Calorimetry and Hess’s Law Experiment 3
Dec 19, 2015
Experiment 3
Goal: To determine heat capacities and
enthalpy changes
Method: Measure heat transferred to/from
various processes using a simple calorimeter
Use Hess’s Law
Definitions
Thermodynamics:The study of heat and its transformations
Energy:The capacity to do work or to produce heat
System:The part of the universe of interest
Surroundings:Everything else (relevant to system)
Universe:System + surroundings
First Law of Thermodynamics
w q
E EΔE initialfinalsys
Energy change in system
= heat disordered energy transferred
+ work
ordered energy transferred
Enthalpy, H
VpΔEqΔH p
Enthalpy change:– heat transferred at constant pressure– most processes on earth
Often:Often: H H EE
Enthalpy changes
1. Thermal enthalpyhigher T greater H
2. Phase enthalpyHsolid < Hliquid < Hgas
3. Chemical (reaction) enthalpyexothermic: heat released
endothermic: heat absorbed
Calorimetry at Constant Pressure
Coffee Cup Calorimeter
q T
qp = CT
H = CT
C: heat capacity
Heat flow (qp)
Heat Capacity
Amount of energy to raise the temperature of a substance 1°C (1 K)Depends on material’s amount and type
c, specific heat capacity: units – energy per gram.oC
C, molar heat capacity: units – energy per mole.oC
CCg
energygTmcqΔH o
op
CCmol
energymolTnCqΔH o
op
Enthalpy of phase change
No T at a phase change water vaporizes/condenses at 100°C
Hvap = –Hcond
water “fuses”/melts at 0°C
Hfus = –Hmelt
q amount and type
Units1. htransition: energy/g
2. Htransition: energy/mol
mol
energymolHnΔH transitionmeasures
g
energyghmΔH transitionmeasured
Measuring H (qp)
Perfect calorimeter is adiabatic:
rcalorimetesample q q
Heat gained/lost by sample = heat lost/gained by calorimeter
~No heat transfer from calorimeter to environment
0 rcalorimetesample q q
so0contentsrcalorimeteΔH
Overview1: Calorimeter calibration each pair
Heat capacity of calorimeter
2: Determine heat of fusion (Hf) of water one pair in Heat required to melt ice small group
3: Determine specific heats of metals other pair in qmetal = Hmetal = mmetal
.cmetal.Tmetal small group
4: Determine heats of reaction (Hrxn) 1) Mg + 2HCl MgCl2 + H2 one pair 2) MgO + 2HCl MgCl2 + H2O other pair Overall) Mg + ½ O2 MgO
EVERYONE does individual report containing results for all FOUR parts of the lab
Part 1 Calorimeter Calibration
Mass dry calorimeter mcal
Add 100 mL room temp H2O
Mass and record temperature mcw,Tcw
Add 100 mL of 35°C H2O Thw
Mass and record temperature mall,Tf
Find heat capacity of calorimeter Ccal
fromwarm water
tocool water + calorimeter
heat
Calculations
0 H H ΔH rcalorimetecool waterhot water
0
TTC
TTc m TTcm
cwfrcalorimete
cwfcwcwhwfhwhw
loss (T dec)
gain(T inc)
gain(T inc)
cwater: 4.184 J/g.°C
Calibration Example Data
mass Tinitial Tfinal
Empty calorimeter 5.00 g 21.2oC28.0oC
Cool water 100. g 21.2oC 28.0oC
Warm water 100. g 35.0°C 28.0oCcwater: 4.184 J/g.°C
! Stop here and calculate your calorimeter’s heat capacity !It should be between 5 and 200 J/oC.If it’s negative or too large, repeat the calibration.
C
Jrcalorimete
orcalorimete
o
Cg
Jo
Cg
J
o
oo
.C
C..C C...g.C...g.
312
022102822102818441000350281844100
Part 2 Heat of Fusion of Ice
Mass dry calorimeter mcal
Add 100 mL water to calorimeter Mass and record temperature mRT,TRT
Add 25 g solid ice Tice = 0°C
Record lowest temperature Tf
Mass mall
fromroom T water &
calorimeter
tomelting ice & heating ice
heat
Calculations
0 H H H ΔH warm icemelt icercalorimeteRTwater
00
CTcmΔh m
TTC TTcmo
ficeicefusice
RTfrcalorimeteRTfRTRT
loss (T dec)
gain(T inc)
gain(T inc)
loss (T dec)
Convert hfus to fus in kJ/mol
Lit value is 6.02 kJ/mol
Heat of Fusion Example Data
mass Tinitial Tfinal
Empty calorimeter5.00 g 21.2°C1.9oC
Water 100. g 21.2°C1.9oC
Ice 25.0 g 0.0°C 1.9oC
C
Jfus
o
C
Jo
Cg
J
o
Cg
Jfus
o
oo
o
h
C... C...g.
