Post on 01-Jan-2016
description
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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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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
Temperatures around the Worldleft picture refers to January temperaturesHeat energy is delivered by solar radiation
Temperature difference is a result of unequal heat delivery
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Global Temperature HistoryShort term “warming”, but long term trend is “cooling”
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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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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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
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
5757
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
58
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
5959
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)