Hot Packs and Cold Packs
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Transcript of Hot Packs and Cold Packs
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1
Hot Packs and Cold Packs• Common Medical “Over the Counter” products
– “Universals” use mechanical heat storage• Put in freezer or microwave, then to injury• Temporary use, but can be recycled
– “Instant” packs involve chemical reactions• Exothermic and Endothermic chemistry• Usually single-use, but no pre-heating or cooling required• We will explore these chemical types in today’s experiment
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2
Thermal Semantics• Temperature
– A quantitative measure of “hot and cold”• Arbitrary scales; Fahrenheit, Centigrade, Kelvin• An indicator of kinetic energy content
– Does not depend on amount of material• Ocean & tea cup can have same temperature
• Heat– A quantitative measure of energy transfer
• Measured in Joules or Calories• Energy flows from hot to cold spontaneously• Transfer by conduction or radiation
– Depends on amount of material involved• More material involves more heat transfer• Ocean has more heat than a tea cup of water
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Temperatures around the Worldleft picture refers to January temperaturesHeat energy is delivered by solar radiation
Temperature difference is a result of unequal heat delivery
3
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Global Temperature HistoryShort term “warming”, but long term trend is “cooling”
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Global WarmingCurrent trend began >10k years ago, are we now “between glacial periods”?
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Changes in Energy• Heat Energy change requires definitions
– Viewpoint is perspective of system changing• Negative Energy considered as LOSS
– Heat flowing OUT OF a fireplace or oven– Oven loses heat when oven door opens
• Positive Energy viewed as net GAIN– Heat flows INTO an ice cube to melt it– Kitchen warms due to open oven door
– Energy change is algebraic difference• Definition: E after – E before = ΔE change
• Depends only on the initial and ending conditions– NOT dependent on the path taken– Ice sample could be melted 10 times and frozen 9
» Same result as melted once
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James Joule experimentdemonstrated equivalence of potential energy and heat
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Energy (Enthalpy) of Thermal Change
• “Enthalpy” or ∆H indicates Thermal energy• Thermal Changes due to Chemical Reactions
– Exothermic reaction = heat generated• Enthalpy sign is NEGATIVE (heat flowing away from system)
– Thermite reaction, neutralization
– burning gasoline, fireplace, hot tub
– body heat from food, rubbing hands to keep warm
– Endothermic reaction = heat absorbed• Enthalpy sign is POSITIVE (heat flows intointo the system)
– Melting Ice, frozen foods
– Evaporation of water & other liquids absorb heat
– Cold can of soda warming on counter
– Choice of direction was arbitrary (like electron charge)• Might not be intuitive, but consistent with other definitions
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ThermodynamicsLosing Energy
• EX OTHERMIC– Reactions which generate and/or lose heat– Energy is transferred to surroundings
• Burning leaves, coffee cooling, moving automobile
– ΔH or “Enthalpy” is term for heat transfer• -ΔH or “Enthalpy” is negative for Exothermic• (-) Enthalpy becomes part of chemical equation• Enthalpy usually in kJ per Mole• Total energy depends on total quantity
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EXOthermic reaction (-ΔH) Producing heat or thermal energy by burning fuel,
converting chemical (or nuclear) into kinetic or heat energy
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1111
ThermodynamicsGaining energy
• ENDOTHERMIC– Reactions which extract and/or gain heat– Energy is transferred into the object
• Melting ice, coffee being made (water heated)• +ΔH or “Enthalpy” is term for heat input
– ΔH “Enthalpy” positive (+) for Endothermic
• (+) Enthalpy becomes part of chemical equation• Enthalpy usually in kJ per Mole• Total energy depends on total quantity
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ENDOthermic reaction (+ΔH)Absorbing heat energy from environment
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Air Conditioning = ENDOthermic cooling results from evaporation reaction absorbing heat
accompanied by exothermic condensation at radiator
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1515
Water EnergyMaking water from elements releases heat energy
splitting water into elements requires electrical energy
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Bond Energy• Heat results from rearranging chemical bonds
– Reducing available energy (reactants-products) releases energy• Burning wood, animal metabolism
– Increasing chemical energy (products-reactants) absorbs energy• Photosynthesis, melting ice
• Impractical to measure ΔH for every known reaction– Billions of chemical combinations– But use of common bonds provides a practical answer …
• Can use common “features” to divide and conquer– Bond breaking energy can be determined for reference cases
• Carbon-Carbon bonds (single C-C, double C=C, triple C≡C)• Diatomic molecules (Cl2, H2, O2, etc.)
