Section 12.2 Changes of State & the Laws of Thermodynamics

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 = -mHfusionmust be removed from the liquid to turn it into a solid. • When a vapor condenses to a liquid, an amount of heat • Q = -mHvaporizationmust 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 = 939,900J • 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. ΔU = 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