Many believe that man s insatiable energy appetite is. destroying the planet. In this unit we will therefore look not

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1 1. Intro 2. data 3. matter 4. the atom 5. electrons 6. periodic table 7. bonding 8. reactions 9. the mole 10. gases 11. solutions 12. Energy 13. Reaction rates 14. equilibrium 15. Acids and bases Energy What changes should we make? Many believe that man s insatiable energy appetite is destroying the planet. In this unit we will therefore look not only at energy from a chemical perspective, but also from an environmental one. We d like to know: Which energy sources are practical, abundant, and environmentally clean? Which energy sources for vehicles are likely to replace gasoline? Which is cheaper, gasoline or electricity? How can energy transfer be measured? How much energy is in a potato chip? How much energy is involved during a phase change? What units are used in energy and how do they relate? What are enthalpy, free energy, and entropy, and why are they important? Tentative Schedule: Day 1: Specific Heat Homework: energyws2: Specific Heat Lab: Specific Heat Capacity of a Metal Day 2: Introduction to Energy Lesson: Why Energy Matters; Definitions; Units Homework: energyws1: Energy Sources and Conversions Day 3: How much energy is in a potato chip: Enthalpy Lab: Potato Chip Calorimetry Homework: Complete chip lab Day 5: Enthalpy Homework: energyws3: Energy with phase change Day 6: Free Energy Lesson: Free Energy; Energy in Connecticut Homework: energyws4 and ws5: Phase change II, Free Energy Day 7: Review Lesson: How to Ace the Energy Test Homework: Review for Energy Test Day 8: Energy Test Lesson: Enthalpy of vaporization and fusion 1

2 Students: Please read this article and then answer the 2 questions at the end. By Jeff Swicord Washington 09 October 2008 Electric Cars Offer Energy Independence With fuel prices still high, the electric car is becoming a more attractive form of transportation. Electric cars were first introduced in the 1970s, but the technology has dramatically improved in the last 10 years. By 2010 automakers like Mercedes and General Motors plan to bring their models to showrooms. Jeff Swicord introduces us to one man who uses electric cars built several years ago as his primary mode of transportation. Like many people in the Washington D.C. area, Brian Murtha commutes five days a week, to downtown and back. But, he does it in an electric car. Brian owns two factory-made electric vehicles: a Toyota Rav 4 EV and a Ford Ranger pick-up truck EV. These were produced in small numbers a few years ago. He charges them from electricity produced by solar panels on the roof of his house. "After I retired from the Air Force, I set a goal not to use energy from anyone else off my property," Brian explained. "To make all my energy myself and be energy independent." Major automakers are betting there will be more and more consumers like Brian in the future. Toyota, General Motors, and Mercedes plan to have an electric vehicle in showrooms within two years. The American made Chevy Volt prototype has received widespread attention at auto shows. For now, fans of electric vehicles like Brian buy their electric cars on ebay. He paid $40,000 for the Toyota. "It gives you 900 pounds on the lowest point of the car, which makes the center of gravity better than the gasoline version," Brian said. "Which makes roll-over less likely and gives you better handling." The inside, with a few exceptions, looks like a gasoline-powered car. Murtha's sun powered car "The electric motor is actually air cooled and produces so little heat it is not really of use in the winter time for heating the passenger cabin. So, Toyota put a heat pump in there," Brian explained. He charges his cars in the garage by plugging into receptors. The electricity runs along wires that are connected to the solar panels on his roof. "It charges about 25 percent an hour," he said. "So if you ran it down a quarter on the fuel gauge, 2