– Use known bond energies to estimate new combinations• Algebraic sum of the bond energy components• Must use balanced equations and appropriate multipliers
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Sample Bond Energy CalculationBurning of Hydrogen in Air, producing heat
Tables of data differ, but have similar values
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A few common bond energies
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Table of Bond Energies combustion heat output
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A home furnace exampleWe can predict heat from burning methane via bond energies
• Burning Methane CH4+ 2O2 CO2 + 2H2O• Reactants:
– 4 * C-H bonds x 414kJ/mol * 1mol= 1656kJ– O=O bond = 498kJ/mol * 2mol = 996– Total reactants bond energies = 2652kJ
• Products: – 2 * C=O bonds x 803kJ/mol = 1606kJ– 2 * H-O bonds x 464kJ/mol * 2 mol = 1856kJ– Total products bond energies = 3462 kJ
• Change = 2652 - 3462 = - 810 kJ– Literature value comparison = - 803 to - 889kJ/mole– Negative energy change means ExothermicExothermic– Products more tightly bonded than reactants– Takes more energy to pull products apart– Excess energy released as Heat
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Standard State
• Must define “state” of material for reference– Gas, Liquids, Solids have different energy content
• Evaporation of water cools (energy loss 44kJ/mole)• Compression of refrigerant heats it (energy gain)
– “STP” is a definition for reference (standard) state• Reference temperature (typically 0 or 25 degrees Celsius)• 1 atmosphere of pressure• Concentration of 1.00 Moles per Liter (usually)
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Heats of reactionsyour home furnace in chemical termsone last step involves the water vapor
Two reactions can be combined (both of these exothermic)Burning of Methane, and condensation of water vapor. A
thermodynamic model of the furnace in your house.
CH4(g) + 2O2(g) CO2(g) + 2 H2O(g) ΔH = - 810kJ/molH2O(g) H2O(aq) a change of state ΔH = - 44 kJ/mol • Evaporation absorbs heat, so condensation yields heat• Stoichiometry requires consistent number of moles2H2O(g) 2H2O(aq) ΔH = - 88 kJoule
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Heats of reactionsCan add the equations, molecules AND reaction energy• Net reaction, adding the two:
CH4(g) + 2O2(g) CO2(g) + 2 H2O(g) ΔH = -810 kJoule2H2O(g) 2H2O(aq) ΔH = - 88 kJoule
-----------------------------------------------------------------------------CH4(g) + 2O2(g) CO2(g) + 2 H2O(aq) ΔH = -898kJoule
• Magnitudes of heat energy combine same as for the molecules in a chemical reaction
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Some mechanisims
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Mechanism for heat of solution
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Heat of solution (dissolving)
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Calorimeter in a cup
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We will use a simple calorimeterstyrofoam cups for insulationswirling better than stirring
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What’s wrong with this picture?(recall James Joule’s experiment)
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Energy Dimensions
• Original definition is “calorie” (small c)– Energy to raise temp.1 gram (1 ml) water 1.0oC– Turned out to be inconveniently small
• Usual quotation in kcal = “Calorie” (big C)– Energy to raise temp 1.00 liter water by 1.0oC– Calories are NOT in S.I. (MKS) dimensions– Commonly used for food products
• SI or ISO unit of energy is “Joule”– 1 watt for one second = 1 Joule– Conversion is 4.184 Joule/calorie– Same thing is 4.184 kJ/kcal = 4.184 kJ/Calorie
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Our Procedure
• Perform an EXOTHERMIC reaction– CaCl2 dissolving in water produces heat
– Make a plot to determine maximum temp.– Use Q=m*ΔT*c = calories
• C is a constant for water = 1.00
– Calculate kcal per mole• Moles from mass of salt & formula weight• Compare to literature values, how close?
