Start Part 2. Tro's "Introductory Chemistry", Chapter 3

Transcription

1 Start Part 2 1

2 Separation of Mixtures Separate mixtures based on different physical properties of the components. Physical change. Different Physical Property Boiling point State of matter (solid/liquid/gas) Adherence to a surface Volatility Density Technique Distillation Filtration Chromatography Evaporation Centrifugation and decanting 2

3 Distillation Figure 3.13 An example of a distillation set up. 3

4 Filtration Figure 3.14 An example of filtration. 4

5 3.7 Conservation of Mass: There is No New Mass 5

6 Law of Conservation of Mass Antoine Lavoisier Matter is neither created nor destroyed in a chemical reaction. The total mass of all the reactants is equal to the total mass of all the products. 6

7 Conservation of Mass. 58 grams of butane burns in 208 grams of oxygen to form 176 grams of carbon dioxide and 90 grams of water. butane + oxygen carbon dioxide + water 58 grams grams 176 grams + 90 grams 266 grams = 266 grams Figure 3.12 A picture of a butane lighter. 7

8 Methane Burns in Oxygen to Produce Carbon Dioxide and Water. methane + oxygen carbon dioxide + water 16.0 g 64.0 g? g 36.0 g. If 16 g of methane reacts completely with 64.0 g of oxygen, and 36.0 g of water is produced. How many g of carbon dixide are produced? 8

9 Methane Burning (cont) mass reactants = mass products mass methane = mass carbon dioxide and oxygen and water g =?g g 80.0 g =?g g 44.0 g =?g = mass of carbon dioxide 9

10 3.8 Energy 10

11 Energy Energy is anything that has the capacity to do work (A force x distance). Physical and chemical changes have energy associated with them. 11

12 Law of Conservation of Energy Energy can neither be created nor destroyed. If energy is constant, how can there be an energy crisis? Transfers of energy are not 100% efficient. Resulting in energy in a non-useful form... 12

13 Matter Possesses Energy An internal combustion engine Energy is stored in chemical bonds of substances. Energy can be converted Example an auto Gasoline burned releasing heat some heat converted to mechanical energy moves auto. 13

14 Kinds of Energy Kinetic and Potential Potential energy is energy that is stored. Gravitational ( a dam) Electrical battery Nuclear atom nucleus Chemical bonds Page 65 A Picture of a dam and reservoir. Kinetic energy is energy of motion, or energy = ½ mass x (velocity) 2 14

15 Converting Forms of Energy Water is stored at a higher level by a dam, creating gravitational energy. Water may flow over the dam and gravitational energy is converted to kinetic energy. Some of the kinetic energy is captured by a turbine to make electricity Eletricity comes to your house and is converted to heat by a toaster The heat of the toaster causes chemiscal changes in bread. Gravitational kinetic electrical heat chemical change. 15

16 Losing Energy. Unfortunately we cannot get a 100% efficient process. The energy lost in the process is energy transformed into a form we cannot use. Often this non useful energy is disapated as heat. 16

17 Example of Lost Energy Most of the energy released in the burning of gasoline is released as sound or heat. This energy does not move your car along 17

18 Units of Energy Calorie (cal) is the amount of energy needed to raise one gram of water by 1 C. kcal = energy needed to raise 1000 g of water 1 C. food calories = kcals = Cal. Energy Conversion Factors 1 calorie (cal) = joules (J) 1 Calorie (Cal) = 1000 calories (cal) 1 kilowatt-hour (kwh) = 3.60 x 10 6 joules (J) 18

19 Energy Use Table 3.3 Examples of amounts of energy use in different energy units. 19

20 Problem 3.58 A bag of chips has 245 Cal. If 1.00 lb of fat is stored for each 1.46 x 10 4 kj of extra energy consumed. How many bags of potato chips contain the food energy to result in 1.00 lb of body fat? J = 1 cal Cal cal J kj bags 20

21 Problem 3.58 (cont.) 245 Cal 1000 cal = 245,000 cal 1 Cal 245,000 cal 4,184 J = 1025,000 J 1 cal 1025,000 J 1 kj = 1025 kj this is kj per bag 1000 J We want to end up with bags per lb. We set it up as below so the kj cancel. Note we are dividing the kj of one lb by the kj for 1 bag. 1 bag 1.46 x 10 4 kj = 14.2 bags 1025 kj 1.00 lb J 21

22 3.9 Energy and Chemical and Physical Changes 22

23 Chemical Potential Energy The amount of energy stored in a material is its chemical potential energy. This energy is due to chemical bonds and the attractions between molecules. During a physical or chemical change ther will be an associated energy change usually heat. 23