3 if you were three quarters and you wanted to fill it all the way up, it would take you an hour." Brian says driving to work with a gasoline powered vehicle would cost him about $8 a day in fuel. "With the electric car it's about 20 to 25 kilowatt hours to go in and back," he said. "And say about.10 a kilowatt hour, that's about $3.00. And there really is not any maintenance." His quest for energy independence includes his house. He has replaced appliances that run on natural gas with electric ones, including the furnace. But his solar array is not big enough to power the entire house and two cars. "Well If I didn't have the electric vehicles to refuel, the 2200 watt array on the roof of my house, right now, almost completely powers the house," Brian said. Brian plans to add another 7,000 watt array on top of his garage. It will give him more than 9,000, and that will be more power than he needs. Questions: 1. One passage in the text states Brian says driving to work with a gasoline powered vehicle would cost him about $8 a day in fuel. With gas currently at 2 dollars per gallon ($2/gallon), and assuming a gasoline powered vehicle gets 30 miles per gallon (30 miles/gallon), how long is Brian s round trip commute? 2. Another passage states With the electric car it's about 20 to 25 kilowatt hours (kwh) to go in and back," he said. "And say about.10 a kilowatt hour, that's about $3.00. Based on Brian s commute from #1, what is the fuel economy of his electric car in kwh/mile? 3. Soon people will have to try to compare gas mileage to electric car mileage. Perhaps the best way would be to calculate the dollars per mile it costs to drive each car. Using the information above, determine what is cheaper to drive by calculating the A. dollars/mile cost of driving a 30 mile per gallon gasoline car at $2/gallon, and B. the dollar/mile cost to drive a 25 kwh/mile electric car car at $.10/kwh The cheaper car to drive is 3

4 sources Eco-cars issues types sources Eco-cars issues types Water, ice, and steam Free energy Hess s Law Water, ice, and steam Free energy Hess s Law Energy units diets Specific heat enthalpy Welcome to planet earth. Please choose your primary energy source Deluxe #1 #2 #3 #4 #5 Meals: Cost (to the Planet): coal oil Natural gas biomass wood $$high Chemical reaction: Specials: All based on C,H CO 2 + H 2 O Medium? Chemical reaction: ValueMenu Chemical reaction: Nuclear fission 235 U + 1 n 236 U 140 Xe + 94 Sr + 21 n Nuclear fusion 3 H + 1 H 4 He All items $1 none wave wind solar geothermal hydro Place your order: Energy: What Changes Should We Make? Energy cost Welcome to planet earth Please choose your ecofriendly car Appearance (1-10) fuel availability Overall (1-10) units diets Specific heat enthalpy Tesla roadster Toyota Prius Chevy Volt Honda Civic GX electric gas Electric or gas CH now now 2010 now $100K 30K 30k 30k Please rate from 1-10: Appearance (1-10) fuel availability cost Overall (1-10) Also: BMW hydrogen 7 E-85 fuel biodiesel H 2 unknown alot Honda FCX Clarity H 2 Limited now 40k BYD F3DM Electric or gas Now (china) 30k Venturi astrolab or eclectic Electric or solar My eco-friendly car choice: Energy: What Changes Should We Make??? 4

5 sources Eco-cars issues types sources Eco-cars issues types Water, ice, and steam Free energy Hess s Law Energy units diets Specific heat enthalpy Why the energy unit matters: Primary sources of energy are usually chemical coal nuclear propane Natural gas gasoline smog Ozone urban sprawl pollution depletion global warming deforestation overpopulation Energy: What Changes Should We Make? Energy units diets Specific heat enthalpy Energy and Weight Loss 2,000 An average diet is about Nutritional Calories per day People burn roughly 1,500 Nutritional Calories per day without exercise One pound of body fat contains 3,500 Nutritional Calories 1,000 One hour of intense cardio exercise burns Nutritional Calories If I eat normal, and run hard one hour per day I should lose Nutritional 500 Calories per day And I can expect to lose 1 pound(s) per week. Water, ice, and steam Free energy Hess s Law Energy: What Changes Should We Make? 5

6 sources Eco-cars issues types Water, ice, and steam Free energy Hess s Law Energy units diets Specific heat enthalpy Specific heat: Q = mc T heat Mass (g) The amount of heat necessary to heat one g of water by one O C. Temp. change o C c = Specific heat constant = j/g o C for water How many joules of heat are needed to heat one cup (237 ml) of water from room temp. (25 o C) to boiling (100 o C)? Q = mc T = (237)(4.184)(75) = 74,371 joules Energy: What Changes Should We Make? 6