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Calculations
• Similar to burning of food experiment– Heat is delivered to measured mass of water
• Calories into water + salt, Q = m*c*∆T• Q = heat in calories• M = actual mass of water + salt ( ≈ 120gram)• C = specific heat of water = 1 cal/(gm*∆T)• Q = 120gm*1cal/(gm*∆T)*∆T = calories• If Q positive, solution gets COLD
• If Q negative, solution gets HOT
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Procedure• Water
– Weigh empty cup and with ≈100mL water– Obtain mass of water in grams
• Salt– Weigh container without & with salt– Obtain mass of salt in grams
• Temperature– take initial temperature of water
• Mix salt and water– Take temp. every 10 seconds for first 3 minutes– Take temp. every 30 seconds for another 2 minutes– Swirl water in cup to mix between readings
• Plot the data
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HEATING REACTION
Heat from mixing H2O + CaCl2
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
0 100 200 300 400
Time in Seconds
Te
mp
era
ture
Ce
nti
gra
de
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Calculation Procedure• Use graph to determine maximum temperature reached• Calculate calories produced
– (water grams + salt grams) * ΔT = calories
• Calculate energy per mole derived from salt– Calories / moles = energy per mole– Convert to kJ/mole for literature comparison
• Calories / 4.182 = Joules• Joules / 1000 = kJoules
– Compare to literature values• - 82.0 kJ/mole for CaCl2 (exothermic) is customary value• How close did you get ?• Calculate error = (literature-experimental) / literature • Answer *100 = percent error
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Sample CalculationMin temp 21.00
Max temp 50.00
∆T 29.0empty
cupwith
contentcontent Grams
Water 5 105 91.06
Calcium Chloride 15 35 20.01
Mass of reactants 111.07
Starting Temperature of reactants (from graph data) = 21.0 oCMaximum temperature (from graph data) = 50.0 oC
ΔT = 29.0
Mass of calorimeter contents (grams of water and salt) = 111.070 gramsSpecific heat of reaction product (given value) = 1.000 calories/(oC*gram)
q = m*∆T*Cp (Cp=specific heat) = 3,221 calories
Grams of salt changing water temperature = 20.01 from data above
Formula weight of CaCl2 [40.078+(2*35.45)] = 110.98 gm/mole
Moles of CaCl2 = 0.1803 moles
Calories per mole of CaCl2 = 17,864 calories/mole
Reaction was exothermic (got HOT), so ∆H must be negative = (17,864) calories/mole
conversion factor for calories to Joules = 4.18 Joules/calorie
heat output in Joules/mole = (74,673) Joules/mole
heat output in kJoules/mole = (74.67) kJoules/mole
Literature value for CaCl2 dissolving in water = (82.0) kJoules/mole
deviation from literature value = -8.9% PerCent
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2nd half of experiment
• Repeat for ENDOTHERMIC reaction– NH4Cl in water absorbs heat
• Measure masses, initial temperature
• Mix and measure temperature changes
• Plot data
• Calc energy absorbed per mole of salt
• Compare to literature value
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COOLING REACTION
