Energy Changes

1
Energy Changes Phase Changes

• Heating Cooling Curves

2
It takes energy to heat stuff up!

• If pure substance in single phase - can calculate
  how much E needed using
• Q mC?T
• Q energy in Joules
• m mass in grams
• C specific heat capacity
• ?T change in temperature Tf - Ti
• On other hand, when something cools down, energy
  is released!

3
Q mC?T

• C specific heat capacity
• (amount heat required to raise temp of 1g of pure
  substance by 1?C)
• C is a physical constant
• unique for every pure substance
• CH2O 4.18 J/g??

4
Heat Flow hot to cold
Environment
5
What about phase changes?

• sometimes more than one phase of substance is
  present
• when melting ice both liquid water ice are
  present at same time
• temperature is constant ?T0
• even though ice water is absorbing heat

6
All chemical physical changes are accompanied
by energy changes

• Phase changes are physical changes
• sometimes energy is absorbed
• sometimes energy is released
• Energy change for given phase change can be
  measured/calculated

7
What are 6 possible phase changes and their names?
8
Potential Energy

• Energy of relative position
• Molecules are always attracted to one another
• put energy into system to pull molecules apart
  from one another
• the farther apart they are, the higher their PE

9
Melting Ice

• Ice water on hot plate ice is melting
• ice absorbing heat from hot plate and using to
  change phase
• temperature of ice-water mix is constant
• heat energy from hot plate going into phase
  change (PE) of system
• heat energy not going to KE of particles
• Remember, temperature is

10
Identify a phase change as endothermic or
exothermic
Endothermic
Exothermic
Gas
Condensation Freezing Deposition
Fusion Vaporization Sublimation
PE
Liquid
Solid
11
Heating Cooling Curves

• way to investigate energy changes
• Measure temperature as function of time at
  constant heating or cooling rate

12
III
IV
V
Liquid Gas
Gas
Solid Liquid
Liquid
Solid
Boil pt.
K.E.?
K.E.?
K.E.?
K.E.?
P.E.?
P.E.?
Temperature
P.E.?
Melt pt.
P.E.?
P.E.?
Time
13
Melting Boiling Points

• Plateaus Phase changes PE changes
• Notice if 2 phases are present, temperature is
  constant
• Melting point, Boiling point

14
What happens to temperature as heat is added at
boiling point?

• Nothing, until only 1 phase is present!

15
To analyze a heating/cooling curve

• Does curve go uphill or downhill?
• Label phases present in each region
• Describe what happens to KE in each region
• Describe what happens to PE in each region
• Locate melting point and boiling point

16
Heating curve of H2O