7 sources Eco-cars issues types sources Eco-cars issues types Water, ice, and steam Free energy Hess s Law Water, ice, and steam Free energy Hess s Law Energy units diets Specific heat enthalpy Enthalpy H = heat change at constant pressure = H or H rxn H vs. q? H = q when pressure is constant Exothermic Reactions H is negative - Endothermic Reactions H is positive + Q Energy: What Changes Should We Make? Energy units diets Specific heat enthalpy 5. Standard Enthalpies of Vaporization and Fusion. For water: H 2 O(l) H 2 O(g) H o vap = 2260J/g gas H 2 O(s) H 2 O(l) H o fus = 334 J/g 2260 J/g H vap H cond J/g liquid 334 J/g H fus H solid J/g solid Energy: What Changes Should We Make? 7

8 sources Eco-cars issues types Water, ice, and steam Free energy Hess s Law Energy Free Energy units diets Specific heat enthalpy G = H-T S Josiah Gibbs New Haven, CT Gibb s Free Energy (j) Enthalpy (j) Entropy (j/k) If G < 0 we have a spontaneous process What is Entropy?? Energy: What Changes Should We Make? 8

9 sources Eco-cars issues types sources Eco-cars issues types Water, ice, and steam Free energy Hess s Law Water, ice, and steam Free energy Hess s Law Energy units diets Specific heat enthalpy Entropy = randomness S = positive = more random Liquid gas + more random Liquid solid - less random 1 particle 2 particles + more random The big bang Energy: What Changes Should We Make? Energy units diets Specific heat enthalpy For a reaction: H=145,000 J S = 322 j/k T= 382K Is it spontaneous? (Apply G = H-T S) G = H-T S =145, (322) = 22,000 Positive: nonspontaneous. Energy: What Changes Should We Make? 9

10 sources Eco-cars issues types Water, ice, and steam Free energy Hess s Law Energy spontaneity units diets Specific heat enthalpy H S Spontaneous? - + always + - never + + Rarely. Depends on T - - Usually Depends on T Energy: What Changes Should We Make? 10

11 lab16.1 Name Period Date Specific Heat Capacity of a Metal 10 Points Please read and complete this pre-lab prior to performing this experiment. Purpose: To use a calorimeter to find the specific heat capacity of a metal. Theory: Have you observed how some metals stay hot longer than others? Specific heat measures this property: it is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius. For example, as a 1 g slug of hot lead (Pb) cools in water, it releases joules (j) of heat to the water for each degree that it cools. We can therefore identify metals by measuring the specific heat of an unknown metal, and comparing it to known values. We will do this by heating a metal to 100 O C, then placing it in room temperature water to see how much hotter the water gets. Metals with a high specific heat will heat up the water a lot. We will then calculate the specific heat (called c ) of the metal to identify it. Questions: 1. Give a definition of specific heat in your own words- don t use the definition above. 2. Why might it be important to use a large mass of metal for these measurements? Procedure: We need 5 pieces of data. Put water in a calorimeter (a Styrofoam cup) to just cover your metal; mass the water. Mass of water in calorimeter: Take the temperature of the water. Initial calorimeter water temperature: Take the temperature of the boiling water (it should be 100 O C) Temperature of metal in boiling water: Carefully put your metal in boiling water. Wait 3 minutes. Transfer to calorimeter (cup). See how hot the water gets Temperature of water in cup (and metal) after heated by metal: Dry your metal and mass it. Mass of dry metal: 11

12 Calculations q = m x c x T where q = heat change, c = specific heat, m = mass in grams, and T = change in temperature in o C.. The key assumption we will use is that heat absorbed by water = heat released by metal (q water = q metal ). q for water = j/g o C. Solve for c and use this to identify your unknown metal. The sample calculation below should be very helpful. Sample Calculation: 125 g water, initial temperature of water = 25.6 o C, initial temperature of metal = o C, Final temp of water = C. Mass of metal = 50.0 g. What is the metal? q water = m water x c water x T water = 125 g x J/g o C x (29.3 o C 25.6 o C) q water = 1900J q water = q metal q metal = 1900J = m metal x c metal x T metal c metal = 1900J/m x T = 1900J/50.0 g x (115 o C 29.3 o C) = 0.44 J/g o C. Comparing to known values, the metal may be Iron (Fe, c = J/g o C). Question: 1. What is the identity of your unknown metal? Show your specific heat calculations below. If more than one metal was identified, use common sense to rule out dangerous or expensive elements. Since this calculation is trickier than most, a guide is presented below based on the following logic. We d like to plug directly into q = mc T, but there are 2 unknowns- q and T. However, since the specific heat of liquid water is known we can calculate the heat gained by the water which, if you think about it, is exactly equal to the heat lost by the metal, since that s what heated the water. So q water = m water x c water x T water = g x J/g o C x ( o C o C) q water = q metal = J rearranging, c metal q ( j) metal o m T metal metal ( g)( C) o j g C By comparing this specific heat to known values, and looking at my metal, I conclude the metal is. 12

