Calorimetry and Hesss Law

1
Calorimetry and Hesss Law

• Experiment 3

2
Experiment 3

• Goal
• To determine heat capacities and enthalpy changes
• Method
• Measure heat transferred to/from various
  processes using a simple calorimeter
• Use Hesss Law

3
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

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First Law of Thermodynamics
Energy change in system
heat disordered energy transferred
work ordered energy transferred


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Enthalpy, H

• Enthalpy change
• heat transferred at constant pressure
• most processes on earth

Often DH ? DE
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Enthalpy change
p

• At constant pressure

p
DV
Vi
Vf
Generally small
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Enthalpy changes

• 1. Thermal enthalpy
• higher T ? greater H
• 2. Phase enthalpy
• Hsolid lt Hliquid lt Hgas
• 3. Chemical (reaction) enthalpy
• exothermic heat released
• endothermic heat absorbed

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Calorimetry at Constant Pressure

• Coffee Cup Calorimeter
• q ? DT
• qp CDT
• DH CDT
• C heat capacity

Heat flow (qp)
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Heat Capacity

• Amount of energy to raise the temperature of a
  substance 1C (1 K)
• Depends on materials amount and type
• c, specific heat capacity units energy per
  gram.oC
• C, molar heat capacity units energy per
  mole.oC

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Enthalpy of phase change

• No DT at a phase change
• water vaporizes/condenses at 100C
• DHvap DHcond
• water fuses/melts at 0C
• DHfus DHmelt
• q ? amount and type

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Units

• 1. Dhtransition energy/g
• 2. DHtransition energy/mol

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Measuring DH (qp)

• Perfect calorimeter is adiabatic

Heat gained/lost by sample heat lost/gained by
calorimeter
so
No heat transfer from calorimeter to environment
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Basic Calorimeter Setup
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Overview

• 1 Calorimeter calibration each pair
• Heat capacity of calorimeter
• 2 Determine heat of fusion (DHf) of water one
  pair in
• Heat required to melt ice small group
• 3 Determine specific heats of metals other
  pair in
• qmetal DHmetal mmetal.cmetal.DTmetal small
  group
• 4 Determine heats of reaction (DHrxn)
• 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

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Part 1 Calorimeter Calibration

• Mass dry calorimeter mcal
• Add 100 mL room temp H2O
• Mass and record temperature mcw,Tcw
• Add 100 mL of 35C H2O Thw
• Mass and record temperature mall,Tf
• Find heat capacity of calorimeter Ccal

from warm water
to cool water calorimeter
heat
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Calculations
loss (T dec)
gain (T inc)
gain (T inc)
cwater 4.184 J/g.C
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Calibration Example Data

• mass Tinitial Tfinal
• Empty calorimeter 5.00 g 21.2oC 28.0oC
• Cool water 100. g 21.2oC 28.0oC
• Warm water 100. g 35.0C 28.0oC
• cwater 4.184 J/g.C
• ! Stop here and calculate your calorimeters heat
  capacity !
• It should be between 5 and 200 J/oC.
• If its negative or too large, repeat the
  calibration.

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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 0C
• Record lowest temperature Tf
• Mass mall

from room T water calorimeter
to melting ice heating ice
heat
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Calculations
gain (T inc)
gain (T inc)
loss (T dec)
loss (T dec)

• Convert Dhfus to DHfus in kJ/mol
• Lit value is 6.02 kJ/mol

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Heat of Fusion Example Data

• mass Tinitial Tfinal
• Empty calorimeter 5.00 g 21.2C 1.9oC
• Water 100. g 21.2C 1.9oC
• Ice 25.0 g 0.0C 1.9oC

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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 mwcal, TRT
• Slide metal into calorimeter
• Record highest temperature Tf

to water calorimeter
heat
from hot metal
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Calculations
loss (T dec)
gain (T inc)
gain (T inc)
Convert cmet to required units
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Metals/Specific Heat Example Data

• mass Tinitial Tfinal
• Empty calorimeter 5.00 g 21.2C 22.6C
• Water 100. g 21.2C 22.6C
• Hot metal 20.0 g 100.C 22.6C

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Metal heat capacities

• Law of Dulong and Petit
• Cmetal (per mol) 3R 24.94 J/K.mol
• where R 8.314 J/K.mol
• Compare your value
• Measured values
• water 75.29 J/K. mol
• Cu 24.44 J/K. mol
• Al 24.35 J/K. mol
• Compare your value

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Part 4 Heat of Chemical Reaction (a)

• Mg 2 HCl ? MgCl2 H2 DHa
• 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

from reaction
to water calorimeter
heat
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Part 4 Heat of Chemical Reaction (b)

• MgO 2 HCl ? MgCl2 H2O DHb
• 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

from reaction
to water calorimeter
heat
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Calculations
loss (T dec)
gain (T inc)
gain (T inc)
mol reactant
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Calculations
loss (T dec)
gain (T inc)
gain (T inc)
mol reactant
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Mg/HCl Example Data

• mass Tinitial Tfinal
• Empty calorimeter 5.00 g 21.2C 28.1oC
• Acidic H2O 75.0 g (mL) 21.2C 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)

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MgO/HCl Example Data

• mass Tinitial Tfinal
• Empty calorimeter 5.00 g 21.2C 24.6oC
• Acidic H2O 75.0 g (mL) 21.2C 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)

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Part 4 Heat of Chemical Reaction

• Mg ½ O2 ? MgO
• Use
• Mg 2 HCl ? MgCl2 H2 DHa
• MgO 2 HCl ? MgCl2 H2O DHb
• H2 ½ O2 ? H2O DHc
• DHc -285.8 kJ

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Part 4 - Hesss Law

• DHrxn SDHindividual rxns
• A ? B DHAB
• B ? C DHBC
• A B ? B C
• A ? C DHAC DHAB DHBC

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Part 4 Hesss Law Application

• Heat of Reaction for Mg combustion
• Mg 2 H ? Mg2 H2 ? Ha
• Mg2 H2O ? MgO 2 H ?Hb
• H2 ½ O2 ? H2O ? Hc
• Mg ½ O2 ? MgO ? Hcomb(or form)
• compare to lit value (-601 kJ)

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Part 4 Example Calculation

• Heat 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/mol
• compare to lit value (-601 kJ)

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Report

• Abstract
• Results
• Sample calculations including
• Calorimeter heat capacity (part 1)
• Heat of fusion of ice (part 2)
• Specific heats of metals Cu and Al (part 4)
• Heats of reactions (part 4)
• Discussion/review questions