HW #1: 1.42, 1.52, 1.54, 1.64, 1.66, 1.70, 1.76, 1.78, 1.80, 1.82, 1.84, 1.86, 1.92, 1.94, 1.98, 1.106, 1.110, 1.116

Transcription

1 Chemistry 121 Lecture 5: Measuring Temperature; Energy and Heat; Specific Heat and the Heating Curve for Water Sections 1.13, 8.15 in McMurry, Ballantine, et. al. 7 th edition HW #1: 1.42, 1.52, 1.54, 1.64, 1.66, 1.70, 1.76, 1.78, 1.80, 1.82, 1.84, 1.86, 1.92, 1.94, 1.98, 1.106, 1.110, Learning Objectives: 1. Identify differences and similarities between the Fahrenheit, Celsius, and Kelvin Scales 2. Convert between Fahrenheit, Celsius, and Kelvin Scales 3. Define heat as a form of energy, thermal energy, which is due to the motion of atoms and molecules 4. Describe the difference between heat and temperature in terms of energy flow 5. Define the calorie and specific heat 6. Use specific heat values to calculate the difference in temperature resulting from the addition of a given amount of energy to a given mass of substance 7. Use temperature changes, energy input, and mass to determine the specific heat of a substance 8. Define heat of fusion and heat of vaporization 9. Use specific heat, heat of fusion, and heat of vaporization to determine the amount of energy required to convert a given mass of ice to steam 10. List the main mechanisms of heat transfer from the body 1

2 1.13: Temperature, Heat, & Energy The relationship between the Celsius ( o C), Fahrenheit ( o F), and Kelvin (K) scales The units are in a ratio of 9:5 between the Celsius and Fahrenheit scales (about 2:1); further, the Celsius scale is built around the phases of water, so 0 o C corresponds to water freezing and 100 o C corresponds to water boiling. I submit Daniel Fahrenheit had been drinking heavily when he built his scale in the early 18 th century - water freezes at 32 o F and boils at 212 o F. (212-32)/(100-0) = 9/5 o F = (9/5) o C + 32 Question: On July 10 th, 1913 the Greenland Ranch in Death Valley, CA recorded a temperature of 56.7 o C 1. What does this correspond to in o F? Question: On January 21, 1971 Prospect Creek Camp, AK recorded a temperature of o F 2. What does this correspond to in o C? 1 This is generally considered to be the hottest temperature accurately recorded on the planet 2 While -80 o F is pretty chilly, it is significantly warmer than the official global record, recorded at the Vostok station on Antartica on 7/21/1983 of o C 2

3 The Kelvin scale is built upon molecular motion, which is what heat is when viewed at the molecular level. As such, there are no negative values on the Kelvin scale. 0 K is known as absolute zero, the point at which all molecular motion ceases The units are the same size in the Celsius and Kelvin (absolute) scale; the only difference is the starting point 0 o C = K o C = K or K = o C Question: What is the value in K for an object of temperature o C? Heat vs. Temperature Heat and temperature are not the same thing; heat is a form of energy and temperature is not o At the same temperature, 2 objects will have different heat contents based on the amount of substance in question and the type of substance o Temperature is a measure of the energy which is available to flow between 2 systems in contact Energy, Heat, & Specific Heat Motion of compounds at the molecular level heat results in collisions. This is one way of transferring energy between objects o The faster the car, the more energy transferred upon collision. Perhaps we could think of heat as the kinetic energy of a large collection of very tiny, very fast 3 cars bouncing off each other 3 Diatomic nitrogen (N 2 ) is the major component of the air we breathe. At room temperature an average N 2 molecule is moving faster than 1000 mph 3

4 Energy always flows from levels of higher concentration to lower concentration o If a fast energetic molecule runs into a slow lethargic molecule, the slow molecule will speed up and the fast molecule will slow down energy will have been transferred Energy is the ability to do work and is conserved; that is, energy cannot be created or destroyed, just converted from 1 form to another o Consider water falling over a dam: gravitational potential energy of water water kinetic energy o Falling water turning a turbine: kinetic energy of water kinetic energy of turbine o Turbine generating electrical potential: kinetic energy electrical potential energy o Plugging in an electric waffle iron and making a tasty Brussels waffle: electrical potential energy kinetic energy (heat) of waffle iron energy for golden brown chemical reaction Question: Let s say you are at home having an ice-cold beverage. This beverage is filled with ice, which you are acutely aware of because your hand is getting very cold from holding it. What direction is heat flowing? What happens to the heat that flowed from your hand? Where did the energy in your hand come from? Since energy may generally be defined as the ability to do work, and we can define and measure work, there should be units associated with energy 4. Enter the calorie The calorie (cal) is defined as the heat energy required to raise the temperature of 1 gram of water 1 o C Do not confuse this with the nutritional calorie (Cal), 1 Cal = 1 kcal 4 The SI unit for energy is the joule (J) J = 1 cal 4

5 The definition of the calorie directly relates to the specific heat of water; that is, S.H. is equal to the heat required to raise the temperature of 1 gram of a substance 1 o C - for water that value is exactly 1 calorie Notice specific heat units are cal/(g o C) Question: How many calories are required to raise the temperature of 1 liter of water from 20 o C to the boiling point of water? Question: Fat has energy content of 9.2 kcal/gram. If a person ate 1 oz of fat that was completely converted to heat, how much water could be raised from room temperature to the boiling point? Phase Changes and the Heating Curve for Water Temperature does not change as a substance transitions from 1 phase to another water boils at o C until there is no more water left to boil then your saucepan is in deep doo doo. Heat of Fusion Defined: The amount of energy needed to convert 1 gram of a substance from solid to liquid Heat of Vaporization Defined: The amount of energy needed to convert 1 gram of a substance from liquid to gas Let s now consider how the temperature of H 2 O in the solid phase (ice) varies as energy is added to it. This is shown for 1 g of water below. Notice the specific heat of ice and steam do not equal the specific heat of water. 5

6 Energy Required to Raise 1.0 g H 2 O from -20 to 120 o C Question: A cryogenically frozen 10 kg head is removed from storage in liquid nitrogen (77 K). As the head is largely water, we can presume it behaves like ice. How much energy is required to restore the head to body temperature of 37 o C? The specific heat (SH) of ice is.48 cal/(g o C), of water 1.0 cal/(g o C), and the heat of fusion is 80 cal/g When approaching problems like this, solve for the energy required for each change and add them together 1. Energy to heat frozen head to melting point 2. Energy to thaw frozen head 3. Energy to warm frozen head to body temperature Heat required = SH ice (mass)(t) + heat fusion (mass) + SH water (mass)(t) 6

7 Mechanisms of Heat Transfer (From the Body) There is one form of energy transfer missing. Can you guess what it is? Hint: What happens on a clear winter evening in the desert? 7