Heat Loss from mixing H2O + NH4Cl
0.0
5.0
10.0
15.0
20.0
25.0
0 100 200 300 400
Time in Seconds
Te
mp
era
ture
Ce
nti
gra
de
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Endothermic data exampleMin Temp 7.90Max Temp 20.00
∆T -12.1 empty cupwith
contentcontent Grams
Water 5 105 92.40
Ammonium Chloride 15 35 20.02
Mass of reactants 112.42
Starting Temperature of reactants (from graph data) = 20.0 oCMinimum temperature (from graph data) = 7.9 oC
∆t = -12.1 oC
Mass of calorimeter contents (grams of water and salt) = 112.420 gramsSpecific heat of reaction product (given value) = 1.000 calories/(oC*gram)
q = m*∆T*Cp (Cp=specific heat) = -1,360 calories
Grams of salt changing water temperature = 20.02 from data above
Formula weight of NH4Cl [14.007+(4*1.008)+35.45] 53.49 gm/mole
Moles of CaCl2 = 0.3743 moles
Calories per mole of CaCl2 = (3,634) calories/mole
Reaction was exothermic (got COLD), so ∆H must be POSITIVE = 3,634 calories/mole
conversion factor for calories to Joules = 4.18 Joules/calorie
heat output in Joules/mole = 15,192 Joules/mole
heat output in kJoules/mole = 15.19 kJoules/mole
Literature value for CaCl2 dissolving in water = 14.7 kJoules/mole
deviation from literature value = 3.3% PerCent
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Now you try it
• Report due next week
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Los Alamos National Laboratory's Periodic Table
Group**
Period 1 IA 1A
18
VIIIA 8A
1 1 H
1.008
2
IIA 2A
13
IIIA 3A
14
IVA 4A
15
VA 5A
16
VIA 6A
17
VIIA 7A
2 He 4.003
2 3
Li 6.941
4 Be 9.012
5 B
10.81
6 C
12.01
7 N
14.01
8 O
16.00
9 F
19.00
10 Ne 20.18
8 9 10
3 11
Na 22.99
12 Mg 24.31
3
IIIB 3B
4
IVB 4B
5
VB 5B
6
VIB 6B
7
VIIB 7B
------- VIII -------
------- 8 -------
11
IB 1B
12
IIB 2B
13 Al 26.98
14 Si
28.09
15 P
30.97
16 S
32.07
17 Cl
35.45
18 Ar 39.95
4 19 K
39.10
20 Ca 40.08
21 Sc 44.96
22 Ti
47.88
23 V
50.94
24 Cr 52.00
25 Mn 54.94
26 Fe 55.85
27 Co 58.47
28 Ni 58.69
29 Cu 63.55
30 Zn 65.39
31 Ga 69.72
32 Ge 72.59
33 As 74.92
34 Se 78.96
35 Br 79.90
36 Kr 83.80
5 37
Rb 85.47
38 Sr
87.62
39 Y
88.91
40 Zr
91.22
41 Nb 92.91
42 Mo 95.94
43 Tc (98)
44 Ru 101.1
45 Rh 102.9
46 Pd 106.4
47 Ag 107.9
48 Cd 112.4
49 In
114.8
50 Sn 118.7
51 Sb 121.8
52 Te 127.6
53 I
126.9
54 Xe 131.3
6 55
Cs 132.9
56 Ba 137.3
57 La* 138.9
72 Hf 178.5
73 Ta 180.9
74 W
183.9
75 Re 186.2
76 Os 190.2
77 Ir
190.2
78 Pt
195.1
79 Au 197.0
80 Hg 200.5
81 Tl
204.4
82 Pb 207.2
83 Bi
209.0
84 Po (210)
85 At (210)
86 Rn (222)
7 87 Fr
(223)
88 Ra (226)
89 Ac~ (227)
104 Rf (257)
105 Db (260)
106 Sg (263)
107 Bh (262)
108 Hs (265)
109 Mt (266)
110 ---
()
111 ---
()
112 ---
()
114 ---
()
116 ---
()
118 ---
()
Lanthanide Series*
58 Ce 140.1
59 Pr
140.9
60 Nd 144.2
61 Pm (147)
62 Sm 150.4
63 Eu 152.0
64 Gd 157.3
65 Tb 158.9
66 Dy 162.5
67 Ho 164.9
68 Er
167.3
69 Tm 168.9
70 Yb 173.0
71 Lu 175.0
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Ice melting classic example of entropy increase described in 1862 by Rudolf Clausius
as an increase in the disagregation of the molecules of the body of ice.
