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CH302 Vanden Bout/LaBrake Fall 2012
Vanden Bout/LaBrake
CH301
THERMODYNAMICS Quantifying Heat Flow – Chemical
Change
UNIT 4 Day 3
CH302 Vanden Bout/LaBrake Spring 2012
Important Information
LM30 DUE T 9AMHW10 DUE T 9AM
EXAM 3 WRAPPER POSTED DUE T 9AM
UNIT4DAY3-VDBThursday, November 15, 20128:19 AM
UNIT4DAY3-VDB Page 1
CH302 Vanden Bout/LaBrake Fall 2012
What are we going to learn today?
Use calorimetry to calculate ΔHrxn
Use different methods to calculate ΔHrxn
Heats of FormationHess’s LawBond Energies
CH302 Vanden Bout/LaBrake Fall 2012
Quiz: CLICKER QUESTION 1
A bomb calorimeter measures heat at constantVolume, which is equivalent to.a) ΔUb) ΔHc) Work
UNIT4DAY3-VDB Page 2
CH302 Vanden Bout/LaBrake Fall 2012
First Law of Thermodynamics –
LAW OF CONSERVATION OF ENERGY
UNIVERSE = SYSTEM + SURROUNDINGS
YOU DEFINE YOUR SYSTEM, EVERYTHING ELSE IS
SURROUNDINGS
ΔU = q + w
CH302 Vanden Bout/LaBrake Fall 2012
Enthalpy Change w/ Chemical Change
ENER
GY
REACTION PATH
2 CH3OH + 3 O2 2 CO2 + 4 H2O + heat
CH302 Vanden Bout/LaBrake Fall 2012
Enthalpy Change w/ Chemical Change
• Thermochemical equation- a chemical change together with the corresponding enthalpy change
• CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890 kJ mol rxn-1
• 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(l) ΔH = -1780 kJ mol rxn-1
• CO2(g) + 2H2O(l) CH4(g) + 2O2(g) ΔH = +890 kJ mol rxn-1
UNIT4DAY3-VDB Page 3
CH302 Vanden Bout/LaBrake Fall 2012
Enthalpy Change w/ Chemical Change
• Thermochemical equation- a chemical change together with the corresponding enthalpy change
• CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890 kJ mol rxn-1
• 2CH4(g) + 4O2(g) 2CO2(g) + 4H2O(l) ΔH = -1780 kJ mol rxn-1
• CO2(g) + 2H2O(l) CH4(g) + 2O2(g) ΔH = +890 kJ mol rxn-1
CH302 Vanden Bout/LaBrake Fall 2012
UNIT4DAY3-VDB Page 4
CH302 Vanden Bout/LaBrake Fall 2012
Standard Enthalpies, ΔHrxnº
• Reaction enthalpy based on all reactants and products being in their standard state (1 bar pressure). Tabulated data can be assumed to be at 25°C.
CH302 Vanden Bout/LaBrake Fall 2012
IT IS POSSIBLE TO CALCULATE THE ENTHALPY OF CHEMICAL CHANGE USING TABULATED DATA
MANY REASONS FOR THIS:
ESTIMATE RXN ENTHALPYTOO COSTLY TO RUN EXPERIMENTWANTING TO PREDICT SPONTENEITY
CH302 Vanden Bout/LaBrake Fall 2012
Hess’s Law – Combine a series of chemical reactions to estimate the change in enthalpy for the net equation.
CO2(g) CO(g) + ½O2(g) ΔHº = + 283 kJ mol rxn-1
C (s) + O2(g) CO2 (g) ΔHº = - 393 kJ mol rxn-1
USE THIS EXAMPLE TO DEMONSTRATE HESS’S LAW
C (s) + ½ O2 (g) CO (g) ΔHº = ?
UNIT4DAY3-VDB Page 5
CH302 Vanden Bout/LaBrake Fall 2012
Hess’s Law – Combine a series of chemical reactions to estimate the change in enthalpy for the net equation.
CO2(g) CO(g) + ½O2(g) ΔHº = + 283 kJ mol rxn-1
C (s) + O2(g) CO2 (g) ΔHº = - 393 kJ mol rxn-1
USE THIS EXAMPLE TO DEMONSTRATE HESS’S LAW
C (s) + ½ O2 (g) CO (g) ΔHº = ?
CH302 Vanden Bout/LaBrake Fall 2012
Hess’s Law – Combine a series of chemical reactions to estimate the change in enthalpy for the net equation.
