Science Department Mark Erlenwein, Assistant Principal

Transcription

1 Staten Island Technical High School Vincent A. Maniscalco, Principal The Physical Setting: CHEMISTRY Science Department Mark Erlenwein, Assistant Principal - Unit 1 - Matter and Energy Lessons 9-14 Heat, Temperature, Heating Curve Prepared by Mark Erlenwein

2 Thermometry (Heat Energy and Temperature) The temperature of a substance is a measure of the average kinetic energy of the particles in its substance. (Remember kinetic energy is energy of motion. When we measure temperature, there is a progressive change in temperature over time, which is noted by the movement of the substance inside the thermometer.) 1.Substances at the same temperature, have the same average kinetic energy (makes sense ). 2.Temperature difference indicates heat flow between two bodies. 2. When two bodies have different temperatures, heat flows from body with higher temp to the body with lower temp. This transfer will occur until the two have the same temp. Heat transfer is spontaneous. 4. A thermometer is used to measure temperature. When a thermometer is calibrated, two fixed reference points of temperature are determined. Between the two fixed points, the distance is divided into a desired number of points. These fixed points are known as the freezing point and boiling point. The Celsius Scale The temperature scale most commonly used for scientific work. Devised by Swedish astronomer, Anders Celsius, Freezing point- 0 C Boiling point- 100 C (Note that 0 C is not the lowest temperature that can exist on the Celsius scale.) The Kelvin Scale Lord William Kelvin, proposed a temperature scale on which the zero point would be absolute zero and the size of the degree would be the same as the Celsius degree. On the Celsius scale, absolute zero is 273C. Freezing point= 273 K Boiling point= 373 K C 0 C 100 C I I I 0 K (absolute zero) 273 K 373 K (K = C+ 273) (melting/freezing point) (boiling/condensation point) Sample Problems Convert the following to Kelvin temperature: a)10 C b) - 45 C c) C Convert 200 K to C K = C C = K 273 a) K = C K = ans. 283 K b) K = C K = ans. 228 K c) K = C K = ans 0 K d) Convert 200 K to C K = C C = K C = 200K 273 C = - 73 C 2

3 Measurement of Energy - The Unit of Measurement Calorie- The quantity of heat that will increase the temperature of 1 gram of water by 1 C. The Kilocalorie is also used. 1 kilocalorie = 1000 calories. Calorimetry- The Process of Measurement Calorimetry- the measurement of the amount of heat released or absorbed during a chemical reaction. These measurements are usually done in a device called a calorimeter. The quantity of heat transferred to or from water in a calorimeter can be determined based on certain factors. the mass of the water (grams of water) the change in the waters temperature ( t, delta t) a constant called the specific heat of water. Specific Heat- The amount of heat energy required to raise the temperature of 1 unit of mass of that substance by 1 unit of temperature. To calculate the calories of heat exchange: Number of 3

4 To calculate the amount of heat exchange: Number of calories = grams of water x temperature change Number of joules = grams of water x temperature change ( # of calories = grams of water x t ) [ t = initial temperature - final temperature] ( # of Joules = grams of water x t x Specific Heat of H2O) [ Specific Heat of Water = 4.2 Joules / g - C ] Sample Problems 1. How many calories a released when 50 g of water are cooled from 75 C to 30 C? # of calories = grams of water x t # of Joules = grams of water x t x 4.2 Joules / g - C grams of water = 50g????? = 50g x 45 C x 4.2 Joules / g - C t = t initial t final = 75 C - 30 C = 45 C # of calories = 50g x 45 C = 2250 calories or 2.0 x 10 3 calories # of Joules = 9450 Joules or 9.5 x 10 3 Joules 2. The temperature of 120 grams of water was raised to 80 C by the addition of calories of heat energy. What was the original temperature of the water? # of calories = grams of water x t grams of water = 120g t = t initial t final =? C - 80 C If we solve for t, we can figure out the original (initial) temperature calories = 120 g x t t = 8.3 C (8.3 C or just 8 C) Well if the change in temperature ( t) is 8.3 C, and the final temperature was 80 C, then the initial temperature will be 8.3 less than the final temperature. 3. 4

