Section 12'2 Changes of State

1
Section 12.2 Changes of State the Laws of
Thermodynamics

•  Objectives
• Define heats of fusion and vaporization.
• State the first and second laws of
  thermodynamics.
• Distinguish between heat and work.
• Define entropy.

2
INTRODUCTION

• You will learn that changing state means changing
  form as well as changing the way in which atoms
  store thermal energy.
•  
• Thermodynamics branch of physics that explores
  the properties of thermal energy. It is the
  study of heat.

3
CHANGES OF STATE

• The 3 common states of Matter
• Solid
• Liquid
• Gas
• Plasma (4th state not as common)
•  
• As the temperature of a solid is raised it will
  turn into a liquid and at even higher
  temperatures it will turn into a gas.
•  
• Go Over Figure 12-10 with the changes of state.
• Read p. 323-324

4
CHANGES OF STATE

• Melting Point temperature at which a substance
  changes from a solid to a liquid.
•  
• Heat of Fusion amount of energy needed to melt
  one kilogram of a substance. (or) The quantity
  of energy needed to change a unit mass of a
  substance from a solid to a liquid at the melting
  point.
•  
• Boiling Point temperature are which a substance
  changes from a liquid to a gas.
• Heat of Vaporization the amount of energy
  needed to vaporize one kilogram of a liquid. The
  quantity of energy needed to change a unit mass
  of a substance from a liquid to a gas at the
  boiling point.

5
CHANGES OF STATE

• Heat Required to Melt a Solid is equal to the
  mass of the solid times the heat of fusion of the
  solid
• Q mHfusion
•  
• Heat Required to Vaporize a Liquid is equal to
  the mass of the liquid times the heat of
  vaporization of the liquid
• Q mHvaporization
• When a liquid freezes (becomes solid), an amount
  of heat
• Q -mHfusion must be removed from the liquid to
  turn it into a solid.
• When a vapor condenses to a liquid, an amount of
  heat
• Q -mHvaporization must be removed.
• The negative sign indicates the heat is
  transferred from the sample to the environment.

6
CHANGES OF STATE

• Table 12.2 p. 324 lists Heats of Fusion and
  Vaporization
• Example Problem 3 p. 325 Heat of Fusion 1
• Q mHfusion
• Q 1.5(334,000)
• Q 501,000 Joules
•  
• Then
• Q mC?T 1.5(4180)(70-0) 438,900 Joules
• So Qtotal 501,000 438,900 939,900 J
• Do Practice Problems p. 325 19-21

7
THE FIRST LAW OF THERMODYNAMICS

• You can increase thermal energy by adding heat to
  the object or by doing work on the object.
• First Law of Thermodynamics states the change
  in thermal energy is equal to the heat added to
  the object Minus the work done by the object. It
  is a restatement of the law of conservation of
  energy.
• ?U Q W (it is measured in Joules)
• Thermodynamics is the study of the changes in
  thermal properties of matter
• Heat Engine device that converts thermal energy
  to mechanical energy.
•  
• All heat engines generate waste heat and
  therefore no engine can convert all of the energy
  into useful motion or work.
• Do Practice Problems p. 328 22-26

8
THE SECOND LAW OF THERMODYNAMICS

• Second Law of Thermodynamics states that heat
  only flows from a region of high temperature to a
  region of low temperature. States that natural
  processes go in a direction that increases the
  total entropy of the universe.
•  
• Entropy the measure of disorder in a system.
• Entropy and the Second Law of Thermodynamics can
  be thought of as statements of the probability of
  events happening.
•  
• Entropy is often used as a measure of the
  unavailability of energy.
•  
• Do 12.2 Section Review p. 331 27-34