• Methanol is a clean burning liquid fuel proposed as a replacement for gasoline. Suppose it could be produced by the controlled reaction of the oxygen in air with methane. Find the standard reaction enthalpy for the formation of 1 mole CH3OH(l) from methane and oxygen, given the following information:
CH4(g) + H2O(g) CO(g) + 3 H2(g) ΔHº = +206.10 kJ
2 H2(g) + CO(g) CH3OH(l) ΔHº = -128.33 kJ
2H2(g) + O2 (g) 2H2O (g) ΔHº = -483.64 kJ
USE THIS EXAMPLE TO DEMONSTRATE HESS’S LAW
CH302 Vanden Bout/LaBrake Fall 2012
Hess’s Law -example
CH4(g) + H2O(g) CO(g) + 3 H2(g) ΔHº = +206.10 kJ
2 H2(g) + CO(g) CH3OH(l) ΔHº = -128.33 kJ
2H2(g) + O2 (g) 2H2O (g) ΔHº = -483.64 kJ
UNIT4DAY3-VDB Page 6
CH302 Vanden Bout/LaBrake Fall 2012
Hess’s Law -example
CH4(g) + H2O(g) CO(g) + 3 H2(g) ΔHº = +206.10 kJ
2 H2(g) + CO(g) CH3OH(l) ΔHº = -128.33 kJ
2H2(g) + O2 (g) 2H2O (g) ΔHº = -483.64 kJ
CH302 Vanden Bout/LaBrake Fall 2012
Standard Enthalpy of Formation, ΔHfº
• ΔH for the formation of 1 mole of a compound from its elements in their most stable form at standard conditions
• 2 C (gr) + 3 H2(g) + ½ O2(g) C2H5OH(l) ΔHfº = -277.67 kJ
CH302 Vanden Bout/LaBrake Fall 2012
Use ΔHfº to calculate ΔHr
º
• Possible because ΔH is a state function: path doesn’t matter, just final minus initial.
• ΔHrº = ΣnΔHf
ºproducts - ΣnΔHf
ºreactants
UNIT4DAY3-VDB Page 7
CH302 Vanden Bout/LaBrake Fall 2012
Use ΔHfº to calculate ΔHr
º
• Possible because ΔH is a state function: path doesn’t matter, just final minus initial.
• ΔHrº = ΣnΔHf
ºproducts - ΣnΔHf
ºreactants
CH302 Vanden Bout/LaBrake Fall 2012
Example: Use ΔHfº to calculate ΔHr
º
• Calculate the standard enthalpy of combustion of methanol from data.
CH302 Vanden Bout/LaBrake Fall 2012
Bond enthalpies
• The heat required to break a mole of bonds at constant pressure.
• Calculate the heat of reaction for methanol using bond energies.
UNIT4DAY3-VDB Page 8
CH302 Vanden Bout/LaBrake Fall 2012
Bond enthalpies
Example: Estimate the ΔHrº for the reaction
CCl3CHCl2(g) + 2HF(g) CCl3CHF2(g) + 2HCl(g)
using bond enthalpy data.
CH302 Vanden Bout/LaBrake Fall 2012
CH302 Vanden Bout/LaBrake Fall 2012
POLL: CLICKER QUESTION 2 FROM ACTIVITY
UNIT4DAY3-VDB Page 9
CH302 Vanden Bout/LaBrake Fall 2012
POLL: CLICKER QUESTION 2 FROM ACTIVITY
CH302 Vanden Bout/LaBrake Fall 2012
POLL: CLICKER QUESTION 3 FROM ACTIVITY
UNIT4DAY3-VDB Page 10
CH302 Vanden Bout/LaBrake Fall 2012
WHAT HAVE WE LEARNED TODAY?
The transfer of heat energy into or out of a system at constant pressure is a state function called Enthalpy.
The change in Enthalpy can be determined experimentally using a coffee cup calorimeter at constant pressure.Then change in Enthalpy can be calculated based on a variety of tabulated data:Heats of formation/Other Heats of Reaction/Bond Energies
CH302 Vanden Bout/LaBrake Fall 2012
Learning Outcomes
Write a formation chemical equation for a compound
Calculate change in enthalpy for a reaction based on calorimetry data
Calculate change in enthalpy for a reaction based on tabulated data (Hess’s law, formation data, bond energy data).
UNIT4DAY3-VDB Page 11