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Calorimetry and Hesss Law

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Measure heat transferred to/from various processes using a simple ... exothermic: heat released. endothermic: heat absorbed. Calorimetry at Constant Pressure ... – PowerPoint PPT presentation

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Title: Calorimetry and Hesss Law


1
Calorimetry and Hesss Law
  • Experiment 10

2
Experiment 10
  • 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

4
First Law of Thermodynamics
Energy change in system
heat disordered energy transferred
work ordered energy transferred


5
Enthalpy, H
  • Enthalpy change
  • heat transferred at constant pressure
  • most processes on earth

Often DH ? DE
6
Enthalpy change
p
  • At constant pressure

p
DV
Vi
Vf
Generally small
7
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

8
Calorimetry at Constant Pressure
  • Coffee Cup Calorimeter
  • q ? DT
  • qp CDT
  • DH CDT
  • C heat capacity

Heat flow (q)
9
Heat Capacity
  • Amount of energy required to raise the
    temperature of a substance 1C (1 K)
  • Depends on materials amount and type
  • c, specific heat capacity per gram
  • C, molar heat capacity per mole

10
Units
  • 1. c (specific) energy/g.C
  • 2. C (molar) energy/mol.C

11
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

12
Units
  • 1. Dhtransition energy/g
  • 2. DHtransition energy/mol

13
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

14
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
15
Basic Calorimeter Setup
16
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
17
Calibration Example Data
  • sample mass Tinitial Tfinal
  • Empty
  • Calorimeter 5.00 g
  • 21.2C
  • Cool water 100. g 28.2
  • Warm water 100. g 35.3C

18
Calculations
loss (T dec)
gain (T inc)
gain (T inc)
cwater 4.184 J/g.C
19
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
20
Heat of Fusion Example Data
  • sample mass Tinitial Tfinal
  • Empty
  • Calorimeter 5.00 g
  • 21.2C
  • Water 100. g ???
  • Ice 25.0 g 0.0C

21
Calculations
gain (T inc)
gain (T inc)
loss (T dec)
loss (T dec)
  • Convert Dhfus to DHfus in kJ/mol
  • Lit value is 6.01 kJ/mol

22
Part 3 Specific Heats of Metals
  • Mass metal mmetal10-30 g
  • Heat in large test tube in
  • boiling water bath 30 min Thw
  • 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
23
Metals/Specific Heat Example Data
  • sample mass Tinitial Tfinal
  • Empty
  • Calorimeter 5.00 g
  • 21.2C
  • Water 100. g ???
  • Hot metal 20.0 g 100.C

24
Calculations
loss (T dec)
gain (T inc)
gain (T inc)
Convert cmet to required units
25
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

26
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
27
Mg, MgO/HCl Example Data
  • sample mass Tinitial Tfinal
  • Empty
  • Calorimeter 5.00 g
  • Acidic H2O 75.0 g (mL) 21.2C ???
  • Mg metal 0.12 g
  • Acidic H2O 75.0 g (mL) 21.2C ???
  • MgO powder 0.30 g

28
Calculations
loss (T dec)
gain (T inc)
gain (T inc)
mol reactant
29
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
30
Calculations
loss (T dec)
gain (T inc)
gain (T inc)
mol reactant
31
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

32
Part 4 - Hesss Law
  • DHrxn SDHindividual rxns
  • A ? B DHAB
  • B ? C DHBC
  • A B ? B C
  • A ? C DHAC DHAB DHBC

33
Part 4 Hesss Law Application
  • Heat of Reaction for Mg combustion
  • Mg 2 H ? Mg2 H2 DHa
  • Mg2 H2O ? MgO 2 H DHb
  • H2 ½ O2 ? H2O DHc
  • MgO ½ O2 ? MgO DHcomb(or form)
  • compare to lit value (-601 kJ)

34
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

35
Energy change
  • At constant volume
  • w 0

pi
pf
36
Measuring DE (qv)
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