13 Name Period energy lab 2 Potato Chip Calorimetry In this experiment we will burn a snack chip and see how many calories of heat it produces. We will then compare it to the nutritional information on the container. We can find how many calories are contained in any combustible material by simply measuring the ability of the burning material to heat water. For every degree it can heat up a milliliter of water that is equal to exactly 1 calorie of heat energy. It is also equal to joules, or Nutritional Calories. To do this we put a burning chip under a beaker of water and measure how much warmer the water gets. This is known as calorimetry. The problem: Every year we perform this experiment we get lousy results. The reason: The transfer of heat from a burning chip is inefficient. Too much heat is lost to the air, and to the beaker. We d like all of the energy from that flame to heat up the water, with no loss. It s a similar problem to that faced at home with your furnace, or to the design of a woodstove. Let s improve it. Draw and describe below a calorimetry apparatus that will be as efficient as possible, while still being safe. Don t forget that your chip needs oxygen to burn, and that your design must be safe. Draw it, and describe it fully below; your homework assignment for tonight is to build your design and bring it to class tomorrow. Note that no construction will be allowed in class. Most common errors Possible solutions -nice design but chip won t burn- not enough air pull air through or open up design -too much water- very little temperature change use less than 5 ml of water -heat loss. insulate Calorimeter Design Blueprint 13

14 Safety Warning: This lab includes flames, burning materials, and potentially hot water. Wear goggles, gloves, aprons, and covered shoes. It can also get smoky. Working in the hood or near a window is a good idea. Be cautious working with burning materials. Please let me know if you need fresh air. Enter your data here: Observations of the Burning of a Potato Chip Mass of water in beaker (For water mass = volume) Initial temperature of water Highest temperature of water Change in temperature Specific heat of liquid water in J/g O C 4,184 Number of joules in a nutritional calorie Calculate the number of nutritional calories in your chip below; show your work including cancelled units for credit Measured Chip Calories Calculation (5 points): Read the nutritional information on the bag and calculate the actual number of nutritional calories in your chip below. Show your work including cancelled units, and explain any estimations you had to make. Actual Chip Calories Calculation (5 points): Why is the measured chip calories different from the actual chip calories? Provide a thoughtful explanation paragraph based on your experimental design and performance. Explanation of results (5 points): 14

15 Write up this lab the in the form of a poster that follows the format shown below: Title Name, Date (For example: Potato Chip Calorimetry Or Energy Analysis of a common Snack Food) Data Q = mc T Q= M= C= T = = ( ) ( ) ( ) = J = Nutritional Calories Schematic drawing With labels Of your calorimeter caption Conclusions Include the Nutritional Calories calculated for your chip, the estimated real nutritional calories for your chip, and an explanation for the difference. Pick a topic: 1. What is calorimetry? 2. Sources of Error in our calorimeter design 3. A better design for the next experiment 10 points: 1. Effort: 5 points -does this represent 45 minutes of effort? 2. Calculations: 3 Points -are they accurate? 3. Analysis: 2 points: Why are the results so bad (or so good). 15

