Energy Changes

1
Energy Changes Phase Changes

• Heating Cooling Curves

2
Heat Flow hot to cold
Environment
3
What about phase changes?

• Sometimes more than one phase of substance is
  present
• when melting ice both liquid water and solid
  water (ice) are present at same time
• temperature is constant ?T0

4
All chemical physical changes are accompanied
by energy changes

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

5
Remember 6 possible phase changes and their names?
6
Melting Ice

• Ice water on hot plate ice is melting
• ice is absorbing heat from hot plate and using it
  to change phase
• temperature of ice-water mix is constant (0C)
• heat energy from hot plate is going into phase
  change (PE is increasing)
• heat energy not going to KE of particles

7
Identify a phase change as endothermic or
exothermic
Endothermic
Exothermic
Gas
Condensation Freezing Deposition
Melting Vaporization Sublimation
PE
Liquid
Solid
8
Heating Cooling Curves

• Measure temperature as function of time at
  constant heating or cooling rate

9
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
10
Melting Boiling Points

• Plateaus Phase changes PE changes
• when 2 phases present, temperature is constant
• Melting point, Boiling point

11
What happens to temperature as heat is added at
boiling point?

• Nothing, until only 1 phase is present!

12
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