to form a “skin” making the
surface less penetrable by solids
Adhesion Attraction of the surface of a liquid to the
surface of a solid• Depends on the material
• Water is attracted to glass• Mercury is not
No adhesion
Cohesion
Molecular attractions within a material(water molecules to water molecules, for
example)Here, cohesion
causes water to
form beads; ad-
hesion causes it
to stick to the web
Capillary Action
A liquid rises in a narrow tube when it breaks the surface tensionMovement of water through paper
Ice Floats!Molecules in a liquid have more movement
than a solid and more energy (particles move apart
Generally, solids are more dense than their ir liquids
Liquid water
Ice
When water becomes fixed points in a solid, hydrogen bonds hold molecules in place
Gives ice an open hexagonal structure
Greater volume means lower density than aliquid
Lower K.E. causes distance between molecules to be less
Higher K.E. causes distance between molecules to be more
Max. density at 4 C
Phase Changes of Water
At 1 atm, water freezes at 0°C and vaporizes completely at 100°C
Liquid phase occurs from 0-100°CChanges from one phase to another will
either require energy or release energy Solid Liquid Liquid Gas Solid Gas
Melting/Freezing
Vaporization/Condensation
Sublimation/Deposition
From Solid to LiquidAs energy is added, K.E. increases
Solid warms up
At 0°C, solid begins to melt and temperature remains at 0 until all solid is turned to liquid
When all is liquid, temperature begins to rise
From Liquid to Gas
As heat is added, K.E. increases (increase in temperature)
At 100C, bubbles form in liquidTemperature remains the same until all
liquid is converted to a gas.Once all is a gas, added energy causes
temperature to increase
Ice melting/water vaporizing
When a substance is in phase, increasing the energy increases the temperature
When a substance is changing phase, increasing the energy does not increase the temperature but is used to break forces of attraction between molecules
Calculating EnergiesEnergy is measured in calories or Joules
1.00 cal = 4.184 JThe amount of energy needed to change
states depends on:1. Type of matter
2. Quantity of matter
Type of matter
Molar heat of fusion (Hfusion)= energy needed to melt 1 mole of a substance
Molar heat of vaporization (Hvap)= energy needed to vaporize 1 mole of a substance
Finding Energy in a Phase Change
Change from a solid liquid q = mHfusion
Change from a liquid gasq = mHvap
q = energy (cal or J)m = mass (g)Hfusion =heat of fusion (cal/g)
q = energy (cal or J)m = mass (g)Hvap =heat of vaporization
Example
How much heat in calories is needed to melt 15.0 g of water?
q = mHfusion
15.0 g water x 80.0 cal = 1.20 x 103 cal 1 g water
Energy and Being In Phase
When all of a substance is in one phase, (e.g. all liquid), the amount of energy required to cause a temperature change depends on:1. type of substance
2. amount of substance
3. range of temperature change
Type of Matter
Specific Heat (c): Energy required to change the temperature of 1 gram of a substance by 1 Celsius degreecg = specific heat of a gas
cl = specific heat of a liquid
cs = specific heat of a solid
Finding Energy When In Phase
q = mcT
T = Tfinal – Tinitial
q = energy (cal or J)m = mass (g)c = specific heat (cal/gC)T =change in temperature (C)
Example
How much energy is required to heat 50.0 g of water from 20.0 C to 85.0 C?
q = mcl TT = 85.0 – 20.0 = 65.0 Cq = (50.0g)(1.00 cal/gC)(65.0 C)q = 3,250 cal
Phase Change Problems1. Draw graph.
2. Mark start and stop points.
3. Every corner means a new equation is needed.
4. Flat sections will use q = mH ( no T means no slope).
5. Find each energy (q1, q2, q3..).
6. Add all energies to get the total energy.
Phase Changes
Vaporization (evaporation): molecules of a liquid escape the liquid’s surfaceRequires energy to overcome intermolecular
forces
Molecules with enough energy to evaporate
Maxwell Boltzman distribution
To evaporate, a particle must:Be at the surfaceHave sufficient energyBe moving in the right direction
Not at surface
Moving in the wrong direction Not enough
energy
I’m free!!
Evaporation produces Vapor Pressure
1. A closed container with a vacuum has a liquid added to it.
2. Molecules begin to evaporate
3. Some particles are recaptured by the liquid
4. Eventually rate of particles leaving = rate of being recaptured
Equilibrium Vapor PressureWhen rate of evaporation = rate of
condensation the pressure becomes constant (Equilibrium vapor pressure)
Vapor Pressure
The vapor pressure of substances varies greatly, depending on the strength of the forces of attraction between particles.
BoilingOccurs when the equilibrium vapor
pressure reaches atmospheric pressureOnly then can a bubble maintain itself
anywhere in the liquidAir Pressure
Particle of gas exerts pressure on surrounding molecules, pushing them out of the way, and therefore up against air pressure
As atmospheric pressure changes, so does the equilibrium vapor pressure necessary for boiling to occur
Therefore, the boiling point depends on the pressure
Vapor Pressure vs. Temperature can give us the boiling point of a substance at any pressure
The b.p. at 1 atm is called the normal boiling point.
P vs. T Phase Diagram
We can expand our graph to see the effect of pressure on melting/freezing and sublimation/deposition
For a “normal” substance:Increasing pressure
raises the melting point (AD)
Important Points on Diagram A: Triple point: temperature
& pressure at which solid, liquid and gas coexist at equilibrium
B: Critical Point: indicates critical temp and pressure
Critical Temp: temp above which substance cannot exist in a liquid state
Critical Pressure: lowest pressure required for substance to exist as a liquid at the critical temperature