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Calories in the food• Calories delivered into water, Q = m*c*∆T
• Q = heat in calories• M = actual mass of water heated ( ≈ 100gram)• C = specific heat of water = 1 cal/(gm-∆T)• Q = 100gm*1cal/(gm*∆T)*∆T = calories
• Calories into water came from food– Calories transferred / mass of food = cal/gram
• If 0.5 gram food (preburn-postburn) yields 2 kcal• 2 kcal / 0.5 gram = 4 kcal/gram for the food• 1.0 pound (454 gm) of this food yields ≈ 1800 kcal
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Carbon FuelsHeat output of fuel results from breaking bonds, releasing energy
• “Heat of Combustion” for carbon fuels (e.g. gasoline, jet fuel)– Called ΔH of combustion, or Combustion Enthalpy
• Source material always contains C and H– Numbers of C & H varies tremendously– Natural products full of variants: linear + branched + ring structures
• Combustion products always contain H2O and CO2
– Sometimes also CO and NOX (N2O, NO, NO2, NO3)– Depends on amount of oxygen available and temperature
• Theoretically possible to calculate heat of combustion for any fuel– Works for simple materials (hydrogen, methane, benzene)– See table for typical values– Not too practical for “real world” bulk materials
• Too many variations and uncertainties with natural products• Dissolved dinosaurs and vegetation don’t yield pure chemical products
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Carbon Fuels
• Fuels have 3 entangled physical properties– Density (grams per cm^3)– Molecular Weight (grams per mole)– Combustion Energy (bond breaking)
• Application defines which is “best”– Higher density (liquid) fuels good for Automobiles
• 5 to 11 carbons in gasoline (depends on season)• More moles per gas tank, drive farther between fillings• Diesel fuel more energy than Gasoline, 11-14 carbons
– Low density (gas) fuels good for domestic use• Vapor state fuels (methane, propane) easy to handle• Constant pressure, simple distribution using pipes• Weight and size of delivery system not important
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Common Fuels
• Burning Hydrogen (proposed by CA)– 1 H-H bond = 436kJ/mol (22.4 Liters or 5.9 gal )
– or 436kJ/gram of H2
• Burning Methane (natural gas)– 4 C-H bond = 1,656kJ/mol, (22.4Liters or 5.9 gal)
– or 1656kJ/mol / 16gm/mol = 106kJ/gm CH4
• Burning Gasoline (octane=C8H18)– 18C-H & 7C-C bond = 7848+2429 =10,277 kJ/mol– Or 10,277kJ/mjol/114g/mol= 90kJ/gram of octane– density = 0.72 gm/mL, 114g/0.72g/mL= 0.16 Liter
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ISO Energy Definition
• Units of Energy, definition of Joule
• Auto data– SUV is 4000 lb= 1842kg– Speed of 62mi/hr = 100km/hr= 27.7 m/sec– kg*(m/s)^2= 1842*27.7*27.7 = 1.41E6 W-S
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• Gasoline efficiency– 18miles/gallon (my Ford Explorer)– 18mi/g/3.84L/g*1.6km/mile 7.4 km/Liter– At 700 gr/Liter, gasoline 10.6 meter/gm– Gasoline energy = 43.6 kJ/gram– Energy expended = 43.6/10.6 = 4.11 kJ/meter
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Energy Unit Conversions
– ISO Definition: 1 Joule ≡ 1 Watt-Second– Units conversion yields 4.184 Joule/calorie– 100 watt device running 1 hour = 36,000 J = 360 kJ
• 100 watts*1 hour*3600 sec/hour = 3.6*10^5 W-s (or Joules)– 360 kJ / 4.18 kJ/kCal = 86 kcal = 86 Cal– One 12 oz can (355ml) Coke Classic = 146 kcal = 146 Cal– 1.7 hour of light bulb use ~ energy in 1 can “Coke Classic”
• Toshiba “Satellite” Laptop, 15V @5A = 75 watts – 75 is 75% of above light bulb example = 2.7*10^5 W-s– 270kJ / 4.18 kJ/kcal = 65 kcal– 2.3 hours laptop energy ~ 1 can of Coke Classic.
– Watt-seconds becoming a commonplace U/M• Direct links between electricity & chemistry U/M• Usual specification units for camera flash
– 50 w-s flash lasts 1/1000 sec, intensity = 50,000 watts !