C...g.g.h
325
022191312221911844100
0091184425025
molkJ
gkJ
molg.
gJ
fus .H 86510001
10218325
Part 3 Specific Heats of Metals
Mass metal mmetal10-30 g
Heat in large test tube in boiling water bath 30 min Thw
(start at beginning) Mass dry calorimeter mcal
Add 100 mL water Mass and record temperature
mw+cal, TRT
Slide metal into calorimeter Record highest temperature Tf
fromhot metal
towater + calorimeter
heat
Calculations
0 H H H rcalorimetecool watermetal
0
RTfrcalorimete
RTfRTwhwfmetmet
TT C
TTcm TTcm
loss (T dec)
gain(T inc)
gain(T inc)
Convert cmet to required units
Metals/Specific Heat Example Data
mass Tinitial Tfinal
Empty calorimeter5.00 g 21.2°C 22.6°C
Water 100. g 21.2°C 22.6°C
Hot metal 20.0 g 100.°C 22.6°C
Cg
Jmetal
o
C
Jo
Cg
Jometal
o
oo
.c
C...C...g.C..cg.
3900
022162231222162218441000100622020
copperforisthis.c molJ
molg.
Cg
J.metal o 8241
55633900
Metal heat capacities
Law of Dulong and Petit:Cmetal (per mol) = 3R = 24.94
J/K.molwhere R = 8.314 J/K.mol
Compare your value
Measured values:water 75.29 J/K. molCu 24.44 J/K. molAl 24.35 J/K. mol
Compare your value
Part 4 Heat of Chemical Reaction (a)
Mg + 2 HCl MgCl2 + H2 Ha
Mass empty calorimeter mcal
Add 25 mL HCl to 50 mL water Mass and record temperature mRT,TRT
Mass 0.10 – 0.15 g Mg; add mMg
Record highest temperature Tf
fromreaction
towater &
calorimeter
heat
Part 4 Heat of Chemical Reaction (b)
MgO + 2 HCl MgCl2 + H2O Hb
Mass empty calorimeter mcal
Add 25 mL HCl to 50 mL water Mass and record temperature
mRT,TRT
Mass 0.30 g MgO; add mMgO
Record highest temperature Tf
fromreaction
towater &
calorimeter
heat
Calculations
0 H H H rcalorimetecool waterrxn
01
1
TTC
TTcm HMM
m
frcalorimete
fcwcwrxnMg
Mg
loss (T dec)
gain(T inc)
gain(T inc)
#mol reactant
Calculations
0 H H H rcalorimetecool waterrxn
01
1
TTC
TTcm HMM
m
frcalorimete
fcwcwrxnMgO
MgO
loss (T dec)
gain(T inc)
gain(T inc)
#mol reactant
Mg/HCl Example Data
mass Tinitial Tfinal
Empty calorimeter5.00 g 21.2°C28.1oC
Acidic H2O 75.0 g (mL)21.2°C 28.1oC
Mg metal 0.12 g
Assume the heat capacity of the solution is that of water (big assumption)
Literature value: -462kJ/mol (compare your value)
gMgJ
a
o
C
Jo
Cg
Ja
.h
C...C...g.gMg.h oo
618779
022112831222112818441275120
HCl/MgforisthisH molkJ
JkJ
molg.
gMgJ.
a 45710001
13124618779
MgO/HCl Example Data
mass Tinitial Tfinal
Empty calorimeter5.00 g 21.2°C24.6oC
Acidic H2O 75.0 g (mL)21.2°C 24.6oC
MgO powder 0.30 g
Assume the heat capacity of the solution is that of water (big assumption)
Literature value: -146kJ/mol (compare your value)
gMgO
Jb
o
C
Jo
Cg
Jb
h
C...C...g.gMgO.h oo
1113
022162431222162418443075300
HCl/MgOforisthisH molkJ
JkJ
molg.
gMgOJ
b 15010001
131401113
Part 4 Heat of Chemical Reaction
Mg + ½ O2 MgO
Use:Mg + 2 HCl MgCl2 + H2 Ha
MgO+ 2 HCl MgCl2 + H2O Hb
H2 + ½ O2 H2O Hc
Hc = -285.8 kJ
Part 4 Hess’s Law Application
Heat of Reaction for Mg combustion
Mg + 2 H+ Mg2+ + H2 ΔHa
Mg2+ + H2O MgO + 2 H+ –ΔHb
H2 + ½ O2 H2O ΔHc
Mg + ½ O2 MgO ΔHcomb(or form)
compare to lit value (-601 kJ)
Part 4 Example CalculationHeat of Reaction for Mg combustion
Mg + 2 H+ Mg2+ + H2 –457 kJ/mol
Mg2+ + H2O MgO + 2 H+ +150 kJ/mol
H2 + ½ O2 H2O –286 kJ/mol
Mg + ½ O2 MgO –593 kJ/molcompare to lit value (-601 kJ)