5 Phases of Matter Matter can exist in three phases: Solid--has a definite shape, a definite volume and a crystalline structure. Has vibrational energy. Liquid--take the shape of the container it is in, has a definite volume, particles move around (the higher the temperature, the more they move) Gas--no definite shape, and no definite volume. Basically, the molecules in a gas are free to go wherever their little legs can carry them. Again, the higher the temperature, the more they move around. Heating Curve Endothermic Boiling Point Heat of Vaporization Melting Point / Freezing point Heat of Fusion Heat of Crystallization Heat of Condensation Cooling Curve Exothermic SECTION TIME A 0 TEMPERATURE 10 STATE S Potential Energy n/a Kinetic Energy Lowest on this incline **A B B 2 50 S S L (M-pt. / Frzg Pt.) Lowest on this plateau *B C S - L C 5 50 L Highest on this plateau Highest on this incline Stay same Lowest on this incline **C D L D L G (B-pt/Cond. Pt.) Lowest on this plateau *D E L - G E Highest on this plateau Highest on this incline Stay same Lowest on this incline **E F G * B C and D - E (Heat energy is being converted to potential energy) **A B and C D and E F (Kinetic energy is increasing) 5

6 6

7 THE NATURE OF SOLIDS Solids have a definite shape, a definite volume and a definite crystalline structure. CRYSTALS A crystal consists of particles arranged in a regular, repeated geometrical pattern. Generally, the relative positions of the molecules in the crystal lattice do not change. Glass is not a true solid. MELTING POINT The temperature at which after energy has been added to a solid, the kinetic energy of the particles increases until there is enough energy to overcome the attractive forces holding them in the crystal--and the substance starts to melt. HEAT OF FUSION Heat of fusion is the amount of energy needed to convert some mass of a substance from a solid to a liquid. The heat of fusion for water is 334 Joules/gram. That means that it takes 334 Joules to change one gram of H 2 0(s) to H 2 0(l) at 0 C and 1 atm pressure (to be discussed later). If you are going the opposite way, freezing it (l s), 334 Joules /g is given off. How much heat energy is absorbed when 7 grams of ice is melted? How much energy is required to freeze 7 grams of water to ice? SUBLIMATION Sublimation is a change from the solid to the gaseous phases without any noticeable liquid phase. Solid carbon dioxide(co2, dry ice), naphthalene (moth balls), the element iodine ( I ). These things are said to sublime ( s g ). CO 2 (s) CO 2 (g) I 2 (s) I 2 (g) 7

8 THE NATURE OF LIQUIDS Liquids take the shape of their container, have a definite volume, and the particles move around. There is a force of attraction among the molecules in a liquid. BOILING POINT As the temperature of a liquid rises, the vapor pressure of the liquid rises until it becomes equal to the pressure on the liquid. At that point any particle in the substance can change from the liquid phase to the gas (or vapor) phase. That's called boiling, and the temperature at which this occurs is called the boiling point. The normal boiling point of a liquid is the temperature at which the vapor pressure reaches standard atmospheric pressure (1 atm, kpa 760 Torr). VAPOR PRESSURE When the molecules of a liquid or solid have enough energy, they can exert enough force or energy to overcome the atmospheric pressure. The substance in question can then become a gas. It can "boil." Liquids do it, but so can solids--dry ice, moth balls, etc. They can become a vapor (a vapor is the term used to denote the gaseous phase of something that is a liquid or solid at room temperature). Evaporation is the process of forming a vapor. HEAT OF VAPORIZATION Heat of vaporization is the amount of energy needed to convert some mass of a substance from a liquid to a gas. The heat of vaporization for water is 2260 Joules/g. That means that it takes 2260 Joules to change one gram of H2O(l) to H2O(g) at 100 C and 1 atm pressure. If the water is condensing (g l), that amount of energy (2260 Joules /g) will be given off. 8

9 1. How much heat (in joules and calories) is released when 120 grams of water condenses from 100 degrees Celsius to 60 degrees Celsius? 2. How many grams of water will require 1200 joules of heat energy when the temperature rises 20 degrees from an initial temperature of 40 degrees celsius? Is heat absorbed or released? 3. If 25 grams of water absorbs 1500 joules of heat energy at a starting temperature of 20 degrees Celsius, what was the final temperature at the conclusion of the absorption? 4. How many degrees will 15 grams of water increase by if 200 joules are initially absorbed at 40 degrees Celsius? 9

10 10

11 Heat of Vaporization Supplement How much heat energy is required when 23.7 grams of water is heated at 100 degrees celsius at 1 atm? Is energy released or absorbed? Endothermic or Exothermic? (q = m Hv) How much energy is required to condense 20 grams of water vapor? At what temperature will condensation form? Is energy released or absorbed? Endothermic or Exothermic? (q = m Hv) What is the Heat of Vaporization of a 227 g substance that absorbs Joules of energy when it boils at sea level? Record your answer using the appropiate number of significant figures. (q = m Hv) 11

