Transcript Document
Section 12.4 Assessment The addition of energy to water molecules will cause them to ____. A. freeze B. change to water vapor D A 0% C D. move closer together A. A B. B C. C 0% 0% 0% D. D B C. form a crystal lattice Section 12.4 Assessment The transfer of energy from one object to another at a lower temperature is ____. A. heat B. degrees D A 0% C D. electricity A. A B. B C. C 0% 0% 0% D. D B C. conductivity Section 15.1 Energy (cont.) energy law of conservation of energy chemical potential energy heat calorie joule specific heat Energy can change form and flow, but it is always conserved. The Nature of Energy • Energy is the ability to do work or produce heat. • Two forms of energy exist, potential and kinetic. • Potential energy is due to composition or position. • Kinetic energy is energy of motion. The Nature of Energy (cont.) • The law of conservation of energy states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed. • This is also known as the first law of thermodynamics. The Nature of Energy (cont.) • Chemical potential energy is energy stored in a substance because of its composition. • Chemical potential energy is important in chemical reactions. • Heat is energy that is in the process of flowing (transferring) from a warmer object to a cooler object. • q is used to symbolize heat. Measuring Heat • A calorie is defined as the amount of energy required to raise the temperature of one gram of water one degree Celsius. • Food is measured in Calories, or 1000 calories (kilocalorie). • A joule is the SI unit of heat and energy, equivalent to 0.2390 calories. • 1 calorie = 4.184 J or 1 J = 0.2390 calories Measuring Heat (cont.) • Example: • A candy bar has 245 Calories. Convert this to calories and then to Joules of energy. Specific Heat • The specific heat of any substance is the amount of heat required to raise one gram of that substance one degree Celsius. • Some objects require more heat than others to raise their temperature. Specific Heat (cont.) • Calculating heat absorbed and released – q = c × m × ΔT – q = heat absorbed or released (in Joules) – c = specific heat of substance – m = mass of substance in grams – ΔT = change in temperature in Celsius Specific Heat (cont.) • Examples: • How much heat does a 20.0 g ice cube absorb as its temperature increases from (-27.0oC) to 0.0oC? Give your answer in both joules and calories. • q = c × m × ΔT • Specific Heat of Ice = 2.03 J/goC • 1 calorie = 4.184 J Specific Heat (cont.) • Example Cont. q=? c = 2.03 J/goC m = 23.0 grams ΔT = FinalTemp(0.0oC) – InitialTemp (-27.0oC) = Change (27.0oC) q = c × m × ΔT q = (2.03 J/goC)(20.0g)(27.0oC)= • Example 2: • A 5.00 gram sample of a metal is initially at 55.0 ºC. When the metal is allowed to cool for a certain time, 98.8 Joules of energy are lost and the temperature decreases to 11.0º C. What is the specific heat of the metal? What metal is it? • q = c × m × ΔT To make the problem easier, solve for the unknown BEFORE you plug in the numbers. Measuring Heat • For Water during a phase change: – The Heat of Fusion (melting) is 334 j/g – The Heat of Solidification (freezing) is 334 j/g • They are the same value (energy in or out) – The Heat of Vaporization is 2260 j/g – The Heat of Condensation is 2260 j/g • They are the same value (energy in or out) • Example – Phase change • Calculate the amount of energy needed to convert 55.0 grams of ice to all liquid water at its normal melting point. • Using the same amount of water calculate the energy needed to completely vaporize the water at its normal boiling point. • Why is there such a large difference in energy needed? Section 15.1 Assessment The heat required to raise one gram of a substance by one degree Celsius is called ____. A. joule A 0% D D. energy C C. specific heat A. A B. B C. C 0% 0% 0% D. D B B. calorie Section 15.1 Assessment Which of the following is an example of chemical potential energy? A. the moon orbiting Earth B. a car battery D A 0% C D. a roller coaster at the top of a hill A. A B. B C. C 0% 0% 0% D. D B C. a compressed spring • Aluminum has a specific heat of 0.897 J/(g ºC). What does this mean?