16 Name Period energy ws1 Energy: Sources, definitions, and conversions Many people, including scientists, believe the Earth, and the people on it, are in trouble. Problems include global warming, air, land, and water pollution, ozone depletion, overpopulation, and urban sprawl, among others. People may disagree as to how serious these issues are, but most would agree that many of the problems relate to energy use. Here we will consider what energy is, how we use it, and how we measure it. Energy is defined as the ability to do work or produce heat. Our primary sources of energy are the combustion of fossil fuels, nuclear fission, and more passive sources including solar, wind, and geothermal energy. Currently, fossil fuels and the nuclear fission of uranium-235 provide most of our energy needs. In Connecticut, about 50% of our electrical energy comes from nuclear fission, 40% from burning oil, gas, biomass, and coal, and the rest is from solar, wind, geothermal, and other sources. Energy is measured using a variety of units. A Nutritional Calorie is one most of us are familiar with. Not to be confused with the scientific calorie, (note the small c), a Nutritional Calorie is the amount of energy required to raise the temperature of a liter of water by one degree Celsius. Unfortunately, lots of different energy units are used, so we need to be familiar with them and know how to convert them. Here are the most common ones: 1 Nutritional Calorie = 4 BTU (British Thermal Units) = 1000 calories = 4184 joules = KWH Use this information above to answer the following questions 1. What is the primary source of energy that powers your rechargeable ipod? a. The battery b. The electrical outlet c. Sunlight d. Power Plants e. Power lines 2. What is the primary source of energy that heats this school? a. Power lines b. Oil c. Natural Gas d. Nuclear energy d. Solar energy e. Radiators 3. What is the primary source of energy that heat our Bunsen burners in class? a. gasoline b. propane c. natural gas d. power plants e. The gas jets 4. Electrical energy in Connecticut comes from a. Electrical outlets b. Power lines c. Nuclear power d. Nuclear power and combustion e. solar energy 5. What is energy? 6. What do you believe is the primary cause of global warming? 16

17 7. How much energy is required to heat a liter of water from 25 O C to 27 O C? Please show your calculation including cancelled units. 8. You just ate a chocolate bar, which will provide your body with 300 Nutritional Calories of sugarrush energy. Please show your calculation including cancelled units. Convert this 300 Nutritional Calories to a. Joules (Hint: there are 4184 joules in a Nutritional Calorie) b. British Thermal Units (btu) c. kilowatt hours (KWH) 9. Here is the calculation that tells me I should buy a plug-in hybrid car as soon as they come out: My gas-powered Mazda gets 30 miles per gallon (that s 30 miles/gallon), and gas currently costs $3.20 per gallon. How much does it cost to drive my Mazda one mile? Each mile driven by a Toyota Prius uses 0.25 kilowatt hours of electrical energy when the combustion engine is off (that s 1 mile/0.253 kwh) and the electricity costs 10 cents per kwh. How much does it cost to drive the Prius one mile? Which is cheaper to drive- the Mazda, or the Toyota? 17

18 Name Period energy ws2 Specific Heat Worksheet Have you ever noticed how some substances feel colder than others in a room? Or have you noticed that some metals like iron retain their heat for a long time, while others like aluminum cool very quickly? Clearly, substances vary in their responses to heating and cooling. The amount of energy needed to heat a substance is known as specific heat, and is unique for each pure substance. The units are j/g O C, which literally means the amount of energy in joules needed to raise the temperature of one gram of the substance one degree Celsius. Liquid water requires over joules, while gold only requires about 1/10 of a joule for each gram to get one degree Celsius hotter. Use the specific heat formula below to learn about the thermal properties of various substances. The first problem is solved for you. The answers are given; you must show the work to get there. All of these problems may be solved using the specific heat equation: q = mc T where q = heat in joules m= the mass of the substance in g c = the specific heat of the substance in j/g o C (2.03 for ice, for liquid water, and 2.01 for steam) And T is the absolute temperature change in o C. Example: How much energy must be absorbed by 20.0 g of liquid water (C water = j/g 0 C) to increase its temperature from C to C? Solution: q = mc T = (20.0 g)(4.184 J/g 0 C)(20 o C) = 1,673.6 J 1. When 15.0 g of steam drops in temperature from C to C, how much heat energy is released? 2. How much energy is required to heat g of water from C to C? (754 J) (11,046 J) 3. If g of steam at C absorbs joules of heat energy, what will be its increase in temperature? (400.6 O C) 18