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Human Energy• At 2000 kCal / day
– 2.00E6 cal/day * 4.184 j/cal = 8.369E6 J/day• same as 8.369E6 watt-seconds/day• 60sec/min*60min/hr*24hr/day=8.64E4 sec/day• (8.369E6 w-s/day) /(8.64E4 sec/day) = 96.8 watts
– Human energy output ≈ 100 watt light bulb!• 20 watts to keep brain going• 80 watts to keep warm, locomotion, organ function
• Issues for A/C and critical environments• 500 people generate 50kW of heat!• Clean rooms adjust A/C to match number of people• Sleeping together keeps us warm (Penguin movie)
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END of Mini-Lecture
• Now to the experiment
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Carbon FuelsHeat output of fuel results from breaking bonds, releasing energy
• “Heat of Combustion” for carbon fuels (e.g. gasoline, jet fuel)– Called ΔH of combustion, or Combustion Enthalpy
• Source material always contains C and H– Numbers of C & H varies tremendously– Natural products full of variants: linear + branched + ring structures
• Combustion products always contain H2O and CO2
– Sometimes also CO and NOX (N2O, NO, NO2, NO3)– Depends on amount of oxygen available and temperature
• Theoretically possible to calculate heat of combustion for any fuel– Works for simple materials (hydrogen, methane, benzene)– See table for typical values– Not too practical for “real world” bulk materials
• Too many variations and uncertainties with natural products• Dissolved dinosaurs and vegetation don’t yield pure chemical products
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Carbon Fuels
• Fuels have 3 entangled physical properties– Density (grams per cm^3)– Molecular Weight (grams per mole)– Combustion Energy (bond breaking)
• Application defines which is “best”– Higher density (liquid) fuels good for Automobiles
• 5 to 11 carbons in gasoline (depends on season)• More moles per gas tank, drive farther between fillings• Diesel fuel more energy than Gasoline, 11-14 carbons
– Low density (gas) fuels good for domestic use• Vapor state fuels (methane, propane) easy to handle• Constant pressure, simple distribution using pipes• Weight and size of delivery system not important
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Common Fuels
• Burning Hydrogen (proposed by CA)– 1 H-H bond = 436kJ/mol (22.4 Liters or 5.9 gal )
– or 436kJ/gram of H2
• Burning Methane (natural gas)– 4 C-H bond = 1,656kJ/mol, (22.4Liters or 5.9 gal)
– or 1656kJ/mol / 16gm/mol = 106kJ/gm CH4
• Burning Gasoline (octane=C8H18)– 18C-H & 7C-C bond = 7848+2429 =10,277 kJ/mol– Or 10,277kJ/mjol/114g/mol= 90kJ/gram of octane– density = 0.72 gm/mL, 114g/0.72g/mL= 0.16 Liter
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ISO Energy Definition
• Units of Energy, definition of Joule
• Auto data– SUV is 4000 lb= 1842kg– Speed of 62mi/hr = 100km/hr= 27.7 m/sec– kg*(m/s)^2= 1842*27.7*27.7 = 1.41E6 W-S
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• Gasoline efficiency– 18miles/gallon (my Ford Explorer)– 18mi/g/3.84L/g*1.6km/mile 7.4 km/Liter– At 700 gr/Liter, gasoline 10.6 meter/gm– Gasoline energy = 43.6 kJ/gram– Energy expended = 43.6/10.6 = 4.11 kJ/meter
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Energy Unit Conversions
– ISO Definition: 1 Joule ≡ 1 Watt-Second– Units conversion yields 4.184 Joule/calorie– 100 watt device running 1 hour = 36,000 J = 360 kJ
• 100 watts*1 hour*3600 sec/hour = 3.6*10^5 W-s (or Joules)– 360 kJ / 4.18 kJ/kCal = 86 kcal = 86 Cal– One 12 oz can (355ml) Coke Classic = 146 kcal = 146 Cal– 1.7 hour of light bulb use ~ energy in 1 can “Coke Classic”
• Toshiba “Satellite” Laptop, 15V @5A = 75 watts – 75 is 75% of above light bulb example = 2.7*10^5 W-s– 270kJ / 4.18 kJ/kcal = 65 kcal– 2.3 hours laptop energy ~ 1 can of Coke Classic.
– Watt-seconds becoming a commonplace U/M• Direct links between electricity & chemistry U/M• Usual specification units for camera flash
– 50 w-s flash lasts 1/1000 sec, intensity = 50,000 watts !
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Human Energy• At 2000 kCal / day
– 2.00E6 cal/day * 4.184 j/cal = 8.369E6 J/day• same as 8.369E6 watt-seconds/day• 60sec/min*60min/hr*24hr/day=8.64E4 sec/day• (8.369E6 w-s/day) /(8.64E4 sec/day) = 96.8 watts
– Human energy output ≈ 100 watt light bulb!• 20 watts to keep brain going• 80 watts to keep warm, locomotion, organ function
• Issues for A/C and critical environments• 500 people generate 50kW of heat!• Clean rooms adjust A/C to match number of people• Sleeping together keeps us warm (Penguin movie)