Objectives Define heats of fusion and vaporization. State the first and second laws of thermodynamics. Distinguish between heat and work. Define entropy.
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.
CHANGES OF STATE The 3 common states of Matter 1. Solid 2. Liquid 3. Gas 4. Plasma (4 th 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 with the changes of state. Read p
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.
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 = mH fusion Heat Required to Vaporize a Liquid – is equal to the mass of the liquid times the heat of vaporization of the liquid Q = mH vaporization When a liquid freezes (becomes solid), an amount of heat Q = -mH fusion must be removed from the liquid to turn it into a solid. When a vapor condenses to a liquid, an amount of heat Q = -mH vaporization must be removed. The negative sign indicates the heat is transferred from the sample to the environment.
CHANGES OF STATE Table 12.2 p. 324 lists Heats of Fusion and Vaporization Example Problem 3 p. 325 Heat of Fusion 1 Q = mH fusion Q = 1.5(334,000) Q = 501,000 Joules Then Q = mC T = 1.5(4180)(70-0) = 438,900 Joules So Q total = 501, ,900 = 939,900 J Do Practice Problems p. 325 # 19-21
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
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