19 4. How much heat (in J) is given out when 85.0 g of lead cools from C to 10.0 C? (C lead = J/g C) (2080 J) 5. If it takes joules to heat a piece of gold weighing g from 10.0 C to 27.0 C, what is the specific heat of the gold? (0.131 J/g O C) 6. If 35 g of a substance absorbs 4000 J of heat when the temperature rises from 25 to C, what is the specific heat of that substance? (0.235 J/g O C) 7. A certain mass of water was heated with 41,840 Joules, raising its temperature from 22.0 C to 28.5 C. Find the mass of water. (1540 g) 8. Calculate the number of joules given off when 32.0 grams of steam cools from 90 degrees celsius to 31 degrees celsius. (3800 J) 9. How many joules of heat are lost by 3580 kilograms of granite as it cools from 41.2 O C to O C? The specific heat of granite is J/g O C (1.56 x 10 8 J) 10. In reality we are making a lot of assumptions for these calculations to be accurate. For example, in many cases we are assuming a perfectly insulated container. List two other assumptions we are making in these calculations

20 Name Period energy ws3 Energy of Heating and Cooling Water In the previous worksheet we learned to use the specific heat equation: q = mc T (where q = joules of heat, m = grams of mass, c = specific heat in j/g O C, and T = degrees of temperature change) To heat or cool water we have to consider the phase it is in, as well as the energy associated with phase changes Symbol and value Symbol What it means c water(l) = 4.18 J/g o C The specific heat of liquid water It takes 4.18 joules of heat energy to raise the temperature of 1 gram of liquid water by one degree Celsius c water(s) = 2.03 J/g o C The specific heat of ice It takes 2.03 joules of heat energy to raise the temperature of 1 gram of solid water (ice) by one degree Celsius c water(g) = 2.01 J/g o C The specific heat of steam It takes 2.01 joules of heat energy to raise the temperature of 1 gram of gaseous water (steam) by one degree Celsius H vap = 2,259 J/g The enthalpy of vaporization of water It takes 2,259 joules of energy to boil one gram of liquid water (convert it from a liquid to a gas). H fus =334 J/g The enthalpy of fusion of water It takes 334 joules of energy to melt one gram of liquid water (convert it from a solid to a liquid). To find out how much energy it takes to heat water, we may have to include the energy of phase changes. For example, there are five separate energy steps involved when ice is heated into steam: 1. The ice warms to 0 O C 2. The ice melts at 0 O C 3. The water heats to 100 O C 4. The water boils at 100 O C. 5. The steam raises to it s final temperature. Note for the last step we assume a closed system that would pressurize. Use these ideas to answer the guided questions below. 20

21 1) A 12 oz. can of soda contains 450 g of water. How many joules are released when a can of soda is cooled from 25 degrees Celsius (room temperature) to 4 degrees Celsius (the temperature of a refrigerator). The specific heat of liquid water is 4.18 J / gram x o C. Hint: q=mc T (39.5kJ, or 39,500 J) 2) How many joules are required to heat 250 grams of liquid water from 0 0 to C? Hint: q=mc T (104.5 kj) 3) How many joules are required to melt 100 grams of water? The heat of fusion of water is 6010 J / mole. Hint: Each mole we melt consumes 6010 J. How many moles of water do we have? (33.4 kj) 4) How many joules are required to boil 150 grams of water? The heat of vaporization of water is 40,670 J / mole. Hint: Each mole we boil consumes 40,670 J. How many moles of water do we have? (338.8 kj) 5) How many joules are required to heat 200 grams of water from 25 0 C to C? The specific heat of steam is 2.01 J / g. 0 C (Hint: there are 3 parts to this) Hint: Here are the parts Part 1: C: q = mc T = ( )( )( )= Part 2: boiling at 100 o C: 40,670J/mole. moles = Part 3: C: q = mc T = ( )( )( ) = Total = = (524.7 kj) 7. How much heat is required to warm 225 g of ice from C to 0.0 C, melt the ice, warm the water from 0.0 C to C, boil the water, and heat the steam to C? (732 kj) 21