24 Energy Changes and Chemical Reactions Chemical reactions happen most easily when energy is released during the reaction. Molecules with high amounts of chemical potential energy are less stable than thoses with lower amounts of chemical potential energy. 24

25 Exothermic Processes When a change results in the release of energy it is called an exothermic process. In exothermic reactions, reactants have more chemical potential energy than the products. Energy that is released is equal to the difference in energy of the reactants and products. Usually the surroundings get warmer 25

26 An Exothermic Reaction A methane flame. A diagram showing how an exothermic process releases energy to the surroundings. Figure 3.16 (a) showing how energy of products is lower than reactants so energy is released. 26

27 Endothermic Processes When a change results in the absorbtion of energy it is called an endothermic process. In endothermic reactions, reactants have less chemical potential energy than the products. Energy that is released is equal to the difference in energy of the reactants and products. Usually the surroundings become cooler. 27

28 An Exothermic Reaction A cold pack A diagram showing how an endothermnic process aborbs energy from the surroundings. Figure 3.16 (b) showing how energy of products is higher than reactants so energy is absorbed. 28

29 3.10 Temperature: Random Molecular and Atomic Motion 29

30 Temperature Temperature is a measure of the random motions of molecules. The faster molecule move the greater the amount energy they have, and the higher the temperature 30

31 Temperature Scales Fahrenheit scale, F. U.S. Celsius scale, C. all other countries. A Celsius degree is 1.8 times larger than a Fahrenheit degree. Kelvin scale, K. Absolute scale. SI unit Figure 3.17 The three temperature scales 31

32 Temperature Scales 100 C 373 K 212 F 671 R Boiling point water 25 C 298 K 75 F 534 R Room temp 0 C C 273 K K 32 F -38 F 459 R 421 R Melting point ice Boiling point mercury -183 C 90 K -297 F 162 R Boiling point oxygen -269 C -273 C 4 K 0 K -452 F Celsius Kelvin Fahrenheit -459 F 7 R Rankine BP helium 0 R Absolute zero

33 Temperature Scales Room temperature is about 72 F and 22 C. 33

34 Fahrenheit vs. Celsius Conversion between Celsius and Fahrenheit must take into account 1 C is 1.8 larger than 1 F. The zero points of the two scales are different. F -32 C

35 F to C Conversions A sunny day is 100 F, what is the temperature in C? C = ( F 32) (100 32) (68) = 37.8 C

36 The Kelvin Temperature Scale Temperature is a measure of the energy content of molecules and should always be positive. Kelvin is an absolute scale that starts at absolute zero, a theoretical value where all molecular motion stops. Kelvin is measured in Kelvins, not degrees. 1 Kelvin has same size as 1 C C = K K is always positive 36

37 3.11 Temperature Changes: Heat Capacity 37

38 Energy and the Temperature of Matter The amount the temperature of an object increases depends on three things heat energy added (q). The mass of material The identity of the material The equation for this change is: Amount of Heat = Mass x Heat Capacity x Temperature Change q = m x C x DT q = heat, m = mass, C = heat capacity, and DT = change in temperature DT = T final T initial 38

39 Heat Capacity Heat capacity is the amount of heat a substance must absorb to raise its temperature by 1 C. cal/ C or J/ C. Metals have low heat capacities; insulators have high heat capacities. Specific heat = heat capacity of 1 gram of the substance. cal/(g C) or J/(g C). Water s specific heat = J/(g C) for liquid. Or cal/(g C). 39

40 Specific Heat Capacity Specific heat is the amount of energy required to raise the temperature of one gram of a substance by 1 C. The larger a material s specific heat is, the more energy it takes to raise its temperature a given amount. Like density, specific heat is characteristic of a substance. It can be used to identify the type of matter.. 40

41 Specific Heat Capacities Substance Specific Heat J/g C Aluminum Carbon (dia) Carbon (gra) Copper Gold Iron Lead Silver Ethanol 2.42 Water (l) Water (s) 2.03 Water (g)

42 The Sign of heat Note heat may be positive or negative. Consider the equation: q = m x C x DT m and C are always positive. DT may be positive or negative, q will have the same sign. If q is negative the process is exothermic. If q is positive the process is endothermic. 42

43 Heat Gain or Loss by an Object q = m x C x DT Problem 3.90 A block of aluminum of volume 98.5 ml absorbs 67.4 J of heat. Initially it is at 32.5 C. What is the final temperature? Density of aluminum is 2.70 g/ml. Heat Capacity of aluminum is J/(g - C) ml g DT using q = m x C x DT T final 43

44 Problem 3.90 (cont.) 98.5 ml 2.70 g = g 1 ml 67.4 J = ( g) J DT g- C 67.4 C = DT C = DT DT = T final T initial 0.28 C = T final 32.5 C 32.8 C = T final 44