12 Heat Problems How much heat energy is required when 23.7 grams of water is heated at 100 degrees celsius at 1 atm? Is energy released or absorbed? Endothermic or Exothermic? How much energy is required to condense 20 grams of water vapor? At what temperature will condensation form? Is energy released or absorbed? Endothermic or Exothermic? What is the Heat of Vaporization of a 227 g substance that absorbs Joules of energy when it boils at sea level? Record your answer using the appropiate number of significant figures How much heat energy is absorbed when 7 grams of ice is melted? How much energy is required to freeze 7 grams of water to ice? What is the Heat of Fusion of a.02 kg substance that releases 1kJ of energy when it melts? 12

13 Challenge Question: A scientist arrived at her log cabin, high in the adirondak mountains. Before she left for the cabin, she picked up some eggs for dinner later. She was hungry after the long trip from Staten Island and decided to cook hard boiled eggs. She put up a pot of water and placed the eggs in the pot. After about 7 minutes, the water was boiling. She remembers from back home that it should take approximately 6 minutes for the eggs to fully cook, once the water is boiling, which will lead to the traditional Hard Boiled texture that some find delectable (yikes!). 6 minutes pass...out come the eggs...and she digs in...so...using your psychic powers and knowledge of Chemistry, predict how delectable, indeed, was the young scientists dinner. 13

14 What information can we deduce, from Table H, concerning these four liquids. What are the trends concerning temperature versus, vapor pressure. If, Sea Level = 1 atm = kpa, then calculate the following conversions:.25 arm = kpa.87 atm = kpa 1.50 atm = kpa 2.30 atm = kpa What do you believe are intermolecular forces of attraction? Which liquid in table H do you think has the strongest and weakest intermolecular forces of attraction? Why? At what temperature do each of the following liquids boil at 70 kpa s of atmospheric vapor pressure? Propanone Ethanol Water Ethanoic Acid 14

15 Table H Notes 15

16 Questions 8 Sample Questions with Solutions More Questions 16

17 Even More Questions Essay Answers 17

18 18

19 Do Now: = substance

20 1. What points (Melting, Freezing, Boiling, Condensating) are present here? 2. What phase change is evident. 3. At what part do the particles have the lowet kinetic energy? 4. At what point(s) is potential energy increasing? 5. At what point is potential energy the lowest? 20

21 SAMPLE REVIEW PROBLEMS 21

22 SAMPLE REVIEW PROBLEMS With Answers How many degrees will 87 grams of water increase by if 865 joules are initially absorbed at 40 degrees celsius? Q = m C T 865 J = 87 g x 4.18 x T Ans. The problem asked for how many degrees will the water increase by = T = 2.4 How many grams of water will require kj of heat energy when the temperature rises 36 degrees from an initial temperature of 40 degrees celsius? Is heat absorbed or released? Q = m C T 12,540 J = m x 4.18 x 36 Ans g Q = m Hv (for vaporization and condensation problems) Q = 49 g x 2260 J / g Ans. 110,740 J How much heat (in joules and calories) is released when 365 grams of water condenses from 84 degrees celcius to 60 degrees celsius? Q = m C T Q = 365 g x 4.18 x 24 Ans. 36,616.8 J For calories, simply remove the 4.18 and use 1.0 for an answer of 8760 calories Q = m Hv (for vaporization and condensation problems) Q = 921 g x 2260 J / g Ans. 2,081,460 J 22

23 Q = m Hf (for melting and freezing problems) Q =7 gx Ans J 334 J / g If 125 grams of water absorbs 15,000 joules of heat energy at a starting temperature of 50 degrees celsius, what was the final temperature at the conclusion of the absorption? Q = m C T 15,000 J = 125 g x 4.18 x T T = 28.7 Ans. The problem asked for the final temperature = = 78.7 degrees celsius Q = m Hf (for melting and freezing problems) Q = 34 g x 334 J / g Ans. 11,356 J THIS IS A FANCY WAY FOR ASKING, what is the melting point or freezing point = ZERO Degrees Celsius FOLKS!! Wuzza Wuzza! Now where and what table might be necessary to determine what the correct answer for this problem is! Look it up folks! 23