22 Name Period energy ws4 Energy with Phase change WS II Useful information (see previous worksheet and powerpoint presentation for a full explanation of terms): q = mc T c water(l) = the specific heat of liquid water = J/g o C c water(s) = the specific heat of ice = 2.03 J/g o C c water(g) = the specific heat of steam = 2.01 J/g o C H vap the energy required to convert liquid water to steam = 2259 J/g H fus = the energy required to convert ice to liquid water = 334 J/g Example: How many joules of energy are required to heat 25 grams of water from -28 to +130 o C? Solution: Heating up that 25 grams of ice involves 5 separate steps: 1. The ice heats from -28 O C to 0 O C: q = mc T = (25 g)(2.03 J/g O C)(28 O C) = 1,421 J 2. At 0 O C the ice melts: 334 j/g x 25 g = 8350 J 3. The water then heats from 0 to 100 O C: q = mc T = (25 g)(4.18 J/g O C)(100 O C) = 10,450 J 4. The water boils at 100 O C: 2259 J/g x 25 g = 56,475 J 5. The steam heats from 100 to 185 (in reality this could only happen as pressure increases); q= mc T = (25)(2.01)(85) = 4271 J Total: 80,967 J 1) How many joules are required to heat 75 grams of water from C to C? 2) How many joules are required to heat a frozen can of ice (360 grams) from -5 0 C (the temperature of an overcooled refrigerator) to C (the highest practical temperature within a microwave oven)? 22

23 3) (Level 1 only) How many joules are released when 450 grams of water are cooled from 4 x C (the hottest temperature ever achieved by man) to 1 x C (the coldest temperature achieved by man). 4) (Level 1 only) How many joules are required to raise the temperature of 100 grams of water from C (the current temperature of space) to 1.6 x C (the estimated temperature of space immediately after the big bang)? 23

24 Name Date Period G = H T S Where G = Gibbs Free Energy Free Energy Worksheet Energy ws5 We have just learned that if the free energy ( G) of a reaction is negative, that reaction will occur spontaneously. This can be calculated using the Gibbs Free Energy Equation shown on the left, named after New Haven s own J. Willard Gibbs. Now, usually a reaction is spontaneous if it is exothermic. Bang. H = Enthalpy in Joules Sometimes however, an endothermic reaction can be spontaneous. The melting of ice is a good example. H is slightly positive, but it has T = Temperature (K) something else going for it: the process produces disorder. This provides enough boost to make the reaction spontaneous at room And S = Entropy in Joules/K temperature. This randomness factor is called entropy (S). Examples of increased randomness include a liquid becoming a gas, something splitting in two, or things spreading out, like the expansion of the universe. As Entropy increases it becomes more positive. If we know the temperature, enthalpy, and entropy of a reaction or system we can use the Gibbs Free energy equation to predict if the reaction will be spontaneous. Example: Calculate DG and indicate if the reaction is spontaneous when H = 2.3 kj, T = 25 O C, and S = 195 J/K Solution: Note that we need to convert kilojoules (kj) to joules (1000 J = 1 kj), and degrees Celsius to Kelvin (K = O C + 273). G = H-T S = 2300 J (298 K. 195 J/K) = -55,810 J = spontaneous Most common errors: Did not change kj to J; did not change O C to K; subtracting before multiplying. Calculate G and indicate if the process is spontaneous or nonspontaneous 1. H = 145 kj, T = 293K, S = 195 J/K 24

25 2. H = -232 kj, T = 273K, S = 138 J/K 3. H = kj, T = 100 o C, S = -268 J/K 4. Calculate the temperature at which G = 34.7 kj if H = 40.2 kj and S = 22.2 J/K. Iron ore can be converted to iron by the following reaction: Fe 3 O 4 (s) + 4H 2 (g) 3Fe(s) + 4H 2 O(g) H = kj For this reaction S = 610 J/K. 5. Is this reaction spontaneous at 298K? What is the value of G? 6. Why is the entropy for this reaction positive? 25

