Download Chapter_7_Energy_and_Phase_Changes_REVISED 2

Energy and Phase Changes (part of Ch.1)
Energy – ability to do work
Kinetic Energy – energy of motion
Potential Energy (PE) – stored energy
Examples of Energy: heat, light, electricity
Law of Conservation of Energy:
- energy may be changed from one form to another, but
the total amount is the same
- energy is never destroyed
- energy is absorbed or given off in chemical reactions
- exothermic reaction: energy is GIVEN OFF
- endothermic reaction: energy is ABSORBED
Measuring Energy:
- measured in joules
- 1 kilojoule = 1,000 joules
Formula to solve heat/calorie problems:
- Calorie is another unit to describe heat energy
q=mCΔT
heat (in joules) = grams x specific heat x ΔT
Note: specific heat of water is 4.18 joules/gramC (which
means that it takes 4.18 joules to raise one gram of water 1
C)
Ex:
How much heat energy in joules is absorbed by 30 grams of
water (H2O) when it is heated from 20C to 30C?
Temperature:
- a measure of the average kinetic energy of molecules
- higher temperature means molecules have MORE kinetic
energy
- lower temperature means molecules have LESS kinetic
energy
RULE: heat flows from a body at higher temperature to a
body at lower temperature until both temperatures are
the same.
Thermometers:
- used to measure temperature
- two fixed points on a Celsius thermometer
- Boiling Point
- Freezing Point
0˚ C = freezing point of water (a.k.a.: ice-water
equilibrium temperature)
100˚ C = boiling point of water (a.k.a.: water-steam
equilibrium temperature)
Kelvin Temperature (Absolute Temperature):
Kelvin = ˚C + 273
Ex: What is the Kelvin temperature for 0˚C?
Ex: What is the Kelvin temperature for 20o C?
Ex: Convert 100oC to Kelvin
Heating Curve Diagram:
- Heat is added at a uniform rate.
- endothermic reactions/process
5
4
100
Temp
(C)
3
2
0
1
Time (minutes)
1: Solid; increase in temperature means increase in
Kinetic Energy (KE), Potential Energy (PE) remains constant.
2: Melting
- Heat of Fusion (heat needed to change solid into liquid)
- Temperature stays the same (no change in KE)
- Increase in PE
- endothermic reaction
3. Liquid; increase in temperature means increase in KE
PE remains constant
4. Boiling
- Heat of Vaporization (heat needed to change liquid into gas)
- Temperature stays the same (no change in KE)
- Increase in PE
- endothermic reaction
4. Gas: increase in temperature means increase in KE
PE remains constant
Heat of Fusion:
- the heat that ice takes in/absorbs (334 joules per gram) to
become water
- the amount of heat needed to change a solid to a
liquid at constant temperature
- while ice is melting, the temperature (average KE of
molecules) stays the same
- Opposite properties when cooling/ releasing energy
Heat of Vaporization:
- the heat that water takes in/absorbs
(2260 joules per gram) to become water vapor
- amount of heat needed to change liquid to gas at
constant temperature
- while water is boiling, the temperature (average KE
of molecules) stays the same
- Opposite properties when cooling/ releasing energy
Solve:
Ex. How much heat energy is needed to melt 100g of ice that
is at the melting point?
Ex. What is the Heat of Vaporization of a substance that
takes 410 joules to boil 20 grams of the substance?
Cooling Curve :
- reverse of the heating curve
- Heat is taken away at a uniform rate.
- exothermic reactions/process
Let’s Draw a cooling curve:
For water:
- gas changes to liquid and gives off 2260 joules of heat per
gram (condensation)
- liquid changes to solid and gives off 334 joules of heat per
gram (freezing)
Lower Plateau: Melting-Freezing Point:
- solid and liquid are at equilibrium
- temperature at which a solid changes to a liquid
if heating/adding energy
- temperature at which a liquid changes to a solid
if cooling/removing energy
Higher plateau = condensation/boiling point
- liquid and gas are at equilibrium
- Temperature at which a liquid changes
to a gas if heating/adding energy
- Temperature at which a gas changes to a liquid
if cooling/ removing energy
Intermolecular force and Hf/ Hv
- The stronger the IMF the higher the Hf and Hv
Sublimation:
- change from a solid to gas without passing
through a liquid phase
- Ex: dry ice (CO2) and Iodine
- Ice and snow can sublime from energy provided by the
sun.
Deposition:
- change from gas to solid without passing through liquid
phase.
- Ex. Snow from water vapor.
Evaporation:
- water changes into a gas
- Different from boiling because evaporation occurs on the
surface of the liquid, when boiling vapor/gas is formed
throughout the liquid.
Boiling-Condensation Point:
- water boils when vapor pressure equals atmospheric
pressure (101.3 kPa or 1 atm)
- Boiling Point of Water = 100C at 101.3 kPa
(see Ref. Table H):
- Vapor Pressure (101.3 kPa) = Atmospheric Pressure
(101.3 kPa)
Vapor Pressure:
- in a closed container, the pressure exerted on the
container and on the surface of the liquid by
the gas phase of a liquid (water)
- With an increase in water temperature,
there is an increase in vapor pressure.
- The weaker the intermolecular forces, the higher the
vapor pressure
Phase Change Animation:
http://mutuslab.cs.uwindsor.ca/schurko/animations/waterp
hases/status_water.htm