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

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

Earth’s Energy from Sunshine

• Plants use sunlight & photosynthesis

• We eat plants & animals which eat plants

• Fossil fuels from ancient plant life– Oil, coal, natural gas

• Temperature changes could modify life

• Vicious cycle– CO2 escapes from heated ocean reservoirs

– More CO2 traps more heat, hotter oceans

Temperatures around the Worldleft picture refers to January temperaturesHeat energy is delivered by solar radiation

Temperature difference is a result of unequal heat delivery

4

Seasons due to earth’s tilt

Global Temperature HistoryShort term “warming”, but long term trend is “cooling”

Global WarmingCurrent trend began >10k years ago, are we now “between glacial periods”?

88

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

9

James Joule experimentdemonstrated equivalence of potential energy and heat

1010

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

1111

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

12

EXOthermic reaction (-ΔH) Producing heat or thermal energy by burning fuel,

converting chemical (or nuclear) into kinetic or heat energy

1313

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

14

ENDOthermic reaction (+ΔH)Absorbing heat energy from environment

15

Air Conditioning = ENDOthermic cooling results from evaporation reaction absorbing heat

accompanied by exothermic condensation at radiator

1616

1717

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 possible 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

1818

Table of Bond Energies combustion heat output

Can use tables to estimate total bond energy

20

Sample Bond Energy CalculationBurning of Hydrogen in Air, producing heat

Tables of data differ, but have similar values

2121

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)

2222

Heats of reactionsyour home furnace in chemical terms

one 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

Heat of Solution

• simply mixing materials can produce heat– Why we add acid to water, not vice versa

• How does it work? – Dissolving salts can either get cold or hot

• Two things are happening– Turning a solid into ions requires energy input

• Pulling ions from solid against electrostatic attraction

– Ions combine with water releasing energy• Bonds form between water and cations or anions

– The difference can be exothermic or endothermic

Dissolving in Water involves breaking and forming bondsthe difference is gain or loss of heat

Heat of Solutionsalt into ions requires energy, hydrating ions liberates energy

difference in energies produces heat or cold

CaCl2 heat of solutionseparating ions requires energy, combining with water produces

energyIf production exceeds expenditure, solution gets HOT (exothermic)Image concept is OK values need improvement, note 3 ions formed

27

Heat of solution (dissolving)

Other examplesincluding experiment material Ammonium Chloride

two hydrated ions formed (versus 3 for CaCl2)

http://www.chem.ox.ac.uk/vrchemistry/energy/Page_25.htm

29

We will use a simple calorimeterstyrofoam cups for insulationswirling better than stirring

What’s wrong with this picture?(recall James Joule’s experiment with stirring)

30

31

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

32

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?

33

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

34

Procedure• Water

– Weigh styrofom cup empty and with ≈100mL water– Difference is mass of water in grams

• Salt– Weigh container without & with salt– Difference is 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

35

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

36

Calculation Procedure• Use graph to determine maximum temperature reached• Calculate calories produced

– Mass (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

37

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

38

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

39

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

40

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

41

Now you try it

• Report due next week

42

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

43

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

4444

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

4545

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

46

Gasoline

• 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

47

Fuels compared

48

Fuel differencesMore hydrogen=more energy, more carbon = more CO2

49

Ethanol as Auto FuelBrazil offers both Ethanol and Gasoline at the pump

50

Automobile dynamics

5151

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

52

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 !

53

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)

5454

Water EnergyMaking water from elements releases heat energy

splitting water into elements requires electrical energy

55

A few common bond energies

58

Some mechanisms

Heat change in a chemical reaction

Burning Alcohol in Air (gasohol)breaking bonds REQUIRES energy, forming bonds RELEASES energy

The difference results in exothermic or endothermic effect

360

61

Calorimeter in a cup

62

Calorimeter in a cup

6363

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

64

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