26 How to ace the Energy exam In this our 12 th unit we investigated Energy, both from a scientific and environmental perspective. We began by considering the primary sources of energy we use, and their environmental consequences, as well as their abundance. This is perhaps the most important information to answer our essential question for this unit: What Changes Should We Make? We looked at some efforts to answer this question in the form of transportation when we took a look at some eco-friendly cars. We then focused on the hard science that relates to energy: what it is, types of energy, Enthalpy, and exothermic and endothermic chemical reactions. During this time we performed two experiments where we measured the specific heat of an unknown metal, and the energy contained in a potato chip, using a calorimeter of our own design. We finished by considering the energy required to heat water, including the phase changes that may be involved, and we familiarized ourselves with Free energy, which includes the esoteric concept of entropy. To ace this unit be familiar with the terminology associated with energy, know how to measure it, and think about the sources of energy we use, and what changes we must make for the sake of our planet. As usual, review your notes, worksheets, and lab experiments, and answer all of the questions below. In our next unit we will ask why some reactions such as explosions are rapid, while others such as rust are slow- this is our Rates of Reaction unit, coming up. Useful information to be provided on test: q = mc T where m = mass (g), c = specific heat (J/g 0 C; see examples for H 2 O below), and T = change in temperature ( 0 C) c water(l) = 4.18 J/g o C c water(s) = 2.03 J/g o C c water(g) = 2.01 J/g o C H vap = 2260 J/g H fus = 334 J/g At 1 atm: Water boils/condenses at 100 o C Water melts/freezes at 0 o C 1 Nutritional Calorie = 4 BTU (British Thermal Units) = 1000 calories = 4184 joules = KWH G = H-T S Where G = change in free energy (J), H = change in enthalpy (J), T = temperature (K), and S = change in entropy (J/K) 26

27 1. Know the meaning and usage of the following terms Energy Enthalpy ( H) Potential Energy Kinetic Energy Exothermic Endothermic Specific Heat Heat of vaporization ( H vap ) Heat of fusion ( H fus ) Heat of condensation ( H cond ) Heat of solidification ( H solid ) Hess s Law calorie Nutritional Calorie Joule Standard enthalpy of formation ( H o ) Free Energy Entropy 2. Understand concepts, with sample questions: 1. Energy Which of the following are exothermic processes: melting, freezing, boiling, condensing, subliming, depositing? 2. Thermochemical properties of fuels 27

28 Based on the table below, how much heat would be generated if 92 grams of ethanol were burned? 3. Energy unit conversions Convert 1000 joules to kj, calories, and Nutritional Calories 4. Specific heat 2. When 15.0 g of steam drops in temperature from C to C, how much heat energy is released? 5. Typical specific heat numbers and their meaning 6. If it takes joules to heat a piece of gold weighing g from 10.0 C to 27.0 C, what is the specific heat of the gold? 6. Enthalpy for temperature changes compared to phase changes. Calculate how much energy is required to convert 1 kg of ice at -10 o C to steam at 101 o C Answer: 28

29 7. Free Energy 1. Calculate G and indicate if the process is spontaneous or nonspontaneous a. H = 145 kj, T = C, S = 195 J/K b. 2. Calculate the temperature at which G = 34.7 kj if H = 40.2 kj and S = 22.2 J/K. 8. Energy and the Environment a. Describe details concerning the energy usage of two of your favorite eco-friendly cars b. How can the energy costs of an electric car be compared to a gasoline car? c. Use this real data to make a comparion of energy costs between a prius and a Matrix Prius: Requires kwh/mile where each kwh costs ten cents Matrix: Requires 1 gallon of gas for every thirty miles, where each gallon costs two dollars 29

30 d. Rate the following energy sources in terms of cost, abundance and damage to the environment Energy Source Coal oil Natural gas Nuclear fission Nuclear Fusion tidal wave geothermal Chemical (if any) involved Cost (cheap, medium, expensive) Abundance (running out, medium, plenty, renewable) Damage to the environment (benign, medium, bad) My opinion (excellent, okay, yuck) plus comments E-85 biodiesel solar wind 8. Depending on your class, you may have learned some additional topics such as protein Kinase Mzeta and the ZIP molecule, the used and abused molecule tetrahydrocannabinol (THC), or details concerning nuclear fusion. Review your notes and have a good general knowledge of each topic. 8. Review the rest of your notes, worksheets, and lab experiments. Good